摘要:

21 Organometallic chemistry of monometallic species By P. K. BAKER Department of Chemistry University of Wales Bangor Gwynedd LL57 2UW UK 1 Introduction Important reviews have been published on Si–H and C–H activation by transitionmetal complexes,1 cationic iridium(III) complexes a step towards the synthetic applicability of C–H activation,2 transition-metal alkane complexes3 and chemical redox agents for organometallic chemistry.4 Extensive reviews have appeared on the organometallic chemistry of carbon dioxide,5 open-shell organometallics as a bridge between Werner-type and low-valent organometallic complexes,6 the organometallic chemistry of halogenocarbonyl complexes of molybdenum(II) and tungsten(II),7 the reactions of 17- and 19-electron organometallic complexes,8 substituent e§ects as probes of structure and bonding in mononuclear metallocenes,9 and diheteroferrocenes and related derivatives of the Group 15 elements.10 Reviews on recent advances in the chemistry of organochromium n complexes,11 the versatile chemistry of arenemanganese carbonyl complexes,12 and metallophosphaalkenes13 have been published.Reviews have also appeared on cationic Group 4 metallocene complexes and their role in polymerisation catalysis,14 enantioselective C–C and C–H bond formation mediated or catalysed by chiral ebthi complexes of Ti and Zr,15 a-agostic interactions and alkene insertion in metallocene polymerisation catalysts,16 chiral CO-emulating ligands from arene chromium chemistry to enantioselective catalysis,17 methanol carbonylation revisited thirty years on,18 and the co-ordination chemistry of carbon dioxide and its relevance to catalysis.19 (n-Allyl)tricarbonyliron lactone complexes20 and unsaturated metallacyclic carbene complexes21 in organic synthesis; asymmetric transition-metal allylic alkylations,22 [Fe(CO) 3 (diene)] complexes as a guide to stereocontrol,23 applications of n-areneruthenium complexes in peptide labelling and peptide synthesis,24 and enantioselective syntheses of organosulfur compounds via 2,3 sigmatropic rearrangements of ylides derived from bis(allyl)-.bis(propargyl)- and bis(benzyl)-sulfide rhenium complexes25 have also been reviewed. 2 Titanium zirconium and hafnium The first examples of a zirconium pentacarbonyl species [Zr(SnMe 3 ) 2 (CO) 5 ]2~ and an arylphosphine Zr0 complex [Zr(SnMe 3 )(CO) 4 (dppe)]~ have been prepared by reacting [Zr(g4-C 10 H 8 ) 3 ]2~ with SnMe 3 Cl followed by carbonylation in the absence Royal Society of Chemistry–Annual Reports–Book A 375 or presence of dppe respectively.26 Treatment of [Ta(CH 2 EMe 3 ) 2 (––CHEMe 3 )- (SiBu5Ph 2 )] (E\C or Si) with PMe 3 gives the bis(alkylidene) complexes [Ta(CH 2 EMe 3 )(––CHEMe 3 ) 2 (PMe 3 ) 2 ] (structurally characterised for E\Si) via preferential silane elimination.However [Ta(CH 2 SiMe 3 ) 2 (–– CHSiMe 3 )(SiBu5Ph 2 )] thermally decomposes to an alkylalkylidyne complex [Ta 2 (CH 2 SiMe 3 ) 4 (k-CSiMe 3 ) 2 ].27 The synthesis and molecular structure of the alkene polymerisation catalyst [Zr(g1- CH 2 Ph)(g2-CH 2 Ph)M(C 6 H 3 ) 2 (NCH 2 C 6 H 4 Ph-4) 2 -2,2@-Me 2 -6,6@N] which has an g2- benzyl co-ordination mode in the solid state and an averaged C 2 symmetry in solution have been described.28 Some new cationic alkylzirconium complexes29,30 such as 1,30 which polymerises both ethene and propene have been described.Zr+ N N CH2 Me3Si Me3Si Me3Si Si Me Me NMe2Ph 1 [B(C6F5)4] O Ti N But H Me Me H 2 Li R R R R CpZr 3 Reaction of [Ti(PMe 3 ) 2 Cp 2 ] with 2,2-dimethyl-5-hexenal in pentane followed by treatment with CNBu5 gave the crystallographically characterised complex 2 which oxidises in air to a§ord a novel fulvene.31 An EPR spectroscopic study of mixtures of [TiR 3 Cp*] (R\CH 3 or 13CH 3 ) and equimolar amounts of B(C 6 F 5 ) 3 or [CPh 3 ]- [B(C 6 F 5 ) 4 ] in the syndiospecific polymerisation of styrene has been made.32 The first example of the catalytic dehydrocyclooligomerisation of primary phosphines by the crystallographically characterised complex K[ZrH 3 Cp* 2 ] was reported at the end of 1995.33 The cationic complex [Zr(CH 2 CMe 3 )L 2 ] (L\Cp*) eliminates isobutene at [75 °C whereas the less crowded complex (L\Cp) is stable in solution at 0 °C but undergoes reversible b-elimination at 25 °C.34 Reaction of [ZrCl 2 Cp 2 ] with three equivalents of LiBu/ yields a 1 1 mixture of [ZrBu/ 3 Cp] and LiCp which reacts with HCl or I 2 to a§ord n-butane and [ZrCl 2 Cp 2 ] or Bu/I and [ZrI 2 Cp 2 ] respectively; reaction of [ZrCl 2 Cp 2 ] with 3.3 equivalents of LiC 6 H 13 and hex-1-ene gives the proposed zirconate 3.35 The synthesis and crystal structure of racemic [TiCl(CH 2 SiMe 3 )Cp(g5-C 9 H 7 )] which is the first chiral-at-titanium (alkyl) complex have been described.36 The preparation of the first example of a crystallographically characterised d1 titanocene cation containing neutral ligands [Ti(py) 2 Cp 2 ][BPh 4 ] has been reported.37 The unprecedented reaction of a [TiF 2 Cp 2 ] in the presence of phenylsilane gives an active catalyst for the hydrosilylation of imines; when M(S,S)- ethylene[bis(g5-tetrahydroindenyl)]Ntitanium difluoride is used as the precatalyst imines are converted to the corresponding amines with very high enantioselectivity.38 Zirconocene–betaine complexes for the development of new catalysts have been prepared and complex 4 has been crystallographically characterised.39 Reaction of [Ti(g2-C 2 H 4 )Cp* 2 ] in benzene with an equimolar amount of Me 3 SiCHN 2 gives the crystallographically characterised diazoalkane complex [Ti(N 2 CHSiMe 3 )-Cp* 2 ] which has the Me 3 SiCHN 2 group attached to the Ti in a side-on manner via the 376 P.K.Baker B(C6F5)2 R F Zr F F F F + – 4 Cp2Ti 5 But But But P Cp2Zr R Ph 6 two N atoms; the complex [Ti(N 2 CHSiMe 3 )Cp* 2 ] decomposes in benzene to the fulvene (R) complex [Ti(CH 2 SiMe 3 )(R)Cp*].40 The preparation of the first titana[3]- radialene and its ring expansion to the titanacyclopentene complex 5 have been reported.41 The first examples of phosphametallacyclobutenes such as 6 have been prepared by the 2]2 cycloaddition of PhC 2 R (R\Me or Ph) with a transient zirconium phosphinidene derived from either [Zr(PMe 3 )MP(C 6 H 2 Bu5 3 -2,4,6)NCp 2 ] or [ZrMeMP(C 6 H 2 Bu5 3 -2,4,6)HNCp 2 ].42 The preparation crystal structure and reactions of the first zirconium–alkyne complex without additional ligands rac-[1,2-ethylene- 1,1@-bis(g5-tetrahydroindenyl)][g2-bis(trimethylsilyl)acetylene]zirconium have been described.43 The synthesis and catalytic activity of the first tin-bridged ansa-zirconocenes [(H 3 C) 2 Sn(C 5 H 4 ) 2 ZrMN(CH 3 ) 2N2 ] and [SnMC 5 H 4 ) 2 Zr[N(CH 3 ) 2 ] 2N2 ] have been reported.44 3 Vanadium niobium and tantalum The synthesis and crystal structures of [VMC 6 H 2 (CF 3 ) 3 -2,4,6N2 Cl(thf)] and [VMC 6 H 2 (CF 3 ) 3 -2,4,6N3 OLi(thf) 3 ] which are the first early-transition-metal complexes containing the p-bonded fluoromesityl ligand have been reported.45 Reaction of [Nb(CO) 2 (g2-PhC 2 Me)Tp*] with NCR (R\Me Et or Ph) a§ords the first mixed (3e)-alkyne–g2-(3e)-nitrile complexes [Nb(CO)(g2-NCR)(g2-PhC 2 Me)Tp*] crystallographically characterised for R\Ph.46 The preparation molecular structure and reactions of [NbCl 2 (NOBu5)Cp] which has the first example of the terminal transition- metal–alkoxylimide ligand system namelyM–– N–OR have been reported.47 The thermal decomposition of [VPh 2 (PMe 3 ) 2 Cp] yields the first isolated and structurally characterised vanadium benzyne complex [V(PMe 3 ) 2 (g2-C 6 H 4 )Cp]; it is best described as a VIII benzometallacyclopropene.48 Thermolysis of the crystallographically characterised complex [Ta(CH 2 Ph) 2 (g4-C 4 H 6 )Cp*] which is a catalyst for the cis-iso specific homogeneous ring-opening metathesis of norbornenes gives a benzylidene species which can be trapped by PMe 3 to a§ord the benzylidene complex [Ta(–– CHPh)(PMe 3 )(g4-C 4 H 6 )Cp*] which has also been crystallographically characterised.49 Reaction of [Ta(CH 3 )(PMe 3 )Cp 2 ] with RN 3 (R\C 6 H 5 p-CF 3 C 6 H 4 or p- Me 2 NC 6 H 4 ) gives rare examples of terminal phenylazido complexes [Ta(CH 3 )(N 3 R)Cp 2 ] which have been structurally characterised for R\C 6 H 5 and p-Me 2 NC 6 H 4 .50 The one-electron reduction of [NbX 2 (g5-C 5 H 3 RR@) 2 ] (X\Cl 377 Organometallic chemistry of monometallic species R\SiMe 3 R@\H; X\Br R\SiMe 3 R@\H; X\Cl R\R@\SiMe 3 ) in the presence of NCRA (RA\Me or Ph) initially a§ords the g1-NbIII complexes [NbX(g1- NCRA)(g5-C 5 H 3 RR@) 2 ] which have been identified by electrochemical methods. However these complexes isomerise at room temperature to the more thermodynamically stable g2-nitrile complexes [NbX(g2-NCRA)(g5-C 5 H 3 RR@) 2 ].51 Reduction of the ansa- NbIV complex [NbCl 2MMe 2 Si(g5-C 5 H 4 ) 2N] with Na–Hg in the presence of RC 2 R (R\Me or Ph) a§orded the ansa-NbIII alkyne complexes [NbCl(g2-RC 2 R)MMe 2 Si(g5- C 5 H 4 ) 2N] which has been crystallographically characterised for R\Me.52 The synthesis and structure dynamics in novel ansa-metallocenes 7 (M\Nb or Ta) crystallographically characterised forM\Nb have been described.53 M CH3 CH3 N N(CH3)2 7 H N Prn Ph Ph (OC)5Cr– MeO + 8 O C N Me W(CO)4 C N O 9 4 Chromium molybdenum and tungsten The first systematic time-resolved infrared and UVspectroscopic study of the photolysis of [M(CO) 6 ] in supercritical Ar Kr Xe and CO 2 has enabled the observation of [M(CO) 5 L] [M\Cr Mo or W; L\Ar (W only) Kr Xe or CO 2 ].54 The first 4]3 annulation reactions of alkynyl Fischer-carbene complexes such as [CrM––C(OMe)C–– – CPhN(CO) 5 ] with 1-azadienes for example PhCH––CHCH––NPr/ initially a§ord a metallated zwitterionic intermediate 8 which has been crystallographically characterised.55 The complex cis-tetracarbonyl[(1-cyclopentyl)(N-morpholino) methylidene](2-hydroxyphenyl isocyano)tungsten(0) undergoes intramolecular cyclisation and subsequent N-alkylation to yield the first a,b-unsaturated mononuclear bis(ylidene) complex 9 which has been crystallographically characterised.56 Treatment of [M(CO) 5 (thf)] (M\Cr or W) with Li[C–– – CC(NMe 2 ) 3 ] followed by BF 3 ·OEt 2 gives the first diaminoallenylidene complexes [MM––C––C––C- (NMe 2 ) 2N(CO) 5 ] which has been characterised by X-ray crystallography for M\W.57 The chelate complexes [W(Ph 2 PCH 2 CH 2 SiHR 2 )(CO) 4 ] (R\Me or Ph) which have W H Si three-centre two-electron bonds have been photochemically prepared from [W(CO) 6 ] and Ph 2 PCH 2 CH 2 SiHR 2 .58 Reaction of [PhP]W(CO) 5 ] (generated in situ from the appropriate 7-phosphanorbornadiene precursor) at 120 °C with enolizable ketones or b-diketones a§ords a range of products such as [W(CO) 5MPhHP(CH 2 )CORN] (R\Me or Ph) as a result of insertion of phosphorus into either the a-CH the enol-OH or the acyl-CH 2 bonds.59 The first examples of complexes containing a tritertiary stibine such as fac-[Mo(CO) 3MMeC(CH 2 SbPh 2 ) 3N] which has been crystallographically characterised have been synthesised.60 Reaction of a stable telluroketone with [W(CO) 5 (thf)] gives [W(CO) 5 L] (L\1,1,3,3-tet- 378 P.K.Baker ramethylindantellone) the structure of which reveals g1,p complexation and trans influence of the weakly bonded telluroketone.61 The synthesis and molecular structure of [KL] 2 [Cr(CO) 5 Te 3 ]·0.5en (L\cryptand 222) which is the first example of a complex with an g1-bonded Te 3 2~ ligand have been described.62 C Mo But OCMe(CF3)2 R¢N (F3C)2MeCO But H C P 10 N B N N W N N N Br X HC Ph 11 H N HB N N W N N N Br X Br Ph C 12 Treatment of complexes of the type [WCl 2 (O)L 2 ] [L\P(OMe) 3 or PMePh 2 ] with 3,3-diphenylcyclopropene (R) gives [WCl 2 (O)L 2 (g2-R)] which react with two equivalents of Li[OC(CH 3 )(CF 3 ) 2 ] to yield [W(–– CHCH–– CPh 2 )(O)MOC(CH 3 )(CF 3 ) 2N2 L] the first tungsten oxo alkylidene complexes to show catalytic activity in ROMPandRCM processes.63 The crystallographically characterised phosphametallacycle 10 has been prepared from the cycloaddition of P–– – CBu5 to the high oxidation state molybdenum complex [MoM––CH(Bu5)N(OR) 2 (NR@)] [R\CMe(CF 3 ) 2 ; R@\C 6 H 3 Pr* 2 -2,6].64 The complex [Cr(NC 6 H 3 Pr* 2 -2,6) 2 (CH 2 CMe 3 ) 2 ] which has been crystallographically characterised readily eliminates CMe 4 at room temperature to yield the hexavalent alkylidene species [Cr(NC 6 H 3 Pr* 2 -2,6) 2 (––CHCMe 3 )].This can be trapped with strong donor molecules L (L\PMe 3 or thf) to a§ord the first stable hexavalent chromium alkylidene complexes [Cr(NC 6 H 3 Pr* 2 -2,6) 2 (––CHCMe 3 )L].65 Reaction of the alkylidene tungsten complexes 11 (X\NCMe 3 1-nitridoadamantane NC 6 H 2 Me 3 - 2,4,6 or O) with Br 2 results in loss of HBr and insertion of the CPh group into one of theW–N single bonds of the tris(pyrazolylborate) cage to give 12 crystallographically characterised for X\1-nitridoadamantane.66 Treatment of cis-[W(–– – CR)Br(CO) 2 - (PPh 3 ) 2 ] (R\C 6 H 4 Me-4) with KTp yields [W(–– – CR)(CO) 2 Tp] whereas photolysis of cis-[W(–– – CR)Br(CO) 2 (PPh 3 ) 2 ] gives trans-[W(–– – CR)Br(CO) 2 (PPh 3 ) 2 ].This complex reacts with KTp initially to a§ord the crystallographically characterised complex [W(CO)(PPh 3 )Tp(g2-OCCR)] which upon reaction with Cl 2 PPh 3 gives the chloroalkyne complex [WCl(CO)(PPh 3 )Tp(g2-ClC–– – CR)] which has also been characterised by X-ray crystallography.67 The preparation and crystal structure of the g2-nitrile complex 13 have been described; it has the tetrafluoroteraphthalonitrile co-ordinated as a four-electron donor ligand.68 Thermal decarbonylation of [WX(CO) 2 Tp@] (X\Cl Br or I) in NCR gives the four-electron donor nitrile complexes [WX(CO)Tp@(g2-NCR)] which react with [NH 4 ][S 2 PR@2 ] to a§ord [W(CO)Tp@(S 2 PR@2 )(g2-NCR)] which has been crystallographically characterised for R\Me R@\([)-mentholate.69 The cationic complexes [Mo(CO) 2 (NO)(g3-allyl)Tp]` (allyl\C 3 H 5 2-methylpropenyl cyclohexenyl or cyclooctenyl) have been prepared and for the case of allyl\C 3 H 5 crystallographically characterised as its [BMC 6 H 3 (CF 3 ) 2 -3,5N4 ]~ salt; the structure in both the solid state and solution revealed a significant g3-g2 distortion of the allyl group.70 379 Organometallic chemistry of monometallic species W CO N NMe2 Me N C F F C F F N F 13 H Me O H Me CO Me Mo+ H F3BF – 14 The first divalent chromium carbene complexes [CrXM––CH(NPr* 2 )N(CO) 2 (g5- C 5 R 5 )] (X\Cl or Br; R\H or Me) have been prepared selectively by adding hydrogen halides to the metal–carbon triple bond in [Cr(–– – CNPr* 2 )(CO) 2 (g5-C 5 R 5 )].These carbene complexes react with isocyanides phosphines and phosphites to yield cationic complexes such as [CrM––CH(NPr* 2 )N(CO) 2 (CNEt)Cp*][PF 6 ] which has been crystallographically characterised.71 Reaction of [W(–– CHPh)(CO) 2 Cp]~ with MeI gives trans-[WMe(–– CHPh)(CO) 2 Cp] which isomerises by an intramolecular methylto- carbene migration to yield the g3-benzyl complex [WMg3-CH(Me)C 6 H 5N(CO) 2 - Cp].72 Treatment of the crystallographically characterised complex [W(CO) 3 (OEt 2 )Cp*]- [BR 4 ] [R\bis(trifluoromethyl)phenyl] with PR@3 (R@\Ph or Cy) gives [WH(CO) 3 Cp*] and the new phosphonium salts [PR@3MEtOC(H)(Me)N][BR 4 ]; the a-CH of Et 2 O has been selectively displaced by phosphines.73 The preparation and molecular structure of the first cyclopentadienyl(halogeno)metal(VI) complex of the chromium triad namely [WF 5 Cp*] have been reported.74 The first crystal structure of a Cp–trihydride d2 system [MoH 3 (dppe)Cp*] has been described; it has a pseudotrigonal prismatic geometry rather than the expected pseudo-octahedron.75 Treatment of [MoMp,g2(3e)-CH 2 C 2 RNMg2(4e)-MeC 2 RNCp] (R\Me or Ph) with CO gives the crystallographically characterised complexes [MoMg2,g3-C(R)C(O)C(Me)C(R)- CCH 2N(CO)Cp] via novel cocyclisation reactions.Upon protonation with HBF 4 ·Et 2 O this complex (R\Me) a§ords 14; X-ray crystallography reveals that protonation occurs on the keto group rather than the g3-allylic CH 2 group.76 Protonation of [MoX(g2-RC 2 R) 2 Cp] using HBF 4 ·Et 2 O gives high yields of the aqua complexes [MoM––C(R)-g3-[C(R)C(R)CHR]NX(H 2 O)Cp][BF 4 ] [X\Cl R\Me or Et; X\Br R\Et (crystallographically characterised); X\I R\Et]; the molecular structure for X\Br R\Et showed the presence of H 2 O and an g4(5e)-butadienyl fragment in an anti-supine conformation.The reactions of these complexes with both neutral and anionic donor ligands are also discussed.77 Reaction of [W(NCMe)(g2- PhC 2 Ph) 3 ] with o-(diphenylphosphino)-styrene or -allylbenzene gives products resulting from either novel reorganisation reactions involving cleavage of C––C and C–– – C bonds or unusual insertion reactions of PhC 2 Ph into C–Hbonds; two of the products 15 and 16 have been crystallographically characterised.78 The asymmetric functionalisation of the benzylic methylene group in [Cr(CO) 3 (g6- PhCH 2 OR)] is achieved in high yield (86 to 96%) and high enantiomeric excess (e.e.) (97 to P99%) using chiral base methodology.79 An important paper published at the 380 P.K.Baker end of 1995 describes the photolysis of [Cr(CO) 3 (g6-C 6 Me 6 )] with Me 3 SiC–– – CSiMe 3 to a§ord the vinylidene complex [Cr(CO) 2M––C––C(SiMe 3 ) 2N(g6-C 6 Me 6 )] which gives the alkyne cation [Cr(CO) 2 (g2-Me 3 SiC–– – CSiMe 3 )(g6-C 6 Me 6 )]` via oxidative isomerization. Cyclic voltammetry and spectroelectrochemical studies show that the neutral alkyne complex reverts to [Cr(CO) 2M––C––C(SiMe 3 ) 2N(g-C 6 Me 6 )] in a process in which the co-ordinated alkyne slips to an g1-bonding mode prior to the transition state.80 The molecular structures of two related redox pairs [Cr(CO) 2 (g2-PhC–– – CPh)(g6- C 6 HMe 5 )]0@1` and [Mo(CO) 2 (g2-PhC–– – CPh)Tp@]0@1` have been crystallographically determined; the results suggest the HOMO of the d6 Cr0 alkyne complex is an antibonding M–alkyne nM orbital.81 The complex [Cr(g6-C 6 H 5 Me)(g7- C 7 H 6 C 6 H 4 Me-4)][PF 6 ] undergoes a reversible one-electron oxidation to the radical dication [Cr(g6-C 6 H 5 Me)(g7-C 7 H 6 C 6 H 4 Me-4)][PF 6 ] 2 ; X-ray crystallographic studies on both complexes showed that the main structural alteration upon oxidation is a small increase in the metal-to-ring distances.82 Ph H Ph Ph Ph W Ph2P C Ph H Ph H 15 H H Ph H W Ph Ph H Ph2P C C Ph Ph Ph H 16 Mn(CO)3 17 5 Manganese technetium and rhenium The novel crystallographically characterised 16-electron complex [Mn(CO)(dppe) 2 ]- [BR@4 ] [R@\C 6 H 3 (CF 3 ) 2 -3,5] reacts reversibly withH 2 to give the first example of an isolated g2-dihydrogen manganese complex trans-[Mn(CO)(g2-H 2 )(dppe) 2 ][BR@4 ].83 A detailed study of the mechanism of the hydrosilation of the manganese acetyl complex [Mn(COMe)(CO) 5 ] with monohydrosilanes is consistent with an autocatalytic process.84 The first example of an anhydrous ReVII peroxo complex [ReMe(O)(g2-O 2 ) 2 (hmpa)] which has been crystallographically characterised catalytically oxidises alkenes with H 2 O 2 .85 The reduction of [ReBrM(C 6 F 5 NCH 2 - CH 2 ) 3 NN]Br with methyllithium under N 2 CO H 2 or C 2 H 4 a§orded [Re(N 2 )M(C 6 F 5 NCH 2 CH 2 ) 3 NN] [Re(CO)M(C 6 F 5 NCH 2 CH 2 ) 3 NN] [ReH 2M(C 6 F 5 - NCH 2 CH 2 ) 3 NN] and [ReM(C 6 F 5 NCH 2 CH 2 ) 3 NN(g2-C 2 H 4 )] respectively.86 A single-step synthesis of the penta(cyclopentadienyl)cyclopentadienyl manganese complex 17 has been achieved by reacting [Mn(CO) 3 (g5-C 5 I 5 )] with CpSnMe 3 ; because of its topology 17 is interesting with regard to the formation of metalated semibuckminsterfullerenes.87 Reaction of [ReBr(PPh 3 )(g2-PhC 2 Ph)Cp][BF 4 ] and the crystallographically characterised complex [ReBr(PMePh 2 )(g2-PhC 2 Ph)Cp]- [PF 6 ] with K[BHBu4 3 ] gives the g2(3e)-vinyl complexes [ReM––C(Ph)CHPhN- 381 Organometallic chemistry of monometallic species Br(PPh 3 )Cp] and the crystallographically characterised complex [ReM––C(Ph)CHPhN- Br(PMePh 2 )Cp]; treatment of [Re(dppe)(g2-PhC 2 Ph)Cp][BF 4 ] 2 with one or two equivalents of K[BHBu4 3 ] yielded the cationic g2(3e)-vinyl complex [ReM––C(Ph)CHPhN(dppe)Cp][BF 4 ] and the cis-stilbene complex [Re(dppe)(g2-ZPhCH ––CHPh)Cp] respectively; both were characterised by X-ray crystallography.Finally reaction of [Re(dppe)(g2-MeC 2 Ph)Cp][BF 4 ] 2 with one or two equivalents of K[BHBu4 3 ] gives the g2(3e)-vinyl complex [ReM––C(Me)CHPhN(dppe)Cp][BF 4 ] and the g2-allene complex [Re(dppe)Mg2-CH(Ph)––C––CH 2NCp] respectively; in the latter the substituted allenic bond is attached to the rhenium centre.88 Treatment of [Re(CO) 2 (g2-RC 2 R@)Cp*] with [CPh 3 ][PF 6 ] a§ords the g3-propargyl complexes [Re(CO) 2 (g3-CHRAC–– – CR)Cp*][PF 6 ] via hydride abstraction.These hydride abstractions were only observed for internal alkynes with a primary methyl or primary alkyl substituent and an unusual regioselectivity for hydride abstraction namely CH 3 CH 2[CH 3?CH(CH 3 ) 2 has been found.89 Reaction of [Re(NH 2 )(PPh 3 )- (NO)Cp] and hexafluoroacetone yields the methyleneamido complex [ReMN––C(CF 3 ) 2N(PPh 3 )(NO)Cp] which gives the p-imine complex [ReMg1- N(H)––C(CF 3 ) 2N(PPh 3 )(NO)Cp][OTf] after addition of TfOH; similarly reaction of [Re(NH 2 )(PPh 3 )(NO)Cp] with trifluoroacetaldehyde followed by TfOH furnishes the p-imine complex [ReMg1-N(H)––C(CF 3 )HN(PPh 3 )(NO)Cp] and occasionally small amounts of the n-trifluoroacetaldehyde complex.90 Treatment of [ReCl 4 Cp*] or [ReCl 2 (O)Cp*] with H 2 S in the presence of pyridine gives the crystallographically characterised chiral dithiolato complex [ReOM(S)(SCH 2 )C 5 Me 4NCp*].91 Reaction of [Mn(CO) 3 (g6-R)][BF 4 ] (R\acenaphthene) with catechol and hydroquinone (L) gives the stable n-bonded complexes [Mn(CO) 3 (g6-cat)][BF 4 ] and [Mn(CO) 3 (g6-L))] [BF 4 ] respectively; the molecular structure of [MMn(CO) 3 (g6-L)N2 ][SiF 6 ] shows an approximately planar arene group with the hydroxide substituents strongly hydrogen- bonded to fluorine in the [SiF 6 ]2~ dianion.92 6 Iron ruthenium and osmium Treatment of [OsH 2 (CO)(PPr* 3 ) 2 (g2-CH 2 CHEt)] with H 2 SiPh 2 gives the crystallographically characterised complex [OsH 3 (SiHPh 2 )(CO)(PPr* 3 ) 2 ] which is the first trihydro–silyl complex of OsIV; similar trihydro–germyl and –stannyl derivatives have also been reported.93 Reaction of cis-[RuH 2 (dmpe) 2 ] with C 6 F 6 at[78 °C a§ords the crystallographically characterised pentafluorophenyl hydride complex trans- [RuH(C 6 F 5 )(dmpe) 2 ] via an exclusive C–F insertion reaction; related C–F bond activations of C 6 F 5 H C 6 F 5 CF 3 C 6 F 5 OCH 3 1,2,3,4-C 6 F 4 H 2 and 1,2,3-C 6 F 3 H 3 are also discussed.94 The ferra-c-ketoesters cis-[Fe(COR)(COCOR@)(CO) 4 ] (R\Me R@\OMe; R\OMe R@\Me) induce thermally either a C–C coupling to give [Fe(COMe)(CO 2 Me)(CO) 4 ] (for R\Me R@\OMe) or a chain–ring isomerisation reaction to give the crystallographically characterised (R isomer) complex 18 (for R\OMe R@\Me).95 The synthesis and crystal structure of the first tris(alkoxycarbonyl) complex K[Fe(CO 2 Bu5) 3 (CO) 3 ] have been described.96 The preparation nucleophilic attack on and rearrangement of several ruthenafurans have been described; the molecular structures of [Ru(CO) 2MC(Ph)NHCMe 3N(CH 2 CO 2 Et)(PMe 2 Ph) 2 ]- [PF 6 ] and [Ru(CO) 2MC(Me)––CHC(O)OEtN(PMe 2 Ph) 2 ][PF 6 ] have also been 382 P.K.Baker OC Fe OC C C OC C O O C O OMe Me O 18 N Ru O N N O C Cl H C O But Pri Pri Cl O But Me 19 N Me N N Me N B N Me N H Me N N Me N N B FeIII H Me N N [BPh4] 20 reported.97 Treatment of [RuCl 2 (pybox)(g2-C 2 H 4 )] with 2,6-di-tert-butyltolyl diazoacetate gives the stable 2,6-di-tert-butyltolylcarbonylcarbene complex 19 in high yield; complex 19 is an active intermediate in asymmetric catalytic cyclopropanation reactions.98 The synthesis and crystal structure of the first hexacarbene iron complex 20 using an important negatively charged tris(carbene) ligand have been described.99 The unprecedentedly simple metallaphosphaalkene complex [RuCl(CO)(PPh 3 ) 2Mg1- P–– CHBu5N] can be reversibly protonated to give [RuCl 2 (CO)(PPh 3 ) 2Mg1- PH––CHBu5N] which is a complex of an otherwise unstable phosphaalkene HP––CHBu5.100 Reaction of [RuCl(CO)(CNC 6 H 3 Me 2 -2,6)(PPh 3 ) 2 (g1-P––CHBu5)] with HBF 4 reversibly gives the phosphaalkene complex [RuCl(CO)(CNC 6 H 3 Me 2 - 2,6)(PPh 3 ) 2 (g1-PH––CHBu5)][BF 4 ] which is transformed by either K[HF 2 ] or [NBu/ 4 ]F to the crystallographically characterised fluorophosphine complex [RuCl(CO)(CNC 6 H 3 Me 2 -2,6)(PPh 3 ) 2MPHF(CH 2 Bu5)N][BF 4 ]·CH 2 Cl 2 .101 Treatment of diphenyl-N,N-dimethylephosium hexafluorophosphate with K[FeH(CO) 4 ] gives the crystallographically characterised zwitterionic chiral complex diphenyl-N,Ndimethylephosium hydridotricarbonylferrate.102 Reaction of the HNMe 2 -stabilised iron–silylene complexes trans-[Fe(CO) 3MP(OR)(NMe 2 ) 2NMSi(OR) 2 (NMe 2 H)N] with the stannylene reagent SnNBu5SiMe 2 NBu5 occurs selectively by insertion into the N–H bond to yield novel types of base-stabilised silylene and stannylene complexes such as 21.103 Treatment of [RuH 2 (g2-H 2 ) 2 (PCy 3 ) 2 ] with pyridine or quinoline having ortho-hydroxy and -amino substituents L–XH gives the new stretched-dihydrogen complexes [RuH(g2-H 2 )(L–X)(PCy 3 ) 2 ]; in the presence of excess of triethylvinylsilane these reactions unexpectedly a§ord the hydrido–vinylidene complexes [RuHMC––C(H)SiEt 3N(L–X)(PCy 3 ) 2 ].104 Reaction of [OsClM(E)-CH––CH(Ph)N(CO)(PPr* 3 ) 2 ] with LiPh furnishes the crystallographically characterised complex [OsHMC 6 H 4 [(E)-CH––CH(Ph)-2]N(CO)(PPr* 3 ) 2 ].However treatment of [OsClM(E)-CH––CH(Ph)N(CO)(PPr* 3 ) 2 ] with methyllithium and LiCD 3 yields [OsHMC 6 H 4 [(E)-CH––CH(CH 3 )]-2N(CO)(PPr* 3 ) 2 ] and [OsHMC 6 H 4 - [CH––CH(CD 3 )]-2N(CO)(PPr* 3 ) 2 ] respectively; both isomerise in solution to give [OsH(CO)(PPr* 3 ) 2 (g3-CH 2 CHCHPh)] and [OsD(CO)(PPr* 3 ) 2 (g3-CD 2 CHCHPh)] respectively.105 n-Allyltricarbonyliron lactone complexes which have aldehyde groups next to the allyl system undergo addition reactions with a range of organoaluminium reagents in 383 Organometallic chemistry of monometallic species Si Fe OC OC CO P OEt NMe2 NMe2 O Et EtO H N But Me2Si Sn N But N Me Me 21 O (OC)3Fe O H Me OH 22 PPh3 Ru PPh3 C SC Cl S Ph 23 moderate to high diastereoselectivity to a§ord complexes such as 22 (d.e.[95%).106 The alkyllithium–carbon monoxide mediated conversion of [Fe(CO) 3 (g4-R)] (R\vinylketone) complexes to [Fe(CO) 3 (g4-R@)] (R@\vinylketene) complexes proceeds without loss of stereochemical integrity.107 Treatment of vinyllithium reagents generated from vinylstannanes [(E)-RCH––CHSnBu 3 ] (R\Ph SiPh 3 SiMe 3 or n-C 5 H 11 ) with [Fe(CO) 4 L] (L\CO or PPh 3 ) followed by quenching with ethyl trifluoromethanesulfonate a§ords vinylketene complexes [Fe(CO) 2 LMCH(R)––CHC- (OEt)C–– ON] in a one-pot reaction in 31–55% yield.108 The first triisopropylstibine RuII and Ru0 derivatives including the crystallographically characterised complex [Ru(SbPr* 3 ) 2 (g3-C 3 H 5 ) 2 ] have been reported.109 Treatment of the alkylidyne complex [RuCl(–– – CPh)(CO)(PPh 3 ) 2 ] with CO 2 CS 2 or MeNCS gives via electrophilic attack by the carbon atom cycloaddition products including the crystallographically characterised thiobenzoyl complex 23.110 Reaction of [Fe(CO) 3 (g4-dba)] with S 2 CPR 3 (R\Pr* or Cy) yields the complexes [Fe(CO) 3 (g3-S 2 CPR 3 )] which have been crystallographically characterised for R\Cy and confirms the g3(S,C,S@) pseudo-allyl attachment of the phosphonio dithiocarboxylato ligand.111 The preparation and characterisation of the stable 17-electron radical [Fe(CO) 2Mg5- C 5 (CHMe 2 ) 5N] have been described.112 The synthesis of a unique family of stable 16- 17- and 18-electron complexes [Fe(dppe)Cp*][PF 6 ] [Fe(dppe)Cp*] (both crystallographically characterised) and [Fe(OSO 2 CF 3 )(dppe)Cp*] has been reported.113 Oxidation of [Fe(g1-dtc)(CO)nCp*] (n\1 or 2) with [FeCp 2 ]X (X\[BF 4 ]~ or [PF 6 ]~) gives the thermally stable 17-electron complexes [Fe(CO)(g2-dtc)Cp*]X which react with first order kinetics with various donor solvents to yield [FeL(g2-dtc)Cp*]X (L\CH 2 Cl 2 thf NCMe or CH 3 COCH 3 ).The complexes [FeL(g2-dtc)Cp*]X react with phosphines to give [FeL@(g2-dtc)Cp*]X (L@\PPh 3 or g1-dppe) whereas anionic ligands X@~ (X@\Cl CN or SCN) give the 17-electron neutral complexes [FeX@(g2- dtc)Cp*]; the synthesis and molecular structure of the 19-electron complex [Fe(dtc) 2Mg5-C 5 (CH 2 Ph) 5N] have also been reported together with detailed EPR and Mo� ssbauer spectroscopic studies of all the complexes.114 Treatment of [Ru(thf)(PPh 3 ) 2 Cp][PF 6 ] with buta-1,4-diyne in the presence of nucleophiles yields alkenylvinylidene or allenylidene complexes including the crystallographically characterised heteroallenylidene complex [RuM–– C––C––CMe(NPh 2 )N(PPh 3 )Cp][PF 6 ] via the cationic trienylidene intermediate [Ru(––C–– C––C––CH 2 )(PPh 3 ) 2 Cp][PF 6 ].115 Deprotonation of [RuM––C––C(Ph)CH 2 RN(PPh 3 ) 2 Cp]` with [NBu/ 4 ]OH gives the neutral cyclopropenyl complexes [RuMC––C(Ph)CHRN(PPh 3 ) 2 Cp] (R\CN Ph CH–– CH 2 or CH––CMe 2 ) crystallographically characterised for R\CN,116 via a novel cyclisation reaction.The preparation of the first indenyl osmium(II) complex [OsCl(PPh 3 ) 2 (g5-In)] and its use as a precursor for the synthesis of novel alkynyl cyclic carbene and vinylidene osmium(II) complexes have been described.117 The 384 P.K.Baker chiral carbon-bonded enolate complex [Ru(CO)Mg2(P,C)-Ph 2 PC 6 H 4 -o- C(O)CHCH 3NCp] is obtained diastereoselectively by deprotonating the corresponding ketone complex with Li[NPr* 2 ]; the major diastereomer (S R6 R C /R R6 S C ) of [Ru(CO)Mg2(P,C)-Ph 2 PC 6 H 4 -o-C(O)CHCH 3NCp] has been crystallographically characterised.118 Reaction of [Fe(SnMe 3 )(CO)MPN(Me)CH 2 CH 2 NMe(OR)NCp] (R\Me or Et) with Me 3 SiOSO 2 CF 3 (TMSOTf) gives the crystallographically characterised stannylene complex [Fe(SnMe 3 )(CO)MPN(Me)CH 2 CH 2 NMe(Me)N][OTf] via P–OR bond cleavage and methyl migration from Sn to P.119 Treatment of [FeMe(CO)MP(OMe) 3NCp] with BF 3 ·OEt 2 followed by PPh 3 furnishes a six-membered carbene phosphite metallacyclic complex 24 which has been structurally characterised and proposed to be formed by a migratory insertion of CO into an Fe–Me bond and an Arbuzov-like dealkylation reaction.120 Reaction of the 17-electron radical [Fe(CO) 2Mg5-C 5 Ph 4 (p-tol)N] with P(OR) 3 (R\Me or Et) gives the Arbuzov rearrangement products [FeR(CO) 2Mg5-C 5 Ph 4 (p-tol)N] [Fe(CO) 2MPO(OR) 2NMg5- C 5 Ph 4 (p-tol)N] and [Fe(CO)MP(OR) 3NMP(O)(OR) 2NMg5-C 5 Ph 4 (p-tol)N].121 The first preparation and crystal structure of the ethene-bridged ferrocenophane 1,2-(1,1@- ferrocenediyl)ethene have been described.122 Reaction of [C 2 Bu5 2 P 2 Sb]~ with [Ru(NCMe) 3 (g5-C 5 R 5 )][PF 6 ] (R\H or Me) gives 25 (crystallographically characterised for R\Me) which are the first examples of complexes derived from a diphosphastibolyl ring.123 FPh3P P O C O B F F OMe OMe Me 24 R R R R R Ru But But P Sb 25 Treatment of the ‘piano-stool’ complex [RuL(g6-C 6 H 6 )][PF 6 ] 2 [L\(2-pyridylethyl)( 2-pyridylmethyl)methylamine] with nucleophiles gives [RuL(g5-C 6 H 6 Y)]- [PF 6 ] (Y\CN~ H~ or OH~) including the first crystallographically characterised (g5-cyanocyclohexadienyl)ruthenium complex (Y\CN~).124 The first Ru0 complex containing a 1,2,4-triphosphole 26 has been prepared and crystallographically characterised; on heating complex 26 undergoes a novel hydrogen migration from the g4-cycloocta-1,5-diene group to a§ord the crystallographically characterised complex 27.125 The preparation molecular structure (for arene\naphthalene) and reactions of the g2-arene complexes [Ru(g2-arene)(NO)Cp*] (arene\benzene toluene or naphthalene) have been described.126 Reaction of solvated Ru2` ions in acetone with a large excess of buta-1,3-diene a§ords the unusual crystallographically characterised cyclooctatriene–cyclooctadienyl complex 28 which arises from the cyclodimerisation and dehydrogenation of two pairs of butadiene molecules at ruthenium.127 The synthesis and crystal structure of the half-sandwich and tripodal chelation complex 29 an example of a complex containing a coelenterand have been reported.128 385 Organometallic chemistry of monometallic species Ru P P PCH(SiMe3)2 26 Ru + 28 N N N N N Ru N 29 2+ 7 Cobalt rhodium and iridium The first homoleptic carbonyl cation of a tripositive metal namely [Ir(CO) 6 ][Sb 2 - F 11 ] 3 has been prepared and spectroscopically characterised; the pentacarbonyl IrIII complex [IrCl(CO) 5 ][Sb 2 F 11 ] 2 has also been synthesised and crystallographically characterised.129 Treatment of the cationic complex [M(CO)(NCMe)(PPh 3 ) 2 ]` (M\Rh or Ir) with singlet oxygen gives the peroxo complexes [M(CO)(NCMe)(PPh 3 ) 2 (g2-O 2 )]`; the iridium complex is stable at room temperature whereas the rhodium complex is only stable below 0 °C.130 The crystallographically characterised diamagnetic complex [Co(CO) 2 TpP3*,M%] spontaneously loses CO in solution to a§ord the paramagnetic complex [Co(CO)TpP3*,M%] which has a triplet spin ground state.In the presence of CO the two complexes are in equilibrium; measurements of the temperature dependence of the equilibrium constant by variable-temperature 1H NMR spectroscopy have been used to determine the thermochemical parameters.131 One-electron oxidation of the crystallographically characterised square-planar 16-electron complex [Rh(CO)(PPh 3 )(i2-Tp@)] with [FeCp 2 ][PF 6 ] affords the square-pyramidal complex [Rh(CO)(PPh 3 )(i3-Tp@)][PF 6 ] (also crystallographically characterised) which has the third pyrazolyl group of the Tp@ ligand N-bonded in the axial site by a three-electron two-centre bond.132 In the presence of catalytic amounts of water the complex [IrEt(CH––CH 2 )(NCMe)Tp@] undergoes intramolecular coupling of the vinyl and acetonitrile moieties to give an unusual iridapyrrole derivative 30.133 Reaction of [Rh(PMe 3 )(g2-C 2 H 4 )Tp@] with thiophene gives a mixture of C–Hand C–S activation products namely [RhH(PMe 3 )(2-C 4 H 3 S)Tp@] and [Rh(CHCHCHCHS)(PMe 3 )Tp@] respectively.In contrast to previous observations 386 P.K. Baker the C–H activation product is thermodynamically preferred.134 Treatment of trans-[Rh(––C–– C––CPh 2 )Cl(PPr* 3 ) 2 ] with CH 2 ––CHMgBr a§ords [Rh(PPr* 3 ) 2 (g3-CH 2 CHC––C––CPh 2 )] which reacts with CO to give the crystallographically characterised complex trans-[RhMg1-C(CH––CH 2 )––C––CPh 2N(CO)- (PPr* 3 ) 2 ]; the coupling of trans-[Rh(––C––C––CPh 2 )Cl(PPr* 3 ) 2 ] with HC–– – CPh yields the crystallographically characterised complex [RhCl(PPr* 3 )Mg3-CH(PPr* 3 )C(Ph)C –– C––CPh 2N].135 Reaction of trans-[Rh(––C––C–– CPhR)(OPh)(PPr* 3 ) 2 ] (R\Ph or C 6 H 4 Me-2) with CO gives the complexes 31 (crystallographically characterised for R\Ph) via migratory insertion of the allenylidene group into the Rh–OR bond to a§ord c- functionalised alkynyl ligands.136 The preparation and crystal structure of the unique dioxo alkene hydride complex 32 have been described.137 Treatment of [Ir(OTf)(PR 3 )(tfb)] (R\Pr* or Cy) withH 2 SiPh 2 gives the first base-stabilised silylene complexes of iridium [IrH 2MSi(OTf)Ph 2N(PR 3 )(tfb)] crystallographically characterised for R\Pr*.138 Ir N H H Me H Et HB(pz)3 30 OC Rh C L L C C OPh R Ph 31 PBut 2 Rh O O H PBut 2 32 The activation of H 2 and light hydrocarbons (CH 4 C 2 H 4 or C 2 H 6 ) by [Ir(CO) 2 Cp*] in supercritical fluid solution has been reported; it is the first use of supercritical CH 4 as a solvent for photochemical reactions.139 Irradiation of [Rh(PMe 3 )(g2-C 2 H 4 )Cp] in pentafluoroanisole yields the metallacyclic complex [RhCH 2 OC 6 F 4 (PMe 3 )Cp] which when treated with one equivalent of [CPh 3 ][PF 6 ] at 220 K a§ords [RhM––C(H)OC 6 F 4N(PMe 3 )Cp][PF 6 ].140 The iridium(III) complex [IrMe 3 (OTf)(PMe 3 )Cp*] undergoes C–H activating methane elimination with ethers (Et 2 O and thf 75 °C; Me 2 O and MeOBu5 25 °C) under mild conditions to yield the cationic hydrido Fischer-carbene complexes [IrHM––C(H)(OR)N(PMe 3 )Cp*][BPh 4 ] (R\Me Et or Bu5) and the crystallographically characterised complex 33.141 Complex 34 undergoes Friedel–Crafts acetylation selectively at the 1,2,5-cobaltadithiolene ring giving chemical evidence for the aromaticity of that ring.142 Treatment of the alkylidene-bridged dithiolene complex [CoMS 2 C 2 (CO 2 Me) 2NMC(CO 2 Me) 2NCp] with P(OMe) 3 a§ords a novel type of sulfonium ylide attached to the cobalt atom in [CoMS 2 C 2 (CO 2 Me) 2NMC(CO 2 Me) 2NMP(OMe) 3NCp] which has been crystallographically characterised.143 The first enantioselective rhodium catalysts for the Diels–Alder reaction between methacrolein and cyclopentadiene has been reported as has the crystal structure of the catalyst precursor [Rh(R-Prophos)(H 2 O)Cp*][SbF 6 ] 2 .144 Reaction of [MRh(k-Cl)ClCp*N2 ] and [MRu(k-Cl)Cl(g6-MeC 6 H 4 Pr*-p)N2 ] with 1,2,3- triphenylguanidine gives the crystallographically characterised complexes [RhClMg2- (NPh) 2 CNHPhNCp*] and [RuClMg2-(NPh) 2 CNHPhN(g6-MeC 6 H 4 Pr*-p)] respectively the first examples of complexes containing chelated guanidine anions.145 The first [2]cobaltocenophane and its [2]cobaltocenophanium ion 35 crystallographically 387 Organometallic chemistry of monometallic species O Cp*(Me3P)Ir H [B{C6H3(CF3)2}4] 33 Co S S Ph 34 But Co But X 35 characterised as its [PF 6 ]~ salt have been described together with the analogous [2]ferrocenophane and [2]ferrocenophanium ions (also structurally characterised as its [BF 4 ]~ salt).146 Several 1,5-dihydro-1,2,3,4,5,6,7,8-octamethyl-s-indacene (H 2 Ic*) and 5-hydro-1,2,3,4,5,6,7,8-octamethyl-s-indacene (HIc*) complexes have been prepared including [RhCp*(g5-Ic*H)][SbF 6 ] which has been crystallographically characterised.147 8 Nickel palladium and platinum The preparation and characterisation of [Ni(CO)(g4-P 7 )]3~ (crystallographically characterised) [Ni(CO)(g4-HP 7 )]2~ and [PtH(PPh 3 )(g2-P 7 )]2~ which has also been structurally characterised have been reported; the dianions [Ni(CO)(g4-HP 7 )]2~ and [PtH(PPh 3 )(g2-P 7 )]2~ are electronically equivalent protonated Zintl ion complexes which have totally di§erent structures.148 The synthesis and crystal structure of the square-planar complex [NEt 4 ] 2 [Ni(C–– –CC–– – N) 4 ] have been described.149 The first reported oxidative addition of an Sn–S bond occurs when the ring compounds (R 2 SnS) 3 (R\Me or Ph) react with [PtMe 2 (4,4-Bu5 2 bipy)]; the luminescent products [PtMe 2 (R 2 SnS) 2 (4,4-Bu5 2 bipy)] have a five-membered PtSnSSnS ring.150 The preparation characterisation and reactions of the seven-membered nickelacycle 36 have been reported.151 The preparation and crystal structures of the cis-bis(homoleptic) complexes 37 and 38 both of which have predetermined helical chirality at the metal Ni N N H H H H 36 S N Me Me S N Me Me Pt 37 S N S N Pt 38 Me Me Me Me centre have been described.152 The molecular structures of the cis-bis(homoleptic) PtII complexes 39 and 40 show a distortion of the square-planar geometry toward a 388 P.K.Baker two-bladed helix.153 When reacted with NaOBu5 the crystallographically characterised palladium(II) metallacycle 41 gives the zero-valent complex [PdMP(o-tol) 3N2 ]; a route to Pd0 from PdII metallacycles in amination and cross-coupling reactions is also reported.154 Treatment of ethene with [Pd(COMe)(NC 5 H 4 CO 2 Me-2)(PPh 3 )][BF 4 ] gives the crystallographically characterised complex [Pd(CH 2 CH 2 COMe)- (NC 5 H 4 CO 2 Me-2)(PPh 3 )][BF 4 ] a rare example of an isolable product from the insertion of an unstrained alkene into a palladium–acyl bond; the unusual structure is square-pyramidal with a weakly interacting oxygen in the apical position.155 The novel seven-membered ring complex [PdCl 2 L] ML\1,2-bis[2-(2,4,6-tri-tert-butylphenyl) phosphanediylmethyl]benzeneN has been prepared and structurally characterised and its reactions with ROH (R\Me or Et) to a§ord chiral cyclometallated N N Pt 39 N S N Pt S 40 Pd P N Et2 H O O Me ( o -Tol)2 41 complexes 42 (R@\C 6 H 2 Bu5 3 -2,4,6) also crystallographically characterised (R\Et) have also been discussed.156 The palladium(II) complex (])-43 which has an amidosubstituted P-chiral phosphine ligand has been prepared and crystallographically characterised; the optically active phosphine ligand can be displaced by dppe.157 The crystal structure of the hydrido(dimethyl)platinum(IV) complex [PtH(Me) 2 Tp@] has been described.158 P Pd R¢ Cl P H RO R¢ 42 Pd N Me Me Me P O Me Me Ph NMe2 S [ClO4] 43 R The synthesis and characterisation of the square-planar cationic PtII complexes [PtMe(phen)(g2-RCH––CH 2 )][BF 4 ] (R\H Me or Ph) which have cis methyl and alkene moieties have been reported.159 The reaction of [Ni(g4-cod) 2 ] with PCy 3 and CO 2 at [20 °C gives [Ni(PCy 3 ) 2 (g2-CO 2 )] which reacts under the same conditions with Me 3 P–– CH 2 to a§ord [Ni(PCy 3 ) 2 (g2-O,C-O––C–– CH 2 )]; the latter is the first example of a ‘Wittig reaction’ on a co-ordinated carbon dioxide nickel complex.160 Treatment of 1,2-dibromocyclohexene with 1% sodium amalgam in the presence of [NiL 2 (g2-C 2 H 4 )] [L 2 \2PPh 3 2PEt 3 or Cy 2 P(CH 2 ) 2 PCy 2 ] a§ords the cyclohexyne –nickel(0) complexes [NiL 2 (g2-C 6 H 8 )] crystallographically characterised for 389 Organometallic chemistry of monometallic species L 2 \Cy 2 P(CH 2 ) 2 PCy 2 ; the reaction chemistry of these complexes is also discussed.161 The first platinum complex containing a [2.2]paracyclophane-1-yne namely 44 has been prepared and crystallographically characterised; the complex can best be described as a platinacyclopropene.162 The molecular structure of [PtMg1- C(PPh 3 )(CO)N(PPh 3 )(g3-C 3 H 5 )] gives the first crystallographic evidence of an g1- ketenyl complex in which the ketene group is also involved in an ylide grouping.163 The b-methyl migratory insertion reactions of the cis-co-ordinated styrene methyl complexes [PdMe(p-XC 6 H 4 CH–– CH 2 )(phen)][BR@4 ] [X\H CH 3 Cl CF 3 or OCH 3 ; R@\3,5-(CF 3 ) 2 C 6 H 3 ] have been studied; the crystal structures of [PdMe(OEt 2 )-(phen)][BR@4 ] and the anti-isomer of [Pd(phen)Manti-g3- CH(CH 2 CH 3 )C 6 H 5N][BR@4 ] have been determined.164 A comparison of the conformation of free dppn and when it is co-ordinated in [Pd(dppn)(g3-C 3 H 5 )][BF 4 ] ·CH 2 Cl 2 indicates that there is a relief of strain upon complexation behaviour analogous to the relief of strain observed upon protonation of proton sponges.165 C C Pt PPh3 PPh3 44 N (cod)Pt N C Ph N Ph Ph 45 NMe2 Ni Si Me Si Me3Si Me3Si Me Me 46 The preparation characterisation chemical vapour deposition and mechanistic studies of the thermal decomposition in aromatic solvents of cis-bis(g2,g1-pent-4-en-1- yl)platinum have been reported.166 The first example of a mononuclear complex containing the triazatrimethylenemethane ligand namely 45 has been prepared by the silver(I) oxide-mediated reaction of [PtCl 2 (g4-cod)] with N,N@,NA-triphenylguanidine; the crystal structure of 45 shows that g2-triazatrimethylenemethane ligand co-ordinates in an essentially planar four-membered PtNCN metallacycle.167 The synthesis and crystal structure of the novel silanediyl(silyl) complex 46 have been described.168 The unusual co-ordinatively saturated cyclopentadienyl palladium complexes [PdLnCp] (HLn\thermotropic 4-R,4@-R@-azobenzene; n\1 R\R@\C 6 H 13 ; n\2 R\ R@\C 6 H 13 O; n\3 R\C 6 H 13 O R@\C 8 H 17 OC 6 H 4 CO 2 ; n\4 R\C 10 H 21 O R@\C 8 H 17 OC 6 H 4 CO 2 ) are dark red solids which melt into isotropic fluids for n\1 or 2; for n\3 or 4 liquid crystalline properties are observed with a stabilization of melting and clearing temperatures up to 70 °C.169 The first palladocene namely 47 (emb) has been synthesised and characterised.170 Me Me Me Me Me Pd Me Me 47 Me Me 2+ Me 390 P.K.Baker References 1 J. 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ISSN:0260-1818    

DOI:10.1039/ic093375

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

22 Organornetallic chemistry of bi-and poly-nuclear complexes By S. DOHERN Department uf Chemistry Bedsun t?udding,Universityof Newcastle-upon-Tyne Newcastle-upon-Tyne,NE1 ?RU,lJK 1 Introduction This review discusses developments in the organometallic chemistry of bi- and poly- nuclear complexes published during 1996. An entire issue (No. 5) of DaIton Trans. dedicated to the first Dalton Discussion on Metal Cluster Chemistry held in January 1995 contains a number of informative articles of interest to organometallic cluster and solid-state chemists.' En addition Dafton Trans.contains several highlight articles either dedicated to polynuclear organometallics or that at least incorporate some aspects of this fieId and include ditungsten hexalkoxides (templates for organometallic chemistry and cataIysis),2 hydrogenation and hydrogenolysis of thiophenic molecules catal ysed by soluble metal c~mplexes,~ the versatile chemistry of arenemanganese carbonyl c~rnplexes,~ synthesis structures and reactivity of homo- and hetero-poly- nuclear complexes of platinum bearing CZR groups as unique bridging ligands5 and finally arene cluster compounds.6 A feature article on heterogeneous catalysis of carbon-carbon bond formation that also focused on the reactivity of di-rhodium-methyIene complexes as homogeneous models appeared early in 1996.' 0ther relevant review articles include the organometaIlic chemistry of CO,,8 open-shell organometallics as a bridge between Werner and low-valent organometallic complexes-the effect of spin state on stability reactivity and structure,' new bonding modes ff uxiond behaviour and reactivity in dinuclear complexes bridged by four- electron donor unsaturated hydrocarbons" and the synthesis of ruthenium and osmium carbonyl clusters with unsaturated rings.I 2 Titanium zirconium and hafnium Insertion of carbon dioxide into the Ti-H bond of ~(Ti(pH)Cg),(p-r]s:$-C,,H,)] gave [(Ti[~-~'-(0)CHO~Cp~z(~-~s: $-CloH,)J containing two monodentate bridg- ing formate ligands. l2 The bis(alkyny1) complex CTi(C~SiMe3)2(~5-C,H,SiMe,),j reacts with [AuR(SMe,)] to form ~(tt-C,H,SiMe,),Ti(C~CS~M~~~zAu(~'-R)J contain-ing a gold atom with an unusual trigonal-planar co-ordination en~ir0nment.I~ The bridging but-2-yne ligand in [(ZrCp2),(p-CH3CCCH,)@-C~CCH3)][BF,] is dis-395 396 S.Doherty placed by alkylisocyanides to afford the novel cationic acetylide-isocyanide bridged dimer [(C~Jr)~(p-q' :q2-C~CCH3)(p-q1 :q2-C=NR)][BF,].'4 The zirconium complex [Zr Me { C Me,SiMe,(NBu')]] containing a rnonocyclopentadien yl-amido ligand reacts with two equivalents of CO to give [{Zr(q2-0,CMe)(p-O,CMe)[(C,Me,)SiMe,(NBu')]),] which exists as a dimer in the solid state but predominantly as a monomer with chelatingacetates in solution. Reaction with a third equivalent of CO results in elimination of Bu'NCO to give [{Zr(q2-0,CMe)(p- O,CMe)r(C,Me,)SiMe,O]~*~ containing a novel bridging ansa-tetramet hylcyc- lopentadienyl-siIyloxy ligand. Fluorination of [Zr(CH,Ph),Cp*] by 1.5 equivalents of trimethylpyridioe*bis(hydroAuoride) [tmpy.(HF),] gave [MF,Cp* J in quantitative yield whereas excess tmpy-(HF) gave [(MF,Cp+),(p-F),] [Htmpy] (M = Zr or Hf).l6 A new approach to the synthesis of constrained-geometry cataIyst precursors in- volves reaction of the SI-CI bond of chlorornethylsilyl-substituted cyclopentadienyl compounds.In one instance LiNHCH(Me)Ph reacted with [ZrCl,(ljt'-C,H,SiMe,Cl)f in the presence of NEt, to give [ZrC13(NEt3){q5-CSH,SiMe,NH(CHMe)Ph}]. In the presence of water the binuclear silyloxide-bridged zirconium derivative [{ Zr[q5 q'-C5H,SiMe2(#1-O)IC12[H,N(CHMe)Ph]),] was isolated.' Several new binuclear ansa-metalIocene complexes of zirconium and hafnium have been prepared and their catalytic activity for the polymerisation of ethene and propene evaluated.For instance Li,[Me,C(C,H4)C9H6)] reacts with two equivalents of [ZrCl,(dme)Cp] to give the ansa-bridged binuclear compounds IZrCI,Cp(~-C,H,(CMe,)C,H,)ZrCI,Cpl. Heterobinuclear ansa-metallocenes of the type [MC1,Cp(~(-CSH,(CMe2)C9H6)MC12cp]have also been prepared from mono-nuclear [MCICp((C,H,)(CMe,)(q5-C,H,))] (M = Zr or Hf)and [M*Cl,(dme)Cp] (M* = Zr or Hf).'* The terminal phosphide [Zr(Me)Cp,(PHC,H,Me,-2,4,6)] generated in situ from the reaction of two equivalents of [ZrMe,Cp,] with PH,(C,H,Me,-2,4,6) reacted with additional [ZrMe,Cp,] to give the binuclear complex [(ZrMeCp,),(p-PC,H,Me,-2,4,6)]. The corresponding reaction of [ZrMe,CpJ with one equivalent of PH,(C,H,Me,-2,4,6) gave [(ZrCp2){CpZr(PHC,H,Me3-2,4,6)}(p-PC6H2Me3-2,4,6)(p-q':q2-C5H,)] which arises from activation of a cyclopentadienyl C-H bond." 3 Vanadium niobium and tantahm The 1,3-dimetallacyctobutanes [(Me3SiCH,),M(p-C(SiMe3))2M(CH,SiMe,),l (M = Nb or Ta) react with carbazdes [N'H = carbazole (Hcb) tetrahydrocarbazole and 3-tert-butylcarbazole to give [(N')2M(p-CSiMe,)},M(N'),1.Addition of 2,6-dimethylphenylisocyanide(XyNC)to [cb,M(p-C(SiMe,)),McbJ resulted in isocyan-ide-alkylidyne coupling to afford [cb,M (p-C(SiMe,))(p-XyNCCSiMe,)Mcb,l which contains a novel bridging amido-alkyne ligand." Solutions of fNbH,(SiMe,),Cp] isolated from the reaction between an excess of HSiMe and [NbH,Cp,] are unstabIe and slowly decompose at room temperature to give [{NbHCp(a:qS-C5H4)),] via reductive elimination of trimethylsilane." Organometallic chemistry of bi-and poly-nuclear complexes 4 Chromium,molybdenumand tungsten The crystal structures of [Cr,Cl(ind),] and [Cr(ind),] have been determined.Both are dimeric in the solid state containing q5-and q3-indenyl groups; toluene solutions of [Cr(ind),] catalyse the polymerisation of ethylene.,’ An improved synthesis of [Cr,(CO),fvj has been reported and its structure determined by single-crystal X-ray crystallography. Reduction of [Cr,(CO),fv] with Na-Hg amalgam or LiBEt,H pro-duced [Crz(CO),fv]2 -which when treated with CF,CO,H gave [Cr,H,(CO),fvj capable of dihydrogenation ofconjugated The reactions of the pentamethyl- cyclopentadienyIchromium(~~~j complex [( CrBr(p-Br)Cp*)2f with a1 kyI and aryl am- ides LLINHR depend markedly on the reactants and reaction conditions.For instance R = C,H,Pr’,-2,6 gave [Cp*Cr(p-NR)(p-Br)CrCp*]and [{ Cr(p-NR)Cp*},] whereas R = C,Hll gave C(CrBr(p-NC,H,,)Cp’~,] and R = C,H,But3-2,4,6 gave [{Cr(p- NR)Cp*)J. In the presence of PhGCPh as a trapping agent reaction of [{CrBr(p-Br)Cp*),] with LiNHR (R = C6H,Pri,-2,6) gave [Cr(NRXPhGCPh)Cp*J while 2,6-xylyl isocyanide gave a monomer containing an unusua1 bis(amid0)-substituted 2,3-dixylyl arninoquinoline ligand via the coupling of two isocyanides and an amido group with the loss of one C,H,Pr’ moiety.24 PhotoIysis of [NbI(CO),tPMe,Ph),tq2-dppa)j prepared from [NbI(CO),(PMe,Ph),] and dppa with [Mo(CO),] gave [{MofCO),),(p-Ph,PC=CHCH=CPPh,)] in which two [Mo(COj,] units are Iinked by a buta-1,3- diene backbone.The butadiene ligand was suggested to form uia P-C bond cleavage to give an intermediate a-alkenyl complex containing CH=C(PPh2)2.25 The dicarbene fulvene complex [( MO,(CO)~)(~’-~~: q3-CH2C~CCH2)fv][BF4],1 has been prepared and its reactivity examined. Mild nucleophiles (Nu-) such as MeOH H,O and PhOMe react to give the monocarbeniurn complexes [(Mo~(CU),)(~-~J~: ‘7,-NuCH,C=CCH,)fv][BF,] 2 via a singIe addition whereas double addition of stronger nucleophiies such as pyridine and triphenylphosphine gave [( Mo2(CO),)(p-q2:q2-NuCH,CZCH,Nu)fv] 3 (Scheme The anion [Mo(CO)~(~,~-C,H is a con- venient precursor to other pentadienyl complexes such as [Hg{ Mo(2,4-C,H ,XCO),),] and [MoI(C0),(2,4-C,H Deprotonation of [Mo,(CO)~~PP~,HX~~-~~’-P,)C~,~ and addition of excess CS affords [Mo,fCO),(p-q3-Ph2PC(HjSP,S}Cp,] 4 containing an unusual four-electron donor CSP,S ring.” One-electron oxidation of [Mo,(p-C,Me,)Cp,] with [FeCp,][PF6] gave [Mo,(p-C,Me,)Cp,][PF,].Further reaction with [FeCpJCPF,] or the trityl radical CPh 1 2 3 Scheme 1 398 S. Doherrty 4 gave [Mo,(p-C,Me,CH,)Cp,] 'most likely via EEC and EC mechanisms respective- ly aIthough in the latter case an EEC process was not discounted. A comparison of the Structures Of [MO,(/L-C8Me&p,] and [Mo,(p-C,Me,)Cp,] [PF,] revealed that the Mo-C bond lengths of the q2-alkene in ~Mo,(~f-C8Me8)Cp,~~PF,1 are significantly longer than those in [Mo,(p-C,Me,)Cp J. EHMO calculations suggest this structural change to be associated with depopulation of an orbital involved in Mo-alkene x*-backdonation namely the HOMO of a" symmetry.'' The cyclic voltammogram of the bis(metal1acyclopentadiene)complex [W,(pC,Me,),Cp,] shows two one-elec- tron oxidations.Reaction of [W,(pC,Me,),Cp,] with one and two equivalents of [FeCp,][PF,J gave [W,(p-C,Me,),Cp,] 'and [W,(p-C4Me,),Cp,-JZ + 5 respective-ly both of which have been crystallographically characterised. Structural studies and EHMO calculations confirm that the HOMO has W-W 6' character. Thermolysis of 5 in refluxing MeCN resulted in carbon-carbon coupling to give [W,(p-C8Me7CH2)Cp2] 6 in which the hydrocarbyl ligand is co-ordinated through one + q3-aIlyI and two p-allyIidene functionaIities. Reaction of [W,(p-C,Me,CH,)CpJ ' with NaBH gave [W,(p-C,Me,)Cp,] 7 formulated as a metalk~cyclonona tetraene 12+ heat 5 6 7 New mono- and di-bridged bis(cyclopentadieny1) imido and 0x0 complexes of molybdenum and tungsten have been prepared.Addition of NBu'H to [(MCI,),(p- Cp"Cp)] (M = Mo or W) gave the MV imido complexes [(MCl,(NBu')),(p-Cp"Cp)] which are easily oxidised by PCl to give [{ MCl,(NBu')),(p-Cp"Cp)]. Reduction of [{ MoCI,(NBu')),(p-CpTp)] and [(MoCI,(NBu'),}(p-Cp"Cp)] with Na-Hg arnal-gam gave [{ MoCl(NBu')),(p-Cp"Cp)] from which [{ Mo(O)(p-NBu')),(p-Cp"Cp)]was isolated after treatment with HgO [Cp":n = 1 Cp' = (q5-CSH,),SiMe,; n = 2 Cp2 = ($-C,H3),(SiMe,),].31 The dicarbomethoxydihydrofulvalene complex [HgMo,(CO),($ q5-(C,H3C0,Me),)J has been prepared by the LiBEt,H reduction of [Mo2(CO),(q5 q5-(C,H3C0,Me),)] to give Li2[Mo,(C0),(q5 q5-(C,H,CO,Me),)] followed by insertion of Hg into the Mo-Mo bond.The elec- Organometallic chemistry of bi-and pofy-nuclear complexes 399 trochemis try of these compounds was investigated. The octahedral w6 cluster [W,(p-H),(H)(CPr')(OPri),(OPri),] is labile to exchange with D, reversibly inserts ethene and in the presence of H, is a catalyst for the hydrogenation of ethene. The chemical inertness of [w&-H),~HXCPr'XOPr'),(OPr')~]enabled Chishoh and Kra~ner~~ to demonstrate that these processes occur exdusively at the terminal W-H site. The solid-state structure of [W,(NMe,),(cot)] has been reported to bear a close resemblance to that previously described for the bis(ally1) complex [W,(p-allyI),(NMe,),].Variable-temperature 'H NMR spectroscopic studies revealed two isomers in rapid equilibrium at room tem- perat~re.~~ Cleavage of the N-N bond of hydrazine by [Mo,(p-Cl)(p-SMe),Cp,] gave the mixed amido-sulfido bridged complex [Mo,(p-SMe),(p-NH,)Cp,l. EHMO cal-culations confirm a ~'6*~6'electron configuration for the quadruply bridged Mo"' d3-d3centre consistent with a M-Mo single bond.35 Condensation of [Mo,(p-R'C=CR2)(CO),Cp2] (R' = RZ = H Me or C0,Me; R' = H R2 = Me Ph or C0,Me) with [Co,(CO),] gave the butterfly clusters [CO,Mo,(p,-R 'CCRZ)(p-CO),(C0),Cp,] with the two molybdenum atoms at the wingtips and the two cobalt atoms forming the hinge.36 5 Manganese technetium and rhenium The dianionic cluster [Mn3(CO),(p-N02),(p-ON0)32 -,isolated from the reaction between [NEt,]cis-[MnCl,(CO),] and [N(PPh,),][NO,] contains both bridging nitro and nitrito groups.Similarly [Mn2(p-C1),(CO),] [MnC13(CO)3]2-and [Mn,(p-CI),(CO),] -all react with [N(PPh,),][NO,] to afford [Mn,(CO),(p-N0,),(p-ONO)12 -. Surprisingly until now rnetaI carbonyl compounds have only reacted with [N(PPh,),][NO,] to give nitrosyl-containing products via oxygen atom tran~fer.~' Addition of AlMe to [Mn(N(SiMe,),),(thf)] gave ["Mn(p-Me)[N(SiMe,),AlMe,])J a methyl-bridged dimer stabilised by Mn-Me interactions involving a methyl group from A1Me3.38 Reaction of [Re,H,(CO),J with dmf yields the unsaturated 44-electron cluster [Re,H,(CO),] -,containing an isosceles Re triangle with two single Re-Re bonds and one double Re=Re bond.This cluster reacts rapidly with donor ligands (CO py PPh or MeCN) to form [Re,W,(CU),L]-. In the solid state two of the hydrides bridge the two longer Re-Re bonds while the remaining two hydrides bridge the Re=Re double bond. In solution NMR spectroscopic studies suggest an alternative structure [Re3(p3-H)(p-H),(C0),j - in which two doubIe Re=Re bonds are de- lo~alised.~~ Several methods for synthesising the novel open cluster [Re,(CO),(p- H)(ReH(CO),)]-have been described. SoIution 'H and I3C NMR spectroscopic studies show conformational freedom about both Re-Re bonds and a dynamic process that exchanges the hydride iigands and the carbonyls &ramto these. The solution exchange processes in [Re,(p-H)(CO),( ReH(CO),)] -were compared with that of the closely reIated complex [Re2(p-H)(CO),(Re(CO)5]],in which the terminal hydride is replaced by a C0.40 Dynamic laser light scattering was used to demonstrate that the newly formed complexes [Re,(CO),(p-OMe),(p-L-L)] (L-L = dppm dppe or dppp) aggregate in solution to form clusters with an average radius of 370nn1.~' Time-resolved infrared spectroscopy was used to probe the photochemical trans- 400 S.Doherty formation of the 3,4-dirhenacyclobutene complex [Re,(CO),(q2-C0,MeC2C02Me)Cp*,j generated from the reactive Re=Re double-bonded com- pound [{Re(CO),Cp*'),] and dmad into the 2,4-dirhena[l,l,O]bicyclobutane [Re,(CO),(p-q2 :q 2-02 MeCCgCO Me)Cp*,]. Conrotatory ring-opening of the 3,4-dirhenacyclobutane was suggested to give a short-lived bis(rnetal1acarbene) intermedi- ate prior to Re-C bond formation to give [Re2(CO),Cp*,(p-q2 q2-0,MeC~C02- Me)].,' Photoextrusion of CO from [Re2(CO),(p-q2 :q2-Me0,CC~CC0,Me)Cp*,] gave the metalla tetra hedrane [Re2(CO)2(p-CO)(p-q :q -MeO,CC=CCO M e)Cp* in high ~ieId.~ Ligand addition reactions of the d2 32-electron dimer [{Re(CO),Cp*),] have been examined.Both CO and MeCN gave stable adducts while those formed with PMe and CH,CH2 at low temperature fragment above -20 "C to give [Re(CO),(thfjCp*] and [Re(CO),(PMe,)Cp*] or [Re(CO),(C,H,)Cp*] respectively. Reaction with HCECH gave [Cp*(CO),Re(p- q1:q3-CH=CHCO)Re(CO)Cp*] containing a dimetallacyclopentenone whereas CH,CzCCH initially gave the 1 1 adduct ECp*(CO),Re(p-COX$-CH,CzCCH,)Re(CO)Cp*J which slowly converts into a mixture of the dirnetallcyc- lopentenone [Cp*(CO),Re(p-q' :q3-(CH,)C=C(CH,)CO)Re(CO)Cp*] and the frag- mentation products [Re(CO),Cp*] and [R~(CO)(CH,CECCH,)C~*].~~ The ring metallated complex [Re(CO),(q'-C,H,Li)] reacts with [Re(CO),Cp] to give the dirhenium acyl anion Li[Cp(CO)2Re(C=O[C,H,Re(CO)3]}] which was pro- tonated and methylated to give the hydroxy and methoxy carbene complexes [Cp(CO),Re=C(OH)(C,H,Re(CO),}] and [Cp(CO),Re=C(OMe)(C,H,Re(CO),)I respectively.Attempted metaltation of the unsubstitured ring in [Cp(CU),Re= C(OMe)(C,H,Re(CO),)] gave the butylcarbene complex /Cp(CU),Re= CBU{C,H,)R~(CO),)]."~ The 3-rnethylthietane substituted compIexes [Re,(CO),(SCH,CHMeCH,j] and [W(CO)5(SCHzCHMeCH2)] catalyse the ring-opening cycIooligomerisation of 3-methylthietane to give the polythioether macrocycle 3,7,11-trimethyl- 1,5,9-trithiacyc- lododecane.Different orientations of the methyt substituents on the ring give rise to two isomers both of which react with [Re,(CO),(NCMe)] to give [Re,(CO),(cis,trans,trans-SCH2CH,SCH,CHMe)~CH,3 and [Re,(CO),-(cis,cis,cis-SCH,CH Me(CH ,SCH ,CH Me),C H J. 46 Adams et id.47 have ako de-scribed the catalytic cyclooligomerisation of b-propiothiolactone by [Re,(CU),(NCMe)]. The known polymer (SCH,CH,O) and the oligomers 1,5,9,13-tetrathiacyclohexadecane-2,6,10,14-tetrone and 1,~,~9,13,17,21-hexathiacyclotet-racosane-2,6,10,14,18,22-hexone have been isolated and fully ~haracterised.~~ Ring opening via S-S bond cleavage and insertion of 5-(ethoxycarbonyl)amino-1,2,4-dithiazole-3-thione into the Re-Re bond of [Re,(CO),(NCMe)] gave [Re,(CO),(p-S2CNC~N(H)C02EtJS}] [Re2(CU),(p-S,CNC(NHC0,Et)S)] and [Re2(C0)81p-S,CNH(CNco~Et)S)I.Minor amounts of the mononuclear complexes [Re(CO),(S,CNHCSNH(CO,Et))] and [Re(COj,(SCN(NHCO,E t)SCSN >]were also is01ated.~~ Several new compounds have been isolated from the reaction of [Re,(CO),(NCMe)] with EtO,CN=C=S including [Re,(CO),(Z-trans-~-C,S-EtO,CN=CS)J [Re,(CO),(p-C,S,N-EtO,CN=CS)] [Re,(CO),{p-C,N,S,-(EtU,C),NC=NCS,j.] [Re2(CO)7(NCMe)(p-C,N,S,-(Et0,C)2NC=NCSz)] and [Re,(CO),(~-C,N,S,-(EtO,CXHXNC~NCS,jl; the first two contain a dimetalIated thioimidate whereas the last three arise from new coupling and rearrangements Organometallic chemistry of bi-and poly-nuclear complexes 401 invoIving two is0 thiocyana tes.Similar1y [Re,(CO),(NCMe)( PMe,Ph)] reacted with EtO,CN=C=S to afford the dimetaIlated product [Re,(CO),(PMe,Ph)(p-Et0,CNCS)J as well as ~rans-[Re,(CO),(PMe,Ph)(~-(EtO,C)N=CN(CO,Et)(S,)}] and cis-[Re,(CO),(PMe,Ph)(p-(Et0,C)N=CN(COt~S2))] both of which are for-med by the coupling and rearrangement of two isothiocyanate moIecules. These Iast two isomers convert into [Re,(CO),(PMe,Ph)(p-N,C,S,-(EtO,C),NC=NCS, }J uia loss of carbon Therrnolysis of [Re,(CU),(NCMe)] with MeO,CHC=C=C(H)CU,Me in the presence of adventitious water gave [Re(C0)4{C(CHzC0,Me~C(H)C0,Me}]which reacts with PMe2Ph to give fac-[Re(CO),(PMe2Ph)(C(CH,C0,Me)=C(H)(C02Me))f.similar reaction with A [Re,(CO),(PMe,Ph)(NCMe)] gave rner-[Re,(CO),(PMe,Ph){p-q3 q'-MeO,C(H)-CCC(H)CU,Me)J and fac-rRe,(CO),(PMe,Ph)(~t-rl~: $-MeO,C(H)-CCC(H)-CO,Me)] two isomers both containing an q3:ql-allene ligand.50 The reactions of [Re,(CO),(NCMe)] and [Re,(p-H),(CO) ,(NCMe) J with S=C(NEt,)N(H)(C,H,Me-p) gave [Re,(CO),(S=C(NEt,)N(H)(p-C,H,Me)}] and [Re,(~i-H)3(CO),ofC'-S=C( NEt2)N(H)(p-C6H4Me)}] repectively.The first of these new compounds contains an S=C(NEt,)N(H)(p-C,H,Me) ligand S-co-ordinated in an equatorial. site while the second product contains an S-bridged thiourea. When heated at reflux in heptane [Re,(CO)g(S=C(NEt,)N(H)(C,H,Me-p)}] loses H to give the non-metal-metal bonded complex [Re,(CO),{p-SCS(N-C,H,Me-p)(NEt,)) J.51 In-sertion of EtO,CN=C=S into the metal-metal bond of [Re,(CU),(NCMe)] gave [Re2(CO),(S-truns-~r-C,S-Et02CN=SC)] CS)j [Re,(CO),(S-trans-p-C,S,N-E~O~C~= and [Re,(CU) { p-C,N,S,-( EtU,C) N=CNCS,)].52 Reduction of [TcO,] -in the presence of BH,.tbfand carbon monoxide in thfgave [Tc,(p-H),(CO),,] the first example of a hydrido-bridged technetium(1) compound. A minor by-product of this reaction was tentatively suggested to be [Tc3(p3-H)(p-H)3(C0)9]-based on IR and 'H NMR spectroscopic data alone.53 6 Iron ruthenium and osmium The room-temperature reaction of [Fe,(CO),] with 2-butyne-1,4-diols gave the buta- triene hexacarbonyldiiron complex [Fe,(CO),(p-q3 q3-R1R2CCCCR3R4)]. Little mechanistic information was provided but [Fe2(CO)P] was suggested to act as a reducing agent as well as a complexing agent.54 The anion [Fe,(cO),(p-SN)] reacts with the carbon-rich carboranes 6-bromopentamethyl- and l76-dibromo-2,5-dibutyI-3,4-diethyl-2,3,4,5-tetracarba-nido-hexaboranes(6)to give N-(6-[Z73,4,5-tetracarba-nido-hexaborane(6)-yl])hexacarbonyldiferraazathiatetrahedrane complexes in which the hexacatbonyldiferraazathiatetrahedraneand carbon cage are linked through a B-N bond.55Oxidation of the vinyl complex [FeL1L2(C(R)=CH2)Cp*](R = OMe 5 L' CO Lz = PPh,; R = OMe L1 = CO L2 = PMe,; R = OMe L1 = L2 = dppe; R = H L1 = L2 = dppe)with [FeCp,][PF,] at -80 "C gave the unstable 17-electron radical [FeL' L2{C(R)=CH,)Cp*][PF,] which undergoes carbon-carbon coupling in the solid state to give the bis(carbene) complexes [(FeL1L2Cp*),(p-=CRCH,CH,RC=)] [PF,] 2.55 The 1,3-diferrio-173-diphosphetane-2,4-diones C(CP'(CO),f+ PC(O)P{Fe(CO),Cp'}C(O)] have been isolated from the reaction between 402 S.Doherty (dme),LiOC=P and [FeBr(CO),Cp']. Further reaction of these diones with rCr(CO),RI (R = Z-cydooctene)] gave C(Cp'(W2Fe)-P(Cr(CO) )C(O)P( Fe(CO),Cp'}C (O)](Cp' = C Me, C H3BuL2-1,3 or C,H2Prij-1,2,4)?' The cationic complexes [(Fp)Ph,PC=CPPh,(Fp)j + and + [(Fp)Ph,PC=CPPh,] [Fp = Fe(CO),Cp] have been prepared by oxidising [Fe,(CO),Cp,] with one and two equivalents of [FeCpJ' in the presence of Ph,PGCPPh,. Addition of one and two mol equivalents of the diphosphine to gave [PPh4J[Fe,(p-H)(C0)g(p3-~2-C=CH2)]the anionic dusters [PPh,] [Fe,(CO),(p,-CCH,)(Ph,PC=CPPh,)]and [PPh,][(Fe,(C0)9(p3-CCH3)}2(Ph2PC~ CPPh,)] respectively.The zwitterionic complex [(Fe,(CO),(p,-CCH3))(Ph2PC:CPPh&Fp)] has also been de~cribed.~' Oxidative addition ofphenyfsifane to [Fe,(CO),] gave the triply-bridged complexes [{ Fe(CO),),(p-SiPhH),(p-CO)] while SiPh,H gave the singly-bridged complex [{(CO),FeFe(CO),(SiHPh2))(p-q2-HSiPh,)]. Themolysis of a toluene solution of the latter product resulted in the formation of [{Fe(CO),}2(p-~2-HSiPh,),l.Both [{(CO),FeFe(CO),(SiHPh2))(p-q2-HSiPh,)]and [{ Fe(CO),),(p-q2-HSiPh2)J con- tain agostic Fe-H-Si interaction^.^' Reaction of [F~,(~-CSXC~-CSM~HCO),C~,~[SO,CF,] 8 with [NBu,jX (X = W or CN) occurs with nucleophilic attack at the p-CSMe thiocarbyne carbon to give 8 9 10 1-co Me Me Md4 11 12 Me' 13 Scheme 2 Organometallic chemistry of bi-and poly-nuclear complexes [Fe,(p-CS)(p-C(SMe)X)(CO),Cp,] 9.Reaction of 9 with MeSO,CF gave [Fez@- CSMe)(p-C(SMe)X)(CO)zCp,][SO,CF,]10 uia selective methylation of the bridging thiocarbonyl ligand. Photolysis of 9 results in loss of carbon monoxide and co- ordination of the thiocarbene sulfur atom to give [Fe2(p-CS){p-$(S)-C(SMe)X}(CO)Cp,] 11,Treatment of this latter complex with MeSO,CF resulted in methylation at the thiocarbonyl carbon to give the mixed carbyne+xrbene complex [Fe,(p-CSMe)(p-q(S)-C(SMe)X)(CO)Cp,]12 which upon addition of CN- gave + [Fe (p-C(CN)SMe1(p-q(S)-C(SMe)X)( CO)Cp,] 13 the first bis(p-t hiocar bene) com- plex (X = H or CN) (Scheme Z).,' H Fhz 14 16 hv -CO1 H Ph2 15 17 Scheme 3 The phosphido-bridged diiron allenyl complex [Fe2(CO),(~-PPh,)(~-~' :q2a,B-C,(H)=C,&H,)] 14 reacts with primary amines to afford the amino-functionalised alkenyl complexes [Fe,(CO),(p-PPh,)(p-q' :q' :q2-(O=C(NHR)CII,C=CH2}] 15 (R = Bu' or Ph) via a novel carbonyl-allenyl-amine coupling sequence and the dime t allac yclopen t adienes [Fez(CU),(p-PPh J1p-q :rj '-CHzC(NHR')CH > ] (R' = Cy or But) via nucleophilic attack at C and hydrogen transfer to Cg.61 The same allenyl complex reacted with dppm to give ~F~,(CO),(~-PP~,)(II~(P): $(C)-Ph,PCH,PPh,C(H)=C=CH,)] 16 containing a dppm-functionalised alIene.Photoly- sis of a toluene solution of 16 resulted in activation of a dppm methylene C-H bond hydrogen migration to the allene and formation of [Fe,(CO),(p-PPh,){p-ql(P):$(C):q'(C)-Ph,PCHPPh,C(H~CCH,31 17 which contains a metal- and car- bon-co-ordinated bis(dipheny1phosphino)methanide (Scheme 3).In contrast reaction of [Fe,(cO),(~-sBu'x~-~~~ :q'-CH=C=CH,)] with dppm gave the isomeric alkenyl complexes [Fe,(CO),(~-SBu')(~-q'(~): q'(C):q2(C)-Ph,PCHPPh,C(H)=CCH3}] and [Fe,(CO),(p-SBu'){p-q (P):q '( C):q2(C)-Ph,PCH PPh,CH ,C=CH ,>]via a similar C-H activation-hydrogen migration pathway.,* The binuclear allenyl complex 404 S. Doherty 18 19 20 Scheme 4 CFe,tCO),(tt-PPh,)(tt-rl' :$z,&z(&~2)] reacts with one fR = Ph) and 0.5 (R = H) equivalents of PPhHR to give [Fe,(C0)6(p-PPh,){p-q1.:q2-(Me)C=CH(PPh,)}] and [(Fe,(CO),(~f-PPh,)[p-~~ :q2-(Me)C=CH]),(p-PPh)j both of which arise from an unprecedented nucleophilic attack of the phophine at C of the p-q' :~~=,~-aIlenyl figand6 The dirnetaIlacycIopentenonecomplex [Fe,(CO),(p-a q3-C(O)CHCH)(p-dppm)] 18 isomerises to the jr-vinyl complex [Fe,(CO),(p-CH,=CP(Ph),CH,PPh,}] 19 uiu intramolecular nucleophilicattack of phosphorus on a carbon of the metallacycle with an associated 1,2-hydrogen migration and C-C bond cleavage.On further heating transfer of a phenyl group from phosphorus to carbon gave the isomer [Fe,(CO),(p-C(CH,Ph)P(Ph),CH2PPh}] 20 together with [Fe,(CO) {pa:q3-C(0)CRCH} (p-PPh,CH,P(Ph)C H,C(CH,)}] the result of a more complicated rearrangement (Scheme 4)-, The redox behaviour of [Fe3(CO)9(pt,-S)]2-has been investigated.The anion [Fe5(CU),4S6]2- was prepared by oxidative condensation of [Fe,(CO),S]2- and [Fe,(CO) 2S6J2- was isolated as a by-product. Alternative procedures for the syn- thesis of these clusters have been described and their crystal structures reported.65 The hexanuclear carbonyl metallate [NEt,],[Fe,C(CO),,] reacts with [(AuCl),(p-L)] (L = dppm or dppe) to give [Fe,Au,C(CO),,(dppm))] and [(Fe,AuC(CO),,),(p- dPP4P The binuclear p-0x0 complex [(RuCl,Cp*),fp-O)] decomposes in chloroform sol-ution by activation of a methy1 C-H bond to give the dinuclear tetrarnethyIfulvene complex [f RuCl,(qS-C,Me4CH2)},j and water. The chloride bridges are cleaved upon addition of donor ligands (pyridhe and dmso) to give [RuCI,L(q6-C5Me,CH,)]. Notably though the bromide complex [(RuBr,Cp*),(p-O)] is significantly less reac- tive toward C-H activation although in the presence of donor ligands both the chloride and bromide complexes react under ambient c~nditions.~' Oxidative addi- tion of the dichalcogenide REER (E = S Se or Te; R = ferrocenyl) to [{RuCp*(p,-Cl)),] gave the ferrocenylchalcogenate-bridgedcomplex [{ RuClCp*(p-ER)),].Re-duction of these complexes with Na-Hg amalgam in the presence of buta-1,3-diene gave { Ru(p-ER)Cp*} (p-s-trans-q :qz-CH,=CH CH,)I whiie react ion with AgSO,CF gave the co-ordinatively unsaturated complex [Ru(p-ER)Cp*][SO,CF,] containing one 16-electron Ru" and one 18-electron metal. centre.,' Reaction of [( RuCl(p-CI)Cp*)J with [MS,] -gave the mixed disulfide-sulfide complexes [(R~CP*),(~(,-S,K~I,-S~~~-S),MS] (M= Mo or W) in which the MS fragment is bonded to the two ruthenium atoms by three suIfur atoms two of which bridge the Ru-M bonds while the third caps a11 three metal atoms.69 The reaction of [( RuCl,Cp*),] with Li(Bu'NSPh) gave two products [RUC~(~~-BU~NSP~)(~~- C,Me,CH,)] and [Cp*ClRu(p-NBu')(p-SPh)RuCp*]; the former has a tetramethyl- Organometallic chemistry of bi-and poly-nuclear complexes 405 fulvene ligand the latter is a sulfido-irnido bridged dimer formed uia S-N bond cleavage of the sulfenamido species.Similarly S-N bond cleavage in the reaction of [{ CrBr,Cp*) J gave [C~(NBU')(SP~)~C~*].~* Various binuclear mixed p-methox- ide-phenolate and bis-p-phenolate complexes of ruthenium have been prepared.In one instance the bridging phenoxide ligand of [(Cp*Ru),(p-OC6H2R1RzR3)2]rear-ranged to give the oxoc yclo hexadien yl complex [RuCp*(q5-C,H ZR R2R3C=U)]. 'SPri 22 The co-ordinatively unsaturated dimer [{ Ru(p-SPr')Cp*),] reacts with excess al- kyne HCsCR [R = To1 or C=CH(CH,),CHJ to give the ruthenacyclopentenyl com- piexes 21 and 22. These complexes both react with Bu'NC to give the it-c-e-alkenyl corn piexes [Cp*(Bu'NC)Ru(p-SPr'){p-q' :q2-C(TolbCHC{C(Tol)=CHSPr')= CH(Tol))RuCp*] and [Cp*(Bu'NC)Ru(p-SPr'){p-q':qz-~-C(C(C=CH(CH,)3CH,)= CHSPr'}=CH(C=CH(CH,),CH,)) RuCp*] respectively via ring-opening of the ruthe- nacyclopentenyl fragment.?' ThermoIysis of [(RuCI(~-SH)C~*)~], isolated from the reaction of [(Ru(p,-Cl)Cp*),] or [{RuCl(p-Cl)Cp*)J with H2S gave the cubane cluster [{ Ru(p3-S)Cp*),]CI2.Reaction of [RhCl(PPh,),] with [{ RuCl(p-SH),Cp*) J gave the trinuclear mixedmetal cluster [(C~*RU)~(~-H)R~(PP~,)CI~(~~-S)J.'~ The (R disulfide-thiolate bridged complexes [Ru,(~-S,)(~-SR)~C~*J = Pr' or PhCH,) have been prepared and structurally characterised and their redox behaviour exam-ined using cyclic v~Itarnmetry.~~ The trinuckar ruthenium pentahydride ~(RUCP*),(~-H)~(~~-H),~ reacts with buta- 1,3-dieneto give [(RUC~*)~(H),{~~-~~-C(M~)CHCH}] which contains a l-rnethyl-1,3- dimetalIoally1 ligand. An intermediate p-q2 :q'-s-cis-isoprene complex was detected by 'H NMR spectroscopy confirming that all three ruthenium atoms are required to activate the i,3-diene; two act as co-ordination sites the other as an activation site for the C-H bond." Insertion of nitriles RCN (R= Me or Et) into the Ru-H bond of [{Ru(p-H),Cp*},] in the presence of an arene gave the novel hydrido p-alkylidenearnido complexes [( RuCp*),(~-areneXp-H~~-~~CHRII 23 containing an q2:$-co-ordinated arene.Addition of ethylene to t(RuCp*)2(~-areneH~-H~p-N=CHR)] gave the bis(ethy1ene) complex f{ RuCp*(q2-C,H,)),(p-HXp-N=CHR)] 24 which upon thermolysis at 80 "C converted into the p-q2:$-s-cis-butadiene complex [(RuCp*)&q2 :q2-CH2=CHCH=CH2)(p-H)(p-N=CHR)] 25 via dehydrogenative coupling ofthe co-ordinated ethylene moIecules (Scheme 5). Possible pathways for this coupling were disc~ssed.~' ThermoIysis of a toluene solution of [Ru,(p-CO)2(CO),Cp,~ and H,SiR',-. (n = 2 or 3; R' = Et or Ph) gave a mixture of the mono-and di-p-methylene bridged 406 S.Doherty d R 23 24 / 25 Scheme 5 complexes [Ru,(p-CH,)(p-CO)(CO),Cp,l and [Ru,(p-CH,),(CO),Cp,] via deoxygenative reduction of a bridging carbonyl with the silane. Labelling experiments with l3C0and D,SiR’,_ confirm that the carbon and hydrogen atoms of the product p-CH moiety arise from CO and the hydrosilane.” Thermolysis of [Ru,(p-CH,)(p-CO)(CO),Cp,] in the presence of HSiMe (170°C 3d) generates methane SiMe, [Ru(H)(SiMe,),(CO)Cp] and [Ru(CO),(SiMe,)Cp]. Reaction of the labile complex [Ru2(p-CH,)(p-CO)(CO)(NCMe)Cp2]with HSiR gave the hydridosiiyl-p-methylene compIex [Ru,(p-CH2)(H)(SiR,)(CO),Cp,3 and [Ru,(p-CH,)(SiR,),(CO),Cp,l both of which liberate CH upon further treatment with HSiR,.Hydrostannanes HSnR, also react with [Ru,(p-CH,)(p-CO)(CO),cp,l firstly to give [Ru,(p-CH,)(H)(SnR,)(CO),Cp,] and then [Ru,(pCH,)(SnR,),(CO),Cp,]. However HSnPh reacts with [Ru2(p-CH,)(p-C0)(CO),Cp2] to afford [Ru,(p-CH,)(SnPh,)(H)(CO),Cp,] which upon standing converts into a mixture of [Ru2(p-SnPh,),(CO),Cp J and [RU,(~-P~){S~(CH,)P~,)(CO),C~~]. A possible pathway for generation of CH4 from [Ru,(p-CO),(CO),Cp,] and hydrosilanes has been for-rn~lated.~~ While addition of CO or PPh to the silylated p-methylene complex CRu,(p-CH,XHKSiR,XCO),Cp,j gave [Ru2(p-CH,)(p-CO)(CO)LCp2],via reductive elimination of SiR,H reaction with [RU~(~-CH,)(S~R,),(CU)~C~~] gave the p-silyl- methylene complex [Ru2(p-CHSiR,)(p-CO)(CO)LCp,l with elimination of SiR,H.Presumably the latter occurs via reductive elimination of p-CH and SIR to give q’-CH,SiR followed by oxidative addition of a C-H of the new silylmethyl group. Addition of HSiR and H to [Ru2(p-CHSiR,)(p-CO)(CO)(NCMe)Cp2]gave [Ru(p-CH,)(SiR,),(CO),Cp,] supporting the involvement of oxidative addition of CH,-SIR in this reaction.79 The binuclear allenyl complex [Ru,(CO),(p-PPh,)(p-$ :q2B,y-C(Ph)=C=CH,}]26 reacts with dipheny1 acetylene to give [Ru,(CO),(p-PPh,){ ,u-$-C,MePh,- Organometallic chemistry of bi-and poly-nuclear complexes 407 26 27 R = Me; 29 R = H 28 Scheme 6 (c(&)(o))] :q2-C=CPh)]28 27. Under the same conditions [R~,(C0)~(p-PPh,#p-q' gave [Ru,(CO),(p-PPh,){ ~-YIS-C,HPh2(C6H,)(o))l 29 (Scheme 6).Reaction of the allenyl dimer with PhCzCH gave two isomers of composition [Ru,(CO),(p-PPh,){p- q5-C,MeHPh(C,H,)(0))] while similar treatment of the acetylide dimer also gave two isomers of [RU,(CO),(~-PP~,)(~-~~~-C~H~P~(C~H,)(~)}~.~* The p-q1:q2g*B-bu-tadiynyl complexes CRu,(CO),(jI-PPh,X~-q' :q2,,,-C~CC~CR)] 30 (R = But Ph or SiMe,) react with the carbene precursors R1,CN (R' = H or Ph) to give the 1-ynyl-allenyl complexes [Ru,(CO),(p-PPh,)(p-q' qZ-C(C=CRbC=CR',}] (R = Bu' R' = Ph). In the case of Ph,CN, attack of the carbenegroup at C gave the ql-indenyl Ph 30 31 R = 8ut 32 R = Ph products [Ru,(CO),(p-PPh,){ q' :y12-CH(C6H4)C(Ph)=CCzCBu')] 31 and [Ru,-:q2-C=C(Ph)C=C(PhXC,H4)CH)] (C0)6(p-PPh2)(jfL-q1 32." Thermolysis of the bi-nuclear acetylide complex rRU2(CO)6(~-PPh,X~-)11 :rf-C=CBu*)] 33 gave fRu,(C0)9(p-PPh,),(~-Zf1:q2-CCBu'),J 34 a 64-electron butterfly cluster containing two bridging p-PPh, one p-ql :q2-C=CButand one ,u3-q1:y12 q2-C=C13u'.Continued thermolysis of 33 led to carbon-carbon coupling to give [Ru,(CO>,(p-PPh,),(Bu'C,Bu')] 35 and [Ru3(CU),(p-PPh,),(Bu")3 36 containing p4-and pc,-butadiyneligands respectively (Scheme 7).* Removal of the chloride from [N(PP~,),][RU,(~-CI)(CO)~(~-P~C=CP~)] in the 408 S.Doherty 36 35 Scheme 7 Carbonyts omitted for clarity 37 38 PhCCH 39 40 Scheme 8 presence of dppm affords the unsaturated 46-electron cluster [Ru3(CO),(p3-q2-1-PhCXPhKp-dppm)] 37 in high yield. Reaction of 37 with CO gives its &electron counterpart [Ru3(CO),(p,-q2-Jj -PhC=-CPh)(p-dppm)] 38 with conversion of the al-kyne to the parallel mode.Cluster 37 reacts with dppm to give [Ru~(CO)~(~-PhCZPhXdppm),] and with H to yield the dihydride [Ru,(p-H),(CO),(,u-PhCzCPh)(p-dppm)] which exists as a mixture of two isomers. Addition of one equivalent of phenylacet ylene to 37 gave the fly-over cluster [Ru,(CU),(p- OrganometalIic chemistry of bi-and poly-nuclear complexes 409 HCC(Ph)C(O)-C(Ph)CPhf(p-dppm)] 39 containing the dialkenyl ketone ligand HCC(Ph)C(U)-C(Ph)CPh together with [Ru2(CO),(p-HCC(Ph)C(Ph)CPh)(p-dppm)] 40 a binuclear ruthenacylopentadiene resulting from alkyne coupling and cluster fragmentation. Cluster 38 readily converts into the vinylidene aIkenyl ketone derivative [Ru,(~-H)(CO),(pC(CPh)C(O)C(Ph)CPh](p-dpprn)] via facile C-H bond activation at 35 “C (Scheme 8).83 The p,-alkyne cluster [Ru,(CO)~(~~-CO)(~,-CF,C=CCF,)Cp J and its labile acetonitrile derivative [Ru3(CO),(p,-CO)(NCMe)(p3-CF,CZCF,)Cp,] react with alkynes.The latter reacts with CF,C=CCF to afford CRU,(CO),(~-CO)~~~-~~-C,(CF,)~)(~~,-CF,)C~,], whereas PhCECPh methylbut-2-ynoate and but-2-yne afford [Ru3(C0),(p-CO),(p3-C,(CF,),R(R’))Cp,] 41 (R = R’ = Ph or Me; R = Me R’ = C0,Me); in which the hydrocarbyl fragment is q’-allyl-p-alkylidene co-ordinated. In contrast refluxing tol- uene solutions of [Ru,(CO),(p-CO)(p,-CF,C_CCF,)Cp,] react with rnethyl-2-ynoate PhCZPh and MeCZMe to give the novel cfoso-pentagonal bipyramidai 41 42 Ru,C cluster [Ru,(CO),(p3-C,(CF,),R(R’))Cp,J 42 which exists in two isomeric forms (R = Ph or Me) resulting from insertion of the alkyne into the CX triple bond of hexafluorobut-2-yne or from straightforward linking of the alkyne with hexa- fluorobut-2-yne.The room-temperature products [Ru~(CO),(~-CO),(~C,(CF,)~-R(R’))Cp,] were shown to be intermediates in the formation of [Ru~(CO)~(~-CO)(~-C3(CF,),}(p3-CCF,)Cp2] and [Ru,(CO),(p3-C,(CF3),R(R‘)}Cp2], the former requir- ing C-C bond cleavage the latter C-C cleavage and C-C regenerati~n.’~ Safarowie and Keisterg5 have determined the kinetics of isomerisation of [Ru,(p-H),(CO),(p,-CSEt)] to [RU,~~-H~(CO)~(~~-~~-CH,SE~)], which involves a double C-H reductive elimination. An inverse dependence of the rate on carbon monoxide concentration is consistent with reversibIe CO dissociation prior to the rate-determin- ing step but following an intramolecular rearrangement proposed as either hydride migration to give an agostic Ru-H-C bond or a change from p,-CSEt to p,-CSEt co-ordination.There is a distinct change in the mechanism of reductive elimination of C-H bonds in clusters of the type [Ru~~~-H),~C~)~(~~~-CX)] (X = Ph C0,Me or SEt) which depends highly upon the nature of the alkylidyne substituent from a CO associative (Ph) to CO independent (C0,Me) to CO dissociative pathway (SEt). The cationic cluster tRu,(~t-HXCO),(lc~-a~pyMp-~~ :q2-PhC=CHPh)]’ 43 catalyses the hydrogenation of diphenylacetylene to cis-and trans-stilbene without hydrogenation of stilbene to 1,2-diphenylethane.The catalytic cycle was proposed to involve dissociation of the olefinic moiety of the alkenyl ligand oxidative addition of H to form a trihydride cluster rapid reductive elimination ofstilbene and insertion of 410 S. Doherty -I+ Ph 43 t l+ PhCGcPh Scheme 9 diphenylacetylene (Scheme 9).86 The triruthenium 0-TL-vinyl clusters [Ru3(p-HXCO),(p -Ph,PC,H S)] and its diphenyl-2-thien ylphosphine substituted counter-part [Ru,(p-H)(CO),(~~(,-Ph2PC*H~~~Ph2PC~H~S)] were isolated from the reaction between [Ru,(CO),,] and Ph,PC,H,S. Prolonged reflux of a toluene solution of [Ru3(p-H)(CO),(p,-Ph,PC,H,S)] gave two tetranudear clusters [Ru4(CO)ll(p4-PPhXp,-C,H,S)] 44 and [Ru,(CO) I(p4-PPh)(p4-C6H4)]45 by elimination of ben-zene and thiophene respecti~ely.~' The P-co-ordinated diphenylvinylphosphine ligand in [M,(CO) ,(Ph,PCH=CH,)] (M = Ru or 0s) readily p-eliminates to give the o-x-alkenyl complexes [M 3(,u-H)(CO)g(p-~1 :q'-Ph,PCH=CH)] containing a phos-phino-substituted vinyl ligand.,' Reaction of the phosphazene chain Ph,SPN=PH,PPPh,=NP(E)Ph with [Ru3(CO) ,3 under oxidative decarbonylation conditions,gave ~Ru3(CO),(p,-Se),(p-PPh,)oh,PNP(Ph)2NPFh2)].8g Ph2 Php 44 45 Organometallic chemistry of bi-and poly-nuclear complexes 411 Both [Fe,Ru(CO),,] and [FeRiu,(CO),,] undergo a phase change from a non- centrosymmetric ordered structure at low temperature (<223 and 173K respectively) to a disordered centrosymrnetric phase at high temperat~re.~’ The unusual q’-l-azavinylidene cluster [Ru,(p-H)(CO),,(p-N=CPh,)] 46 has been prepared by reacting [Ru~(CO)~~] with LiN=CPh followed by protonation with trifluoroacetic acid.Notably this compound could not be prepared directly from benzophenone imine and [Ru~(CO),~]. Reaction of the azavinylidene cluster with dppm gave [Ru,(p-H)(CO),(p-q’-N=CPh,)(p-dppm)]and [Ru3(p-H)(CO),(t-N=CPh,Xp-dppm)(ql-dppm)]; -the former loses CO to give [R~,(p-H)(cO)~(p~-q 46 47 N=CPh,)(p-dppm)] 47 which contains a p3-q2-1-azavinylidene ligand. Similarly [Ru,(p-H)(CO),(PPh,)(p-N=CPh2)J loses CO to give [Ru,(p-HXCO),(PPh3Xp3-q2-N=CPh,)f also containing a p,-q2-l-azavinylidene ligand. Further substitution of carbon monoxide with PPh in [Ru3(~~-HHCO),(PPh,Hj~-N=CPh,)l gave [Ru,(p-H)(C0)8(PPh,)2(p-N=CPh2)l which is thermally unstabIe and reversibly orthometd- lates at a azavinyIidene phenyl ring to give fRu3(p-H),(Co),(PPh,),( jl-N=CPh(C,H,))].” The reaction of [Ru,(CO),,] with 3,N-diphenylprop-2-enimine gave [Ru3(p-H)(C0)9(ji,-q2-PhCH2CH2C=NPh)] as the major product together with [Ru2(CO)~( PhC=CHCH,P h)] [R u ,(CO),( PhC=CHCH=NPh)J and [Ru4(CO) (PhC=CHCH=NPh),].ExampIes of these latter three clusters are well known whereas clusters analogous to ~Ru,(p-H)(CO),(p,-q2-PhCH,CH,C=NPh)J, containing a ~13-q2-imine ligand in which the C=C double bond has been hydrogenated have never been isolated from the reaction of [Ru3(C0),,] with azadiene.” DodecacarbonyItriruthenium reacts with SnR (R = C6H,Pri,-2,4,6) to afford fRu,(CO) &-SnRJJ and [Ru,(CO)~(~-S~R,),].Reaction of the pentanudear clus- ter [Ru,(CO),,(p-SnR,),] with additional diorganotin reagents SnR’ [R’ = R or CH(SiMe,),] gave the corresponding hexarnetallic cluster [Ru,(CO)&SnR,),(p- SnR’,)] while [Ru,(CO),,(p-dpprn)] reacted with [(SnR,),] to give [Ru~(CO)~(~-SnR,),(p-dppm)].93 Reaction of SnR (R = C,H,Pri,-2,4,6 C,H,Et,-2,6 or C,H2Ph,-2,4,6) with [Fe,(CO) J each gave [Fe2(CO),(p-SnR,)] whereas prolonged treatment with SnR’ (R’= Me$,) ultimately gave the known compound spiro-[{Fe2(CO),),(p4-Sn)J Reaction of the tin or lead reagents [MCH(PPh,),] (M = Sn or Pb) gave [Fe,(Co),(pCo)(dppm)] in near quantitative yield.94 Facile fragmentation of [Ru,(p-H)(CO),(p,-q2-SCNHPhNPh)J in the presence of excess diphenylthiourea gave [Ru(CO),($-SCNHPhNPh),] containing two biden-tate diphenyhhioureato ligands.The room-temperature reaction of [RuJp-H)(C0),(p,-q2-SCNHPhNPh)] with PPh results in CO substitution whereas two equivalents ofPPh leads to facile P-C bond cleavage to give the sulfido compound 412 S. Doherty [Ru,(CO),(PPh,~p-q2-C,H,~p-PPh,Xcl,-S)],suggesting that the thioureato Iigand is in fact ineffective as a cluster stabilising ligar~d.'~ Substitution of PPh for CO in the hexanudear cluster CRU,(~-HXCO),~(C~~-SX~~-~~~-SCNHP~~P~)] occurs at the api-cal p-S-bonded ruthenium atom to give [Ru,(p-HXCU) ,(PPh3)(p-S)(p-q2- SCNHPhNPh)]. While the diphenylthioureato ligand remains intact the ruthenium cluster framework experiences substantial bond Iength elongations.In contrast other two-electron donors such as Bu'NC P(OMe), P(OPh), PBu" and Me$ substitute for CO at the nitrogen-bonded ruthenium atom leaving the metal framework essen- tially ~nchanged.'~ Reaction of CI,PNPr' with [Ru,(C0),J2-and the reaction product of Na,[Os(CO),] and [Os,(CO),,] provides a convenient route to the phosphinidene- stabilised clusters [M,(C0)13(p3-PNPr'2)] (M = Ru or 0s)-Thermolysis of [M3(CO)i,(p3-PNPri2)] effected the loss of carbon monoxide to give the closo-five vertex M4P polyhedral cluster ~M,(C0),2(p3-PNPr'JJ 48. Hydrolysis of this product led to facile P-N bond cleavage with formation of CN~I,Pri2][M,tCU),2(PO)~ 49 containing a rare example of a triply-bridging phosphine monoxide liga~~d.~~ NPri2 I ... M (CQ3 M= RuorOs M = Ru or 0s 48 49 Thermolysis of [RU,(CO)~~] with cycloocta-1,4-diene gave [Ru,(CO) 2(p4-~2-C,Hl0)j and an isomer [Ru,(CU),,(p,-q2 q2-C8Hlo)],butterfly clusters that contain 60and 62 electrons respectively.The HOMO-LUMO gap of the SO-electron cluster is large enough for it to be thermodynamically stable even though a closed butterfly structure contains a more stable [RU,(CO),~] fragment.98 ThermoIysis of [Ru3(C0),,] and 9-anthraacylphosphine gave several products including [Ru,(p-4H7PPh2)]and [Ru,(CO) 1(p4-C1 H)(CO)g(p3-C1 .H,PPh,)J anthracyne compIexes arising from double metallation of one of the unsubstituted rings and [Ru,(CO) 3(p5-ql:q2:q3:q3-C,,H9-q1-PPh,)] 50 a bowtie cluster with an anthracene unit co-ordina- ted to the Ru cluster framework viaa q2-C=Cdouble bond two ,+-ally1 groups and a ql-a-interaction." Reaction of [Ru3(CO>,,] with 1,3,5-triisopropenyl benzene in refluxing octane gave two isomeric clusters of the formula [Ru,(CO),,(Cl,Hzo)].Both contain a tetrahedral arrangement of metal atoms but differ in their hydrocarbyl bonding modes due to hydrogenation ofdifferent carbon atoms in the C framework. In one isomer the two hydrogen atoms add across two of the unsaturated side arms to give q3-co-ordination of the C,,H, hydrocarbon whiIe addition across only one double bond affords the other isomer with an q2-co-ordinated C,,H, hydrocar-bon."' Johnson and co-workers"' isolated [Ru,(p-H)(CO),(p(,-ql 9' q1-C7H8)]51 and [Ru,(CO) r(,u,-ql qi:qz:q2:q2-C,H6)] 52 from the reaction of [Ru~(CU)~~] with Organometallic chemistry of bi-and poly-nuclear complexes 413 50 51 52 norbornene and norbornadiene respectively.The first contains a trianguIar ruthenium cure with norbornadiene bonding through its alkenic bond and an agostic C-H -.Ru interaction. This mode of co-ordination was proposed to parallel that of norbor-nadiene absorbed on a Pt(ll1) surface. The other a tetranuclear butterfly framework of ruthenium atoms contains norbornadiene co-ordinated through both double bonds.lO' Clusters ranging in nuclearity from five to seven were isolated from the reaction between 1,4-diisopropenylbenzeneand [Ru,(CO) J-Of those isolated [Ru,H(CU) 5(C1,HI and [Ru,H(CU),,(C,,H,,)] are partidarly noteworthy be- cause their duster atom frameworks have not previously been reported.Both consist of a distorted edge-bridged tetrahedron of ruthenium atoms with a ruthenium spike connected to the edge bridging ruthenium. The unsaturated organic iigand is q6-bonded through the arene ring to the spike ruthenium atom q'-co-ordinated through one isopropenyl unit and 0-bonded through two carbon atoms and is overall an eleven-electron donor."' The two cluster ally1 compounds [Ru,(p-H)(CO),(y,-ql:ql:q2-C,H,Ph)] and [Ru,(p3-H)(CO),,(p,-q1 q' q3:q3-C,H,Ph)] have been iso- lated from the reaction between [Ru,(CO),,] and isopropenylbenzene. Thermolysis of an octane solution ofthe former cluster with [Ru,(CO),,] gave the hexanuclear alkyne cluster [RU~C(CO),~(~,-~~ '1, q2-C,HPh)].'03 Addition of M'(CO) (M' = Ru or Fe) to nido-[Ru,(CO),(p,-PC(C0)3~')~~ gave the phosphinidene dusters [Ru,M'(CO),,(p-CO){p,-PC(CO)Bu'}2] tion.'O4 (M = Ru or Fe) uia a duster expansion reac- Gaseous nitric oxide reacts with various high nuclearity ruthenium clusters. The carbide cluster [N(PPh~)2]2CRuSC(CO),61 gave [N(PPh,),][Ru,C(CO),,(NO)] which reacted further with NO to give [RusC(CO),,(NOXNO,)l. Similarly the ally1 derivative [N(PPh,),][Ru,C(CO),,(C,H,)] reacts with NO to give [Ru,C(CO) ,(C,H,)(NO)] and [Ru,C(CO) ,(C,H,)(NO),(NO,)] clusters contain- ing NO and NO,. The NO was thought to form by disproportionation of two NO ligands co-ordinated to an unstabIe electron-rich intermediate a process accompanied by reduction in cluster nu~learity."~ A range of p-nitrene clusters has been prepared by therrnolysis or pyrolysis of nitrosyl clusters.Methylation of [Ru,(CO),,(NO)]-gave [RU~(F~-HXCU),,~~-CO~(~~-NH~{~~-~~-C(U)OM~~] and [Ru,(CO),,(NUMe)]. Hydrogenation of the latter cluster gave [Ru~(~-H)~(CO),(~,-NH)] and [Ru& H),(CO),(NOMe)J while in the presence of [Ru3(CO),,] [Ru,(p-H)(CO),,(p-CO),(p4-NH)(p-OMe)] and [Ru,(~-Hj3(C0),,(~,-NHX~r,-OMe)]were isolated. Thermolysis of [Ru,(CO),,(NOMe)] at 90 "Cgave [Ru,(CO) ,(p,-NXp-OMe)] and [RU~(C~),~(~L-CO),(~,-NH~~-~M~),~, and pyrolysis of [Ru,(p-II),(CO),(NOMe)] at 140"c gave CRU~(~~-H)~(C~),,~~-CU~~~~~-~~-NC(O)OM~~], [Ru6(CO) 414 S. Doherty Co)2(y,-NH)(p-oMe)(p-~2-NfH)C(0)0MeSI and [RU6(Co),6(pU-Co)2(p4-N~~~-OMeWp-NCO)]. The crystaI structures of these compounds were reported.The C carbide fragment in [Ru,(y,-C2)(p-PPh2)2(p-SMe)2(CO) couples with Me,SiC=CSiMe via a vinylidene intermediate to give [Ru (p4-CCCCH(SiMe3)}(p- SMe)(p,-SMe)(p-PPh2)2(CO)l,,] which has been converted into [Ru,fp,-CCCCH,)(p,-SMe)(p-SMe)(p-PPh,),(CO),53 via alkafi hydrolysis to afford the 0] first structurally characterised butatrienylidene cluster. The Ru open envelope con- formation of 53 converts into a spiked rhomboidal Ru core upon carbonylation to ,]54."' [Ru,(ps-CCCCH,)(p-SMe)2(p-PPh2)2(CO) Addition of PhGCR (R = H or Ph) to [Ru5(C0)1 ,(pcs-C,~p-SMe)2(~-pPIPh,),l gave LRu5(CO)]o{p5-H I H I /.c4 C-H -C\\ \/ S Me 53 54 CCC(Fh)C(R)}(p-SMe),(y-PPh,)J via facile carbon-carbon coupling between the exposed dicarbide and PhCECR.These two clusters (R = H or Ph) differ in the mode of co-ordination of the unsaturated fragment. In one (R = H) the metal atoms retain a pentagonal skeletal framework while the other (R = Ph) contains an extra Ru-Ru bond to give an envelope-type ruthenium atom framework."' Addition of CNBu' to [Ru,(CO) I(p5-C2)(p-SMe)2(p-PPh,),lgave two products [Ru,(CU) ,(CNBu'Xp,-C2)(p-SM e),(p-PP h2)J and [R us(CO) o( CNBu')(p5-C2)( p-SMe),( p-PPh )2 1 which can be interconverted by loss or addition of carbon monoxide. Overall the addition of CNBu' results in a flattening and expansion of the Ru metal atom framework and movement of the C unit into the plane of the Ru skeieton.log Thermolysis of [Ru,(CO),,] in ethanol gave the hydridoruthenium cluster anion [Ru~,H,(CO),~]~- while thermolysis in a methano1-water mixture gave [Ru 1H(C0)2,]3-.Both complexes have been crystallographically characterised. Treatment of [Ru,H(CO),,J-with [Ru~(CO)~~] in refluxing digIyme also gave [Ru,,H2(C0),,]2 in good yield. These transformations support a proposal for the ~ formation of high nuclearity ruthenium clusters via a build-up series involving [Ru6H(CO),,I- CRU,H,(CO),,~~- CR~,~H,(CO)~,I~- and C~u11H(CO)2,13-.110 Reaction of the mixed-rnetal cluster CCU2RU,(CO),6(p6-C)(NCMe),]with 1,5,9-trithiacyclododecane resulted in abstraction of the copper atoms to give [Cu(q3-[121 aneS&' -[12]aneS,)] [Ru,( CO),6(p6-C)].1 The osmiurn(iI1) complex [Os,Br,Cp*,] is a convenient starting material for the preparation of mono(pentamethy1cyclopentadienyl)osmiumcomplexes in the +2 oxi-dation state.For instance reaction with PR (R = Me or Ph) or cod gave [OsBr(PR,),Cp*] and [OsBr(cod)Cp*] respectively which upon treatment with NaBH gave the hydride [OsHL,Cp*] (L = PPh,). Surprisingly reaction of OrganometaIlic chemistry ofbi-and poly-nuclear complexes 415 [UsBr(PMe,),Cp*] with NaBH gave [OsH,(PMe,),Cp*] 'which deprotonated with MeLi to give [OSH(PM~,),C~*].~'~ The primary secondary and tertiary alkane thiolate osmium(vi) nitrido complexes [{ OS(NXCH,S~M~,),(~-SR)~),] have been pre- pared from [{ OsCl(N)(CH,SiMe,) 1 J and the corresponding alkali-metai thiolate (R = CH,CH, CMe, CHME, CH,CHMe or CH,Ph).Il3 Three novel osmium clusters were isolated from the reaction of ~{Os(CO),(SnMe,)},J under vastly different conditions.Firstly pyrolysis at 170 "C gave [(Os(CO),(SnMe,)),] containing a Os,Sn triangulated raft-like metal cluster framework whereas UV irradiation gave [Os,(CO)l,(SnMe,)4] characterised by a central Os,Sn rhomboidal framework linked to two outer Os,Sn triangles via the osmium atoms. Finally treatment of a solution of [{ Os(CO),(SnMe,)]J with Me,NO gave [0~,(CO),,(p,-0)~(SnMe,),l containing a central six-membered (OsOSn) ring. l4 Several mixed osmium-germanium clusters have been prepared and struc- turally characterised including [{Os(CO),(GeMe,)) J [Os,(CO) 1(GeMe2),] [Os,(CO),,(GeMe,),J and [Os,(CO),(GeMe,),]. The majority of these clusters have structural analogues among the binary carbonyis ofosmium.' l5 Three clusters were isolated from the reaction between [Os,(CO),,(NCMe),J and C,F,N=NNHC,F, two isomers of [OS,(CI-HXCO),,(NCMe)~(~-~'-C6F,"NC6F5)] and [OS,(CI-HXCO),~(~~~-C~F~NNNC,F~)~.The former clusters are thermodynamically unstable and readily convert into the latter uia 0s-0s bond formation,' l6 Reaction of RN=NNHR with [RU~(CO)~,] at 80 "C gave [Ru,(p-H)(CO),,(p-RNNNR)] (R = p-C,H,X; X = F C1 Br I or H) whereas the linear triosmiurn duster [Os,Cl(CO) l(q2-RNNNR)] was isolated from the reaction of RN=NNHR with [Os,(CO) ,(NCMe)] in CH,CI,. Both clusters contain the triazen- ide ligand the first bridging a Ru-Ru bond ina closed Ru cluster the latter chelating the termina1 osmium atom in a linear Os array.'17 The Schiff base NC,H,CH=NC6H,0C,,H3 reacts with tos,(~-HXCo),(lL,-Ccl)~, in the presence of DBU to give [~s,(p-~),(~~)~(p,-~CI)("C6~~~~16~30~~ and with [Os,(CO),,(NCMe),] to give the orthometaHated cluster [Os,(p-H)(CO),,(p,-NC,H,CH=NC6H,OCl,H,,)~-The low-energy visible absorptions of the former cluster display a marked negative solvatochromism whiIe the absorptions of the latter are relatively insensitive to the nature of the s5 56 56 416 S.Doherty The transformations of bicyclic trinitrogen heterocycles on triosmium clusters have been investigated. Indoline reacts with [Os,(CO),,(NCMe),f to give [Os,(p-H)(CO),o(p-~2-C,H,NH)] 55 which decarbonyIates to give a tautomeric mixture of [OS,(~-H)~(CO),(~,-~~-C~H~N)] 56; the first tautomer has a p-alkylidene-imino bonding mode the other a p-amido-aryl bonding mode.Continued thermolysis of these tautomers gave the dehydrogenated clusters [Os,(p-H),(CO),(p3-q2-C,H,NH)]. Direct analogues of 55 and 56 have also been isofated from the reaction between tetrahydroquinoline (thq) and [Os,(CO),,(NCMe),] in addition to [Os,(p-HXCO),,{p-q '-C,H ,,(CH,)CN)] the product of nucIeophilic attack of thq on MeCN. The dehydrogenation product [Us,(p-HXCO),(p-q2-C,H,N)] was obtained by thermolysis of [Os3(p-H),(CO),(p-rj2-CgH,N)].' l9 Treatment of a dichloromethane soIution of [Os,(CO),,(NCMe),] with [N(PPh,),][NO,] at 40 "C gave [N(PPh3),]fOs,(CO),,(p-q2-N02)] which was sub-sequently protonated to give neutral [Os,(p-H)(CO),,(p-q"-NO,)] in high yieId.l** Reaction of [Os,(CO),,(NCMe),] or [Os3(C0),,(NCMe)] with HD or a mixture of H,-D gave a11 three possible isotopomers.Similarly all three isotopomers were generated by mixing [Os,H,(CU),,] with D,. This behaviour is consistent with the formation of a highly fluxionaf 48-electron tetrahydrido-deu terido complex [OS~H,D~(CO),~~, followed by rapid reductive elimination to afford the observed isotopomer distribution.'" The red unsaturated cluster [O~,(CO)~(~i-dppm)(p~-q~-~-PhC~CPh)] reacts with phosphorus donor ligands L [L = PBu, PPh, PMe,Ph or P(OMe),] to afford the saturated 48-electron clusters [Us3fCO),(p-dppm)L(p,-q1:rj'-JI-PhC=CPh)] which exist in several isomeric forms. The P(OMe) derivative EOs,(CO),(P(OMe),)(p- dppm)(p,-q2- 1I-PhCrCPh)J undergoes facile decarbonylation to give the unsaturated cluster [Os,(CO),{ P(OMe)3}(p3-q':q2-I-PhCZPh)] containing a p3-q2-l-aI-kyne.Iz2Triosmiurn clusters have been introduced into bovine serum albumin (BSA) by acylation of the free amino functionality in [OS,(CO),,(~~-~~-L~ (L = succinimido-4-pentynoate) 23 pyMe 57 4-methylthiazole 59 Organometallic chemistry ofbi-and poly-nuclear complexes 417 Several triosmium and triruthenium clusters of 4-methyithiazole have been pre-pared and their reaction with 4-methylthiazole and PPh examined.The lightly sta bilised cIuster [Os,(CO),,(NCMe),] reacts with 4-methylthiazole to give [Os,(p-H)(CO)lo{p-C~NCMeCH$)] 57 which reacts with PPh to give the mono- and bis-phosphine substituted products [OS,(~H)(CO)~(~-C=NCM~=CHS HPPh,)] and [Os,(p-H)(CO),(p-C=NCMe=CH~XPPh3)J which exist in a number of isomeric forms.Further reaction of 57 with 4-methylthiazole gave [Os,(p-H),(CO),(p-C=NCMe=CHS)(p-C=CMeN=CHS)]58 and [Os,(p-H),(CO),(p-C=NCMe=CHS),I 59 each containing two methylthiazoIe Iigands; the first has one C,N-and one C,S-bound thiazole while the latter contains both C,N-co-ordinated. The reaction of [Ru,(CO),,] with one equivalent of 4-rnethylthiazole in the presence of a catalytic amount of benzophenone-sodium promoter gave [Ru,(p-H)(CO) J2,3-q2-CNCMe=CHS)] while two equivalents of thiazole gave fR~,(p-H),(CO)~(2,3-q~-CNCMeCHS),].'24 Several products have been isolated from the reaction of 1-hyd r oxypy r idine-2- thi one with [Os,(CO) (NCMe)] incl uding [Os,(p-H)(CO)I {p-{q2-Sc, '-SC,H 4N(O)}3 [OS~(P-HXCO)~O H4N(O))3 and [Os&-H)(CO) { P-4' ul 2-SC,H,N(O)} 3.Thermolysis of [Os,(p-H)(CO),,{ p-q'-SC,H,N(O)}] results in N-0 bond cleavage to give [0~,(p-HXC0)~(p,-pyS)]and [OS,(CO),(J~-OH)(,Y~-~~S)]. The complex [Os,(p-H)(CO),(p-q :q2-SC,H,N(0))] was shown to be an intermediate in this tran~formation.''~ Diphenylmercury reacts with [Os,(p-H)Br(CO),(p-q2-C=N(CH2) 31 to give [0s,( CO)I &-q -C,H 5)(p-q -C=N(CH )3} 1which read ily con-verts into [OS,(CO)~~~~-~~-C=N~CH~~,~(~-~': ?,r6-C6H,)]when heated at reflux in n-octane. 26 cu co The solid-state structures of both isomers of the a-x-vinyl cluster [Os,(p-H)(CO),(PPh,)(p-ql :$-CH=CH,)] have been determined by singIe-crystal X-ray crystaIlography.Proton NMR spectroscopic studies revealed slow equilibration of these isomers. Variable-temperature 13C NMR spectroscopic studies were used to probe the basis of the non-degenerate 0-Kinterchange of the vinyl group in 60; by analogy the line shape broadening associated with isomer 61 was tentativeIy proposed to be the result of a similar 0-Kexchange albeit with a higher energy barrier.127 Thermolysis of the activated tetraosmium hydride cluster [Os,(p-H),(CO),,(NCMe),J with cyclohexa-1,3-diene produced the known clusters [Os& H).2(CO)1,(rl2-c,H,)I> [0s&tp-H)3(C0)1 l(p-ql:112-C6H9)3 [os4(p-H),fCo) 1(q4-C6H8)I 7 [0S4(pL-H)2(C0)10('16-c6 Hfj)land [os4(co)9(q4-c6 8~~6-c,Hc5)1 together with the previously uncharacterised compounds [OS,(~-H)(CO),~(~~-~~ :q2:7'-and C6H8Xq3-C6H,)I [0s4(~~(-H)2~C0~1~(~6-C~H~C6H~)~ [Os,(p(-H)2(Co),3(q4-C,H,)].Dehydrogenation of the cyclohexyl moiety in [Os4(p-H)3(CO)1l(p-~l :q2-C6H,)] gave low yields of a compound formulated as the new cyclohexyne cluster [o~,(p-H)~(Co), L(p3-q q2:q1-C6H8)]. This work serves to demonstrate that the 418 S. Doherry cyclohexa- 1,3-diene moiety can undergo hydrogenation to form cyclohexyl and allylic complexes and isomerisation to yield cyclohexyne rings or dehydrogenation via C-H activation to afford $-benzene derivatives.I2* Treatment of the unsaturated cluster ~Os,H,(CO),,] with [Hg(CzCPh),] gave two new 0s-Hg containing clusters [Os(CO),( HgOs,(CO) &-q :q2-CH=CHPh)),] and [(Os3(CO),,(p-q1 :q'-CH-CHPh)),(p4-Hg)]; the latter contains two Os,Hg butterfly clusters linked through a wingtip atom.In refluxing thf [{Os,(CO),,(p-r,71 q2-CH=CHPh)),(p,-Hg)] undergoes a redistribution reaction with [Hg{ Mo(CO),Cp) J to afford [{OS,(CO),~(~-~~-CH=CHP~](~~-H~)(M(CO)~C~)]. The reaction of [Os,H,(CO),,] with RHgCZHgR (R = Me Et or Ph) also involved facile Hg-C bond cleavage and gave [{Us3(CO)lo(p-q1 :q2-CH=CH2))(p4-Hg){Us3(CO),,(p-H))] and [fOs,(CO),,(p-q':q2-CH=CH2)),(p,-Hg)].'29 Thermolysis of a solution of [Os,(CO) 1(PH3)] and [Os,(CO) ,(NCMe)] results in the formation of [Us,(tt-r-rXCO),,(~-PH,)J which contains two 0s clusters linked through a phosphido bridge together with the by-product phosphinidene cluster [Os,(p-H),(CO),,(p-PH)].Similarly reaction of [Os,(CO) ,(pH,)] with [Os,(CO),,(NCMe),] gave [Os,(p-H)(CO) I(NCMe)(p-PH,)]. Deprotonation of [Os,(p-H)(CO),,(p-PH2)] in the presence of [N(PPh,),]CI led to the formation of a mixture of anions [Os,(p-H)(CO) l(p-PH)]-and [Os,(CO),,(p-PH,)] -,which when heated at reflux in xylene decarbonylate to give [os,(co),&,-P)]-. The same interstitial phosphido cluster was obtained by heating [Os,(p-H)(CO),,(p-PH,)] in xylene while thermolysis of [Os,(p-H)(NCMeXCO) 1(p2-PH2)]gave [Os6(p-H)(p6-P)(CO) Braga el d.13 have examined the crystal structures of several organometallic complexes and found that a number contain M-H + 0 hydrogen-bonding interac- tions that involve the metal hydride and a carbonyl oxygen atom. The donor capacity of the M-H group appears to be similar to that of C-H and M-H --CO bonds are of comparable strength to those of C-H -.0 hydrogen bonds.'31 An alkoxide-alcohol mobile phase has been used to obtain electrospray mass spectra of various neutral metal carbonyl complexes including; [M,(CO),,] (M = Mn or Re) [RU~(CO)~,] [RU6(C)(Co)1 717 L1r4(c0)121 ~Ru,(c)(co)14(~6-C,H,Me)l and [0s4(c0),0-(q6-c,El6)].3 2 7 Cobalt rhodium and iridium Kerr and co-workers133 have developed a Me,NO-promoted Pauson-Khand reac-tion that uses gaseous ethylene both under atmospheric pressure and autoclave conditions; in the Iatter case reduced yields were obtained as the pressure approached 50atm. This methodology has been used to perform a key transformation in the synthesis of ( +)-taylorione.A range of alkynepentacarbonylcobalt complexes of (R)-(+)-Glyphos has been prepared both under thermal conditions and at room ternpera- ture using N-methylamine-N-oxide as promoter.' 34 Thermolysis of the diyne-bridged complexes ~(Co,(CO),),(PhC,Ph)] with brna gave [Co,(CO),(bma)(PhC,Ph)Co,(C~),] a thermally unstable product that readily loses [Co,(CO),] to give [Co,(CO),{p-)lr2 9' q':q'-Z-Ph,P(Ph)C=C(PhC,)C= Organometallic chemistry of bi-and poly-nuclear complexes 419 C(Ph,P)C(O)OC(O))Jt via P-C bond cleavage and functionalisation of the diyne with the transient bma and phosphido moieties. Therrnolysis of [Co,(CO),(bma)(PhC,Ph)-Co,(CO),] with excess bma gave additional binuclear complexes [Co,(CO),(bma)J and [CO,(CO),(~~~)(~-C=CPP~,C(O)O(CO))(~-PP~~)]~ the former containing two intact bma ligands the latter a phosphido bridge and a a-co-ordinated bma.Cyclic voltammetry data are consistent with low-potential redox couples associated with low lying x* orbitals of the bma ligand.135 Thermolysis of [CO,(CO)~(C~H,M~,-J,~,~)] with bma gave four products; [Co,(CO),(bma),~ [Co,(CO),(bma)(p-C=C(PPh,)C(O)OC(U))(p-PPh )] [CO3(CO) (p-q2:9 -P(Ph)C-C(PPh,)C(0)OC-(0)}] and [Co,( CO),( PPh ,){ p-q :q1-P(Ph)C=C(PPh2)C( O)OC( O)}]. The PPh ligand in the last cluster has been suggested to arise from P-C (phenyl) bond cleavage and transfer to a bridging p-PPh, itself generated nia P-C (maleic anhydride) cleav- age. Both trinuclear clusters are redox-active and show an irreversible one-electron oxidation and two one-electron reductions.The second one-electron reduction is associated with dissociation of the C=C bond of maleic anhydride from the cluster. EHMO calculations suggest that the instability associated with this reduction is due to unfavourabIe rnaleic anhydride x*-CO interactions present in the LUM0.'36 Protonation of [Co,(p3-qz:$-arene)Cp,] (arene = isopropylbenzene 1P-diethyl- benzene 1,2-diphenylethane or 1,l-diphenylethane) yields the hydrido clusters [Co3(p3-H)(p-q2:q2:q2-arene)Cp,J + whereas derivatives containing unsaturated groups (2-methyhtyrene p-methylstyrene or p-methoxystyrene) attached to the arene ring protonate at the fl-carbon of the side chain to give CCo3(~,-r,l-r12:2-4-r13:4-6-~3(R3)C,H4-1-C(CH,RiXR2)~Cp,]+(R' = R2 = H R3 = Me or OMe; R1= it2 = Me).The HOMO of the former cluster is localised largely on the cobalt atoms while the latter has a large LCAO amplitudeon the P-carbon atom of the styryl group which also carries a substantial negative charge. It appears that both charge and overlap control the preferred site of pr~tonation.'~~ Reaction of LiMe with [M(acac)Cp*] gave [M,(p-H)(p,-CH)Cp*,] (M = Ni or Co) the crystal structures of which are isornorphous. Both compounds contain metal atoms in oxidation state 7/3 with identicai co-ordination numbers but different electron counts. The compIex [Co,(p,-CH)(p-H)Cp*,] reacts with dihydrogen to give diamagnetic [Co3(p3-CHXp -H),Cp*,].'38 The trinucIear cluster [Co 3(C0)9(p-CR)] reacts with 1,3,5- tri thiane 2-methyl-2,4- dimethyl-2,4,S-trimethyl- 2-benzyl- and 2,4,6-tribenzyl-1,3,5-trithianeto give the trisubstituted products [Co3(CO),&,-CR)(p3-SCHR1SCHR2SCHR3)J.In a11 cases the trithiane ligand adopts a chair conformation and caps a triangular CO,face using all three sulfur atoms.In contrast the nine-membered crown thioether 1,4,7-trithiacyclononane affords [CO,(~-CO)(CO),(~~,-CR)(S(CH,CH,)~)] in which all three sulfur atoms of the ligand are co-ordinated to a single cobalt atom.139 The diphenylvinylphosphine-substituted clusters [Co3(CO),~,(Ph,PCH=CH,),(p3-CR)] (R = Me or CO,Me n = 1 or 2) have been prepared; in the case of [Co3(CO),(Ph,PCH=CH2)(p3-CR)] Ioss of carbon monoxide results in co-ordination of the vinyl moiety of the Ph,PCH=CH ligand to give [Co3(CO),(Ph,PCH=CH,)(p3-CR)].140 Condensation of [Co,(CO),(p-PPh,),] with PhC-CPh gave [Co(CO),( PPh,CPh=CPhCOC(O)CPhCPh >I,containing a diphenylphosphine vinyl substituted lactonyl ring q3-q1(P)-co-ordinated to cobalt.Insertion of CO into the lactonyl ring gave [Co(CO),(Ph,PCPh=CPhCC(O)OC(O)CPhCPh}]in which the 420 S. Doherty lactonyl group has been converted into a cyclic anhydride also q3-co-ordinated to cobalt. '41 Methylation of the heterobinuclear complex [IrRh(CO),(pdppm),] gave [lrR h(C H,)(CO),(p-dppm) j[CF,SO Jwith the methyl and two terminal carbonyls co-ordinated to the iridium. At ambient temperature [IrRh(CH,)(CO),(p-dppm),] [CF,SO,] reacts with H to liberate methane and give [IrRh(H)(p-H),(CO),(p- dppm),][CF,SO,] via the intermediate dihydride [IrRh(H),(CH,)(CO),(p-dpprn),l ECF,SO,].Reaction of [IrRh(CH3)(CO),(p-dppm)2~~CF3S03] with SO promoted metal- to-me tal me thy1 migration to give [IrR h(C(0)C H )(CO),(p-SO,)(pi-dppm),] [CF,SO,] presumably via an aIkyl-bridged complex; reaction with Bu'NC gave the irninoacyl [IrRh(CU),(p-Bu'N=CMe~p-dppm),][CF,SU,] via migratory inser-tion.', The low-temperature reaction between [Ir,H(CO),(p-CH,)(p-dppm),l [CF,SO,] and acetylene and phenylacetylene yields the alkyne- and vinyhdene- bridged complexes ~1r,(CH,)(CU),(p-HC=CH>(~~-dppm)23[CF,SO3] and [I r,(CH XCO) {p-C=C(H)Ph)(p-dppm),] [CF,SO,] respectively the former via the acetylide-hydride [Ir,(H)(CH3)(CO),(p-C~CH)(p-dppm)2][CF,S03]. Under similar conditions the mixed-metal dimer [IrRh(CH,~C0),(p-dppm),][CF3SO3] reacts with acetyIene to afford [TrRh(CH,)(CO)3(p-HC~CH)(~~-dpprn),j[CF,S03] and then fIrRh(CH,)(p-$ q2:~i-HCC(R)PPh,CH,PPh2}(p-dppm)2][CF,S0,]via P-C bond formation between the bridging dppm and the rhodium end of the alkyne.PhenylacetyIene reacts with [1rRh(CH,)(CO),(p-dppm),][CF3SO3]to give the un- stable methyl-hydrido-ace tylide [IrR h(H)(CH,)(CO),(p-C~CPh)(p-dpprn),l [CF,SO,] which reacts with excess phenylacetyIene to give [IrR h(H)(CO),(C=CPh)(p-CSP h)(p-dpprn) ][CF,SO 3. 43 Addition of excess [MgCI(aHyl)] to rac-[Rh,(nba),L][BF,] gave [Rh,(rj~~-aIlyl),L] [L = (Et,PCH,CH,)PhPCH,PhfCH,CfX,PEt2)] which exhibits slow non-selective hydroformylation activity contrasting with the dication [Rh,(nba),L][BF,] which is a highly active and regioselective hydroformylation cataIyst for alk-1-enes.In the presence of carbon monoxide the bis(acy1) product [Rh,(CUC,H,),(CU),L) is for-med presumably via an q'-allyl intermediate. Further reaction with H,-CO under pressure eliminated the unsaturated aldehyde to give the unsymmetrical binuclear complex [Rh,(p-CO)(CO),(q3:q1-L)].144 Addition of secondary silanes SIH,RR' to [(Rh(dippe)),(p-H),] gave [Rh2-(p-H)(p-q2-HSiRR')(dippe)2] containing an agostic Rh-Si-H interaction. Addition of one equivalent of carbon monoxide then led to loss of H and the formation of [Rh2(~-SiRR'Xlr-COXdippe),f.The hydride compIex [Rh,(p-H),(dippe),] is an effective catalyst precursor for the hydrosilylation of olefins by diphenylsilane to give Ph,SiEt and Ph,SiBuH at ambient temperature and pressure and a catalytic cycle has been proposed.Addition of one equivalent of HBF to the binuclear acetylide complexes [Rh,(C0)2(PCy,),(p-02CMe){p-q' :q2-C,C(OH)R,)] (R = Ph C0,Me or SiMe,) affords the p-m,o-allenylidene complexes [Rh,(CO),(PCy,),(p-O,CMe)(p-o,a-C=C=CR,)J[BF,] formally unsaturated and containing 30 valence electrons and a single Rh-Rh bond. A study of the bonding in the model p-c a-allenylidene complex [Rh,(CO),(PH3),(p-O2CMe)(p-~: a-C=C=CH,)] + successfuIly rationalises the 30 Val- ence electron count and reveals a net acceptor behaviour for the unsaturated ql-hydrocarbyl ligand. 146 Treatment of [NBu,j[Ir,Br(CO) with [Fe(zl5-P,C,Bu',)tl;l5-P,C3Bu',)1gave Organomatatlic chemistry of bi-and poty-nuclear cornpiexes 421 [Ir,(CO) {Fe(qS-P3C,Bu',)(q5-P2C3Bu',))] via co-ordination through one phos-phorus atom of the q5-P,C,Bu' ring.Similarly [NBu,l[Ir,Br(CO) ,] reacts with [FeCp(qS-P3C,Bu',)j to give [Ir4(CO)1 l(FeCp(qs-P,C,Bu',))j. Further treatment of [Ir4(CO) {FeCp($-P,C,Bu',)}] with [NBu,][Ir,Br(CU) in the presence of AgSbF gave [ir H(C0) o( FeCp(q Ir,(CU)But)))(CMe,C ,(HP3C-3 via metal la t ion of one of the C2P ring But groups.14' The monodentate 2-(diphenylphos-phino)pyridine ligands in [Ir,(CO),,(PPh,py),] are both co-ordinated at basal irid- ium atoms one axia1 and the other equatorial. Reaction with [Cu(NCMe),][BF,] and AgPF gave [Ir,M(CO),o~PPh,py),]Y (M =Cu Y = BF,; M = Ag Y = PF,) with M occupying an apical site of a trigonal bipyramidd IrM4 ~1uster.l~~ Reaction of the dinuclear Ir" compound [(Ir(p-SPr')Cp*},] with S gave the novel p-S9 nonasulfido- bridged complex [{ 'fr(p-SPr')Cp*),(p-Sg)] which reacts with NaBPh to give the paramagnetic h"l-Ir'v complex [{ Ir(p-SPri)Cp*)z(p-S2)-J-[BPh,] .49 8 Nickel Palladium and Platinum The new cyclopentadienyhickelamido complexes [(Ni(p-NHRH$-C,Me,R')),] (R = Ph p-tolyl 2,6-xyIyl or But;R' = Me or El) are dimeric in both solution and the solid state and undergo cis-trans isomerisation via Ni-N bond cleavage rotation of the amido group and reco-ordination of the arnido ligand. Reaction of [(Ni(p-NH(p-Tol)]Cp*,)},] with CO and CNBu' gave the insertion products [Ni(CO)(C(O)NH(p-Tol)}Cp*] and [Ni(CNBu')(CN( Bu')NH(p-Tol))Cp*] respectively.' 50 The haiide- and pseudo-halide-substituted vinylidene-bridged binuclear A-frame (X complexes ~Ni,X2(~~-C=CH,)~PR,CH,PR,)I = C1 Br or I R = Me; X :C1 Br NCS or OCN R = Ph) have been prepared and characterised.The bonding between the [Ni,CI,(PH,CH,PH,),]2 framework and the vinylidene fragment (C=CH2)2-was examined using EHMO calculations. The HOMO was found to be primarily meta1 based with significant da* character and b symmetry with a minor contribution from a vinylidene 7t orbital. The LUMO has substantia1 vinylidene x* character and b symmetry. This MO picture was used to account for the trends in the electronic absorption spectra of these corn pound^.'^' Addition of TePPr" to a solution of [Ni,(dppm),] gave [Ni,(p-Te),(p-dppm),] a highiy redox-active metal cluster with three reversible electrochemical couples.Treatment of [Ni,(p-Te),(p-dppm),I with one and two equivalents of [FeCp,][PF,] gave [Ni,(p-Te),(p-dppm),] and [Ni,(p-+ Te),(p-dppm),12 containing 49 and 48 electrons respectively; the monocation has been crystallographically characterised. 152 The diplatinum and platinum-palladium complexes [RPt(p-H)(p-dpprn)MR'1 [PF,] (R R' = Me Et or Ph; M = Pt or Pd) react with HCl with metal-carbon bond cleavage. Reaction of the dipalladium compIexes is more rapid than with the di- platinum counterparts while the reactivity of the mixed platinum-palladium com-plexes is complicated yielding products of Pd-C and Pt-C bond cjeavage. In the latter case Pt-C bond cleavage is followed by migration of the remaining R group from palladium to platinum.The phosphine ligands in [Pd,(p-PBu',)(PR,),]+ (PR = PCy,H or PMe,) can be substituted with CO or isoprene to give [Pd,(p-PBu',)(PR,),(CO)] and [Pd,(p- + 422 S. Doherty PBut2)(PR,),(p-qZ $-H,C=CHC(Me)=CH,)] + respectively. In the case of [Pd2(PBui2)(PCy,H),(CO)] bridge-terminal exchange of PCy,H for PBu' gave a + mixture of isomers with that containing a p2-PCy group and a terminal PBu*,H figand the most stable,'54 In refluxing toluene the dinuclear Pt" complex [(Pt(H)(PBu',H)(p-P3u')},] reacts with CO to give the 44-electron Pt',Pt" triangular complex [Pt,(H)(CO),(p-PBu'),] via reductive elimination of PBu',H.'~~ Oxidatively induced reductive elimination of PPh and the terminal phenyl group in [Pt,Ph(FPh,),(p-PPh,),] in the presence of I, gave the cationic cluster [Pt3(p- I)(PPh,),(p-PPh,),] +-While the conversion of tertiary phosphine ligands into phos- phido bridges via thermal P-C bond cleavage is known this is the first example of the cluster-mediated conversion of a M-C bond into a P-C bond of a tertiary phos- phine.15' A series of p-ally1 PdIPd' complexes has been prepared from [{Pd(p-Cl)[p-CH,C(R)CH,]),f and [Pt(C,H,)(PPh,),](R = H 1-Me I-Ph I-CO,Me 1-CI 2-C1 2-C02Me,2-CN or 2-S02Ph).Ally1 ligands containing withdrawing substituents were found to co-ordinate to the Pd'Pd' dimer more strongly than those with less electron- withdrawing substituents. A distinct preference for an anti configuration of the 1- substituted ally1 ligand was also noted.Ab initio MO/MP2 calcuIations performed on [Pd,(p-Br)(p-CH,CHCH,XPH,),] revealed donation from the ally1 non-bonding n-orbital to the da* orbital of the Pd,(p-Br)(PH,) fragment together with back donation from occupied da-da and dn-dn bonding Combinations to the ally1 E* orbital.' 57 A range of neutral anionic and cationic butadiene Pd'Pd' dimers has been prepared by the addition of buta-1,3-diene to a mixture of Pd" halide and Pdo complexes. Mixtures of [Pd,X,(PPh,),] (X = C1 or Br) and /Pd,(dba),] gave [Pd,X(p-X)(PPh ,)(,u-H C=CHCH=CHJ] of [Pd2( p-CI) (PPh J4] [PF6] and [Pd,(d ba)J gave [Pd,(p-CI)(PPh,),(p-H2C=CHCH=CH2)][PF6] and of [PdCI,(NCPh),] [PPh4]C1 and [Pd,(dba),] gave [PPh,][Pd2(p-Cl)Cl,(p2-H,C=CHCH=CH2)].158 The reaction of cis-[Pt(C,F,),(C_CR)~]'-or [Pt(CrCR),]*- with [(PdC1(q3-al- lyl)),] is a convenient route to the zwitterionic complexes Qcis-[(C,F,),Pt(p-q' q2-C=CR),Pd(~3-allyl)] (Q = PMePh, R = Ph; Q = NBu, R = Bu' or SiMe,) and [NBu,][(RC=C),Pt(p-q' :q2-C-CR)2Pd($-ally1)] (R = Ph Butor SiMe,).Addition of one equivalent of [Pt(C=CR)J2 -to [{ PdC1(q3-allyi)),] gave the trinucjear acetylide [(Pt(p-$ :$-C=CR),){ Pd(q3-alJyl)),]. 59 Addition of czs-[Pt(C,F,),(thf)J to a di-chloromethane solution of [NBu,J,[{ Pt(C,F,),(p-PPh,)),] gave [NBu,] IPt,(C,F,),(p-PPh2)21 62 which contains a PPh ligand that donates six-electrons through three Pt-P interactions and an $-co-ordinated phenyl group.160 Phenyl- vinylsulfide (PhSCH=CH,) reacts with [PdX(C,F,XNCMe)J (X = C1 Br or I) to give [{ Pd(p-XXPhSCHCH,C6F,))2] a three-membered dimeric (pheny1thio)aIkylpallada-cycle which sIowly isornerises to the heteroatorn complex [( Pd(p-Cl)(p-o-K-PhSCHCH,C,FS)>,] containing a bridging p-a-K(pheny1thio)alkyi fragment.De- composition of [(Pd(,u-CI)(PhSCHCH,C6F,)),1 in refluxing toIuene gave vinylpenta- fluorobenzene consistent with a 1,2-hydrogen shift and Pd-SR B-elimination. At room temperature in the presence of tetrahydrothiophene hydrolysis of the C-S bond gave C6F,CH,CH0 and PhSPdBr.16' Double directed lithiation of the diaryIplatinum complex cis-[Pt(PEt~),(C6H3(CH,NMe2),-3,5~~~ followed by transmetallation with [PtCl,- Organometallic chemistry of bi-and poly-nuclear complexes 423 62 (SEt,),] gave the trinuclear platinum complex ~is-[Pt(PEt,),(C,H,(c~~~Me~j~-~,5-PtCI),] which reductively eliminates [PtC1(2,6-(Me,NCH2),c6~~-C,H2(CH2NMe,),-2,S)PtCl] containing the forma1ly anionic biphenyl bridging ligaod [2,6-(Me,NCH,),C6H2C6H2(CH2 NMe j2-2,612 Palladium complexes of nitrogen-donor Iigands have been prepared with both symmetrical terminal and asymmetrical bridging isocyanide ligands.The palladium dimer [Pd2CI,(CNR),J reacts with bipy phen and dmphen to give [Pd,{CNR),L,] [PF J2 which contain square-planar paliadium atoms with terminal isocyanides and chelating N-N ligands (L). In contrast reaction with bquin and napy affords [ad,@-CNR),(bquin),][PF,] and [Pd(p-CNR),(napy),j[PF6J2 co-ordinated by bi- and mono-dentate nitrogen-donor ligands respectively; the former contains a symmetri- cally bridging isocyanide while the isocyanide in the latter asymmetrically bridges the two palladium atoms.'63 9 HeterometaIiics Addition of phenylazide and diazoacetate to [Cp,Zr(p-NBu')IrCp*] gave [Cp,Zr(p- NEh')(p-N,Ph)lrCp*] and [Cp2Zr(~c-NBu'){,u-N2C(H)C0,Et}IrCp*] respectiveIy.Thermolysis of the former at 75 "C resulted in loss of N and formation of the mixed amido complex [Cp,Zr(p-NBu'>(p-NPh)IrCp*]-Cross-over experiments showed that loss of N occurred without fragmentation a mechanism has been proposed for the transformation of an organoazide complex into a bridging imido complex. 164 63 The titanium complexes [Ti(sR),(~?'-c,H,pPh~~~~ have been used as metallo bi- and tri-denta te ligands to prepare a variety of heterometaliic complexes.Reaction with one equivalent of [Mo(CO),(nda)] gave [(CO)4Mo(p-Ph2PC,H,),Ti(SR)2] (R = Et or Ph) while three equivalents gave [((CO),MO),(~-P~,PC,H,)~T~(~-SR),] 63. Co-ordination of the thiolate ligands in [(CO),MO(~-P~,PC,H,),T~(SR)~]with [M(C,F,),(thf)J (M = Pd or Pt) gave the trinuclear complexes [(CO),Mo(p-Ph,PC6H,),Ti(p-SR),Pt(C,F,),]. 165 Terminal acetylenes react with the hetero- 424 S. Doherty bimetallic complex [Cp*NiM(CO),Cp”] (M = Mo or W Cp” = C,H or C,H,Me) to give [Cp*Ni(p-~3-~’-C(H)C(Ph)CO}M(CO)2Cp’’] containing a five-membered me- tallacycle. Upon protonation or methylation these metaIlacycIes rearrange to give the four-membered ring systems [Cp*Ni{p-y3 q’-C(H)C(Ph)COR)M(CO),Cp”]’ (R = H or Me).’66 Reaction of the tripodal amido-supported complex [MCI(MeSi[SiMe,N(C,H,Me-4)j3}](M = Ti Zr or Hf) with K[M’(CO),Cp] (M’ = Fe or Ru) gave [(MeSiCSi- MeN(C6H,Me-4)],)MM‘(CO),Cp] containing a highly polar unsupported metal-meta1 bond.Reaction with MeNC led to rapid insertion into the metal-metal bond to give [(MeSi[SiMe2N(C,H4Me-4)]3)M(p-q2-C=NMe)M’(CO)2Cp] 64 de-monstrating the mixed eIectrophiIic-nucleophilic character of their metal centres. 16’ Insertion of a heteroallene X=C=Y into the unsupported early-late heterobimetallic [HC(SiMe,NC,H,F-2)3ZrM(CO),Cpl (M = Fe or Ru) gave [(HC(SiMe,NC,H,F-2),}Zr(XCY)M(CO),Cp] (X = 0 or S Y = 0,S or NR).l6* Me 64 Photochemical Ioss of CO from [Cp”CpTa(CO)(p-PMe2)W(CO),I,generated from racemic [Ta(COXPMe,)Cp”Cp] and [W(CO),(thf)] yields [Cp”CpTa(p-CO)(p- PMe,)W(CO),] (Cp” = 1-Bu1-3,4-Me,C,H,).Addition of the optically active phos-phine neornen thyldiphenylpbosphine gave a pair of diastereoisomers which could be 69 separated by fractional crystallisati~n.~ The chiral anionic ligand [Mo(CO),(PPhH)] reacts with [PtCI,(L-L)] to give the neutral trimetallic mooo-phosphido-bridged compIexes fPt(p-PPhH),( Mo(CO),),(L-L)] (L-L = dppe dpae dppee) the first examples of heterometallic complexes that contain two chira1 primary phosphido bridges. ’* The mechanism of palladium-catalysed metal-carbon bond formation was inves- tigated by reacting [MI(CO)3(~5-l-F>h,P-2,4-Ph,C6H2)] (M = Mo or W) with stoichiometric amounts of Pdo to give [M(C0),(qs-l-Ph,P-2,4-Ph2C5H2)PdI(PPh3)] (oxidative addition step).Reaction with representative ethynyl tin derivatives then gave [M(CU)3(qS-1-Ph2P-2,4-Ph2CSH2)Pd(PFh3>(Cz CC6H4N0,-p)] a model for the transmetallation step in Pd-mediated C-C bond formation.’ 7’ The trinuclear 0x0-acetylide cluster [Cp*W(O)Re,(CO),(p-C=CPh)] reacts with thiophenol to give the fragmentation product [Cp*W(O)ReH(CU),(p-C-CPh)]. Oxidative decarbonylation of this complex in acetonitrile first gave [C~*W(O)R~H(CO),(~-CECP~)] and then the head-to-tail dimer [{ Cp*W(O)ReH-(C0)3(p-CZPh)},].172Refluxing toluene solutions of [Cp,W,Ru,(CO),,] slowly lose CO to give the oxo-carbido cluster [Cp*zW2(0)Ru,C(CO) If 65 which adopts a wingtip bridged butterfly arrangement of metal Reaction of the cationic carbyne complex [Mn(CO),(=CPh)Cp][BBr,] with 425 Organornetallicchemistry of bi-and poly-nuclear complexes [NEt4],[Fe2(C0),] gave the heteronuclear carbene-bridged complex [Mn(CO),Cp(p-C(C0Et)Ph) Fe(CO),] the tetranuclear dicar bene [{(CO),CpMn(=CPh)},Fe,(CO),1 and [Mn(CO),Cp].Possible mechanisms for the formation of these compounds have been described.' 74 Deprotonation of the spiked triangular cluster [Re2Pt(p-H),(CO),{ HRe(CO),)] with [NEtJOH affords the anion [Re,Pt(p-H),(CO),{ Re(CO),}] -which is thermally unstable in solution readily converting to [Re,Pt(p-H),(CO),( HRe,(CO),) J -_Alter-native synthetic routes to these dusters have been described.I7* 65 A number of reactions of siIylated dinuclear Fe-Pd acyl complexes have been reported.Addition of CNR to the heterobimetallic complex [(CO),Fe(p-Si(OMe),(OMe)>(p-dppm)Pt{C(O)Me}] results in cleavage of the Pt-OMe bond to give [(CO),fSi(OMe),}Fe(p-dppm)Pt(C(O)Me}(CNR)f whereas dmad inserts into the Pd-acyl bond to give the alkenyI complex [(CO),Fe{ p-Si(OMe),(OMe)}(p- dppm)Pt{( MeO,C)C=C(CO,Me)C(O)Me) J isolated as its isocyanide adduct [(CO) {Si(0Me) } Fe(p-d ppm)Pt ((MeO,C)C=C( CO Me)C(O)Me>(CN R)] Excess PhCgCH and Bu'GCH react with [(C0)3Fe{p-Si(OMe),(OMe))(p-dppm)Pt{ C(U)Me)] to give the vinylidene-bridged complex [(CO),Fe(p-C=CHR)(p-dppm)Pt(CO)] (R=Ph or Bu'). The additional carbony1 ligand was suggested to arise from the acyl ligand while the fate of the siIyl group was less certain.The 11-Si-0 bridge in [(C0)3Fe{p-Si(OMe),(OMe)}(p-dppm)PtPh] opens under a purge of carbon mon- oxide to give [(CO),{Si(OMe),}Fe(p-dppm)PtPh(CO)] which rather surprisingly does not undergo insertion into the Pt-phenyl bond but loses CO reversibly under Facile methoxy-dimethylamino exchange during the reaction of [Fe(CO),{ P(OMe),)] with HSi(NMe,) gave the amine-stabiIised iron-silylene com-plex [Fe(CO) { P(0MeXN Me,),} {=Si(O Me) NH Me,)]. Deprotonation with excess KH and reaction with [CuCI(PPh,)] gave the heterobimetallic ~(CO),{P(OMe~NMe2)2)Fe(C1-Si(OMe),(NMe,)~Cu(PFh3)~.177 Differences in the reactivity of mer-[FeH(CUj,(Si(OMej,~{ Ph,PCH,C(O)Ph}] and mer-[FeH(CO),{ Si(UMe),}{ Ph,PCH,C(O)NPh,)] have been observed. The former con- taining a ql-(PI-co-ordinated diphenylphosphino ketone reacts with SnX,Bu (X = C1 Br or O,CMe) in the presence of NEt, to afford [(CO),((MeO),Si)Fe{p- Ph,PCH=C(O)Ph}SnBu,] uia spontaneous deprotonation of the functional phos- phine to give an enolate Iigand.In contrast the N,N-diphenyl-2-diphenylphos-phinoacetaminde complex mer-[FeH(CU),(Si(OMe),} { Ph,PCH,C(O)NPh,}] reacts with SnX2Bu (X = C1 or B) but not with [S~(O,CM~),BU,].'~* 17' reduced pressure. 426 S. Doherty Substitution of the bridging carbonyi in [Fe,(CO),(~~-CO)(p-Ph,PXPPh,)Pt(PR,)] (R = Ph or Tol X = CH or NH) by the cycIic amides ENBu'SiMe,NBu' (E = Ge or Sn) gave [Fe(CO),(p-ENBu'SiMe,NBu')(p-Ph,XPPh2)Pt(PR3)J In the presence of excess GeNBu'SiMe,NBu' [Fe(CO),(p-GeNBu'SiMe2N3u')(p-Ph2XPPh2)Pt(PR3)] and the tetranuclear complex [Fe(CO),(~-GeNBu'SiMe,NBu'~p-Ph,XPPh,)Pt(p-GeNBu'SiMe,NBu')] are in equilibrium.In contrast reaction of [Fe(CO),{p-Si(OMe),(OMe))(p-dpprn)M(Me)] (M = Pd or Pt) with ENBu'SiMe,NBu' leads to the terminal base-stabilised complexes mer-[Fe(CO),(p-Si(OMe),(OMe)}(ENBu'SiMe,NBu')(p-dppm)M( Me)]. 79 OctacarbonyIdiosmacycIobutane reacts with [Pt(C,H,)(PPhJ,] to give [Os,Pt(CO),(PPh,),] which exists in solution as three interconverting isomers. Two distinct mechanisms for their interconversion have been described (i) a low energy restricted trigonal twist motion at a phosphine-substituted osmium centre (AH* = 10.1kcal moI-l)and (iij an olefin-type rotation of the Os fragment about the platinum centre (AH* = 10.7kcal mol-i)-180 The reaction between [SiCl( Fe(CU),Cpf ,I TIPF and OP(NMe,) gave [Bi(OP(NMe,)3)2{Fe(CU)2Cp),lCPF,I which has a Bi centre most aptly described as four-co-ordinate with an equatorially vacant trigonal-bipyramidal geometry; the [Fe(CO),Cp] fragments occupy equatorial sites.The structure was compared with that of [BiCI,{Fe(CO),Cp),]- and those of the aryl bismuth compounds [BiR,- L,] The diethoxycyclopropenylidene complex [M(CO),(C,(UEt),) J (M = Mo or W) reacts with K[Fe(CO),Cp] replacing only one ethoxide to give [M(CO) (C,(OEt)[Fe(CO),Cpj >I.A zwi tterionic cyclopropenium-type structure was favoured based on the available structural data.182 Reaction of [W(CO),(CrCC=CH)Cp] with one equivalent of [Ru,(CO),,(NCMe),] gave the atkyne-bridged cluster [Ru,(CO),(p-CO)((,u-q' yll y12-HC=CC=C)W(CO),Cp)] which reacts with a further equivalent of the tungsten-acetylide complex to give [RU,(CU)~ W(CO),Cp~p3-Cp(CO),WC,cC,(0)] C,CH(O))] via co-dimerisation of two CO Iigands and two molecules of the diyne fragment.' 83 Several complexes and clusters containing the 1,3-diyne unit have been prepared and the unco-ordinated carbonxarbon triple bond used to prepare a range of novel mixed-metal cIusters.Such complexes include [Mo,(p-q2-Me,SiC,C=CSiMe3XCO),Cp2] [Pt(q2-Me,SiC,C~CSiMe,XPPh3),] [Ru,(p-q*-SiMe,C CKSiMe,)(p-CO)(CO),] [(Mo2rCu)4Cp,} {co2(co)6)(~"f12' p-q2-SiMe,C,C,SiMe,)] [Re,(y-H)((y-q :q2;p-q2-C2C,SiMe,)[Co2(p-dpprn)(CO)4]) (CO)81 CRe,(tc-H){(~,-~':112;~-vlZ-C,C,SiMe,)CCo~(Co)~)~-dppm)j)(CO),I and ~CO,(~-~~-RC,C~C~W(CO),C~~)(~-~~~~~CO),~ (R = H or SiMe,).IB4 An electrochemical analysis of [Os,(CO),,(CpFe(C,H,C_CH))] and the bisfferro- ceny ])-subs ti tuted cluster [Os3(C0)9{ (CpFeC H,CEC H),CO}] showed reversible one-electron oxidations associated with the ferrocenyl moiety; the two well separated redox processes of the latter cluster are consistent with inequivalent ferrocenyl sites.18' The activated cluster [0s3(C0),,(NCMe),J reacts with [Fe(C,H,CzCSiMe,),] to yield the p-q2-alkyne bridged cluster [Os,(C0),,(p3-q2-Fe[C5H4(C2SiMe3)]2)] which readily decarbonylates to afford the butadiendiyl clusters [Os,(COj,(p3-q4- Fe[C,H,(C,SiMe,)],)] the product of carbon-carbon bond formation and a 1,2-SiMe shift. A cyclic voltammetric study of the last cluster revealed a single reversible one-electron oxidation associated with the ferrocenyl group and an irreversible two- 427 OrganornetaIIicchemistry of bi-and poly-nuclear complexes electron reduction assigned to the osmium fragment.Reaction of [N(PPh,),] [Re(C0)J with [N(PPh J2] [Fe,( CO) { CCOC(0)CH,} 1gave the pace tylide cluster [Fe,(CO),(CzCRe(CO),)1 together with the by-product [N(PPh3),I2 [Fe,(CU),CCU]. Addition of acyl chloride to the reaction mixture appears to reduce the concentration of the ketenylidene by-product presumably by regenerating the acyIketenylidene precursor [Fe,(CO),( CCOC(O)CH,) 1.'87 Ionic coupling of [Ru(NCMe),Cp*] and [OS,(CO),,]~-gave the heptanuclear cluster [OS~R~,(CO),~C~*,] best described as either a bicapped trigonal bipyramid with the two RuCp* units at the apical sites or as four Os,Ru tetrahedra sharing four common faces.' 88 Condensation of [Ru3(CO),,] with [Mo,(CO),(p-HC=CR'))Cp,] (R' = H Me Ph or C0,Me) affords the p-vinylidene clusters [Mo2Ru(p3-C=CHR')(CO),Cp,] together with low yieIds of [Mo,Ru,(p,-CH)(p,-CR1)(C0),2Cp,].189 Condensation of [RU,(CO)~ 2I with [Mo2(p3-S),(p-SR),Cp,3 gave the tetrahedral cluster [Mo,Ru,(p3-S),(p-SR),(CU),Cp2 3.' 90 The pentaosmium carbide cluster [Os,C(CO),,j has been used to prepare new mixed-metal 0s-miurn-palIadium clusters.Its reaction with [Pd(PPh,),] in dichloromethane gave [Os,PdC(CO),,(p-CO),( PPh,),] whereas its reaction with [PdCl,(PPh,),] in reflux- ing chloroform gave [Os,PdC(CO),,(p-Cl),(PPh,)]. The first of these clusters consists of a square-based pyramid of osmium atoms with the [Pd(PPh,),] group capping a triangular face; the latter cluster consists of a butterfly arrangement of four osmium atoms with the palladium capping a wingtip itself bonded to the remaining [Os(CO),(PPh,)] moiety.19' Reaction of [Mo,(CO),Cp,] with [Ru,(CO),,] affords moderate yields of the carbido-oxo cluster [MO,RU,(~~-C)(~-O)(CO)~~C~,], which crystaliises with two distinctly different molecules in the asymmetric unit. These clusters obey the 18-electron rule but contain two electrons less than that predicted for a eloso-octahedral skeletaI framework.192 Reaction of [Pt,Ru,(p-H),(CO), 3,-with [Ir(NCMe),Cp*]' + and HgI gave [PtJR~6(p3-H)2(C0)2 ,(p3-IrCp*)] and [Pt,Ru,(p,-H),(CO) '(p3-Hg1)] respectively.Both clusters comprise an alternating Ru,Pt,Ru layered metal-atom framework with the heterornetal atom p3-triply bridg- ing one Ru,Pt trianguIar face. Both cornpiexes comply with the expected electron count for monocapped face shared bioctahedra (I32 cluster valence electron^).'^^ In situ IR and NMR spectroscopic studies showed that the heterobimetallic com- plex [($-C,R,)Ru(p-CO)~(p-L-L)RhX~] (R = H or Me; L-L = dppm dppe or dppee; X = C1 or I) readily and reversibIy dissociates to give [Ru(CO),(q'-L-L)Cp] + and [RhX,(CO),]- in the presence of CU. In situ 31PNMR spectroscopic studies of the reaction provided evidence for an intermediate species [Cp(CO) Ru(p dppm)RhCl,(CO)]. lg4 A mixture of [{ Ircl(cod)),] and the chiral oxazolylferrocene-phosphine ligand dipof catalyses the hydrosilyfation of simpIe ketones to give the corresponding see-alcohols after acid hydroIysis.The first step of the reaction presumably involves exchange of cod with dipof to give a heterobimetallic Fe-Ir complex."' Both isomers [Ir(q4-2,5-drnt)Cp*j and [Ir(C,S-2,5-dmt)Cp*] react with [Ru3(CO),,] to afford [{Ir(q4-2,4-dmt)Cp*}Ru,(CO) J while two products were isolated from the reaction with [Re,(CO),,] the first [fIr(q4-2,4-dmt)Cp*)Re,(C0)9] containing an S-co-ordinated thiophene in an equatoriat position the other [(Ir[q4-SC,H,MeC(=O)Me]Cp*)Re,(CO)9]7 resulting from ring opening to give an S-co-ordinated acyl-thiolate. A similar compound [(Ir[q4-SC,H2MeC(=O)Me] 428 s.Doherty Cp*)Mn,(CO),] was isolated from the reaction with [Mn,(CO),,j.196 Cobaltocene readily desuIfurises the 2,5-dmt figand in [Ir(q4-2,5-dm t)Cp*j and [Ir(C,S-2,5-dmt)Cp*J to afford [Cp*IrC( Me)=CHC(H)C(Me)CoCp] containing a planar iridacyc- lopeotadiene q4-co-ordinated to a CpCo fragment.Reaction of the ring-opened iridathiabenzene [Ir(C,S-2,5-dm t)Cp*] with [CoJCO) ,] and [Co,( CO),] gave C{ Jr(q4-2,5-dm t)Cp*)Co,(CO) J in which the 2,5-dmt ligand is co-ordinated through sulfur to Co and y4-co-ordinated to the IrCp* fragment. At high reaction temperatures the desulfurised linear tetranuclear cIuster [{ Cp*Ir[C(MebCHCH=C(Me)](p-CO),Co),] is formed in high yieIds. In contrast [Co,(CO),(y6-C,H,Me,)l reacts with [Ir(C,S-2,5-dmt)Cp*] to give the q6-iridathiabenzene cluster [{ys-Cp*Zr(C,S-2,5-dmt))Co,(CO),] together with low yields of another isomer of [(Ir(q4-2,5-dmt)Cp*)Co,(CO) 3.* Refluxing [WIr,(CO) &p,] with triphenylarnine gave low yields of [W31r4(p-WXCU),,CpJ with a metal-atom framework based on a seven-vertex bicapped trig- onal bipyramid.”’ The heterobimetalhc complex [WCo(CO),Cp] has been used as a single-source precursor for the preparation of polycrystalline WCoO on Si(100).200 References 1 J. Chem. Soc. Dcrlton Tmns.,1996 issue 5 555-800. 2 M. H.Chisholm,J. Chrrn.Soc. Dalton Trans. 1996 1781. 3 C. Bianchini and A. Meil J.Chem.Soc. Dollon Tram 1996 801. 4 S. Sun. C.A. Dullaghan and D.A. Sweigart J. Chem. Soc. Dalton Trans. 1996,4493. 5 J. Fornies and E. Lalinde J. Chem. 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Sauvageot 0.BIacque M. M. Kubicki S. Juge and C. Moise Orpnomrtaflics,1996,15,2399. 170 A. J. Deeming and S. Doherty Polyhedron 1996.15 1175. 171 P. Cianfriglia V. Narducci C. L. Steroz E. Viola G. BocelIi and T.A. Kodenkandath Organomerollics I996,15,5220. 172 H. L. Wu G. L. Lu Y. Chi L.J. Farrugia S. M. Ferry and G.H. Lee inorg. Chem. 1996,35,6015. 173 C.J. Su P.C. Su T. Chi S.M. Peng and G. H. Lee J. Am. Chem.Soc. 1996,118,3289. 174 Y.Yu,J. Chen J. Chen and P. Zheng J. Chem.Soc. Dalton Trans. 1996 1443. 175 M. Bergarno T. BeringheIli G. D'Alfonso G. Ciani M. Moret and A. Sironi Organometallics 1996 15 1637. 176 M. Knorr C. Strohmann and P. Braunstein Organumrtallics 1996,15,5653. 177 P. Braunstein C. Stern C. Strohmann and N. Tong Chem. Commun. 1996,2237. 178 P. Braunstein C. Charles A. Tiripicchio and F. Ugozzoli J. Chem.Soc. Dalton Trans. 1996,4365. 179 M. Knorr E. HalIauer U. Huch and M. Veith and P. Braunstein Organumetallics 1996,15,3868. 180 3. Cooke R. E. D. McClung J. Takats and R.D. Rogers Orgonumrtallics 1996,15,4459. 181 C.J. Carmault L. J. Farrugia and N.C. Norman J. Chem. Soc. Dafron Trans.,1996,443. 182 M.S.Morton J. P.Seiegue and A. Carrillo Oi-~qunomrtallics,1996 15,4664. 183 M. I. Bruce B. W. Skelton A. H. White and N. N. Zaitseva J. Chan.Soc. Dalron Trans. 1996,3151. 184 M. I. Bruce P. J. Low A. Werth B. W. Skelton and A. H. White J. Chrm.Soc. Dalron Trans. 1996 1551. 185 S.L. Ingharn B. F.G Johnson P. R. Raithby K. J. Taylor and L. J. Yellowlees J. Chem. Soc. Dalton Trans. 1996,352I. 186 L. P. Clarke J. E. Davies P. R. Raithby and G. P. Shieids J. Chem. Soc. Dalron Trans. 1996,4147. 187 D. M. Norton R. W. Eveland J. C. Hutchinson C. Stern and D. F. Shriver Urgunomerdlics I996,15,3916. 188 J. Lewis C.A. Morewood P.R. Raithby M. Carmen and A. de Arellono J. Chem. Soc. Dalron Trans. 1996,4493. 432 S. Doherty 189 H. Adams L. J. Gill and M.J.Morris Organomerallics 1996 15,4182. 190 H. Adarns N. A. Bailey S. R. Gay L. J. Gill T. Hamilton and M. J. Morris J. Chem. Soc. Dalton Trans. 1995,2403. 191 J. W.S. Hui and W.T. Wong J. Chem. Soc, DaIron Trans. 1996,2887. 192 H. Adams L. J. Gill and M. J. Morris Orgunomrrallics 1996,15,464. 193 R. D. Adams T.S. Barnard J. E. Cortopussi and L. Zhang Organometallics 1996,151,2664. 194 P.S.Bearman A. K. Smith N. C. Tong and R. Whyman Chem. Commwn. 1996,2061. 195 Y. Nishibayashi K. Segawa H. Takada K. Ohe and S. Uemura Chem. Cornmun. 1996,847. 196 J. Chen V. G. Young jun. and R.J. Angelici Organometallics 1996 15,2727. 197 J. Chen L. M. Daniels and R. J. Angelici OrganomeraIlics 1996,15 1223. 198 J. Chen V.G. Young jun. and R.I.AngeIici Organumetdlics 1996,15 1414.199 S. M. Waterrnan M. G. Humphrey and D.C. R. Hockless Organometallics 1996 IS 1745. 200 S. G. Shyu J. S. Wu S. H. Chuang K. M. Chi and Y. S. Sung Chem. Commun. 1996,2239.

ISSN:0260-1818    

DOI:10.1039/IC9969300395

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

22 Organometallic chemistry of bi- and poly-nuclear complexes By S. DOHERTY Department of Chemistry Bedson Building University of Newcastle-upon-Tyne Newcastle-upon-Tyne NE1 7RU UK 1 Introduction This review discusses developments in the organometallic chemistry of bi- and polynuclear complexes published during 1996. An entire issue (No. 5) of Dalton Trans. dedicated to the first Dalton Discussion on Metal Cluster Chemistry held in January 1996 contains a number of informative articles of interest to organometallic cluster and solid-state chemists.1 In addition Dalton Trans. contains several highlight articles either dedicated to polynuclear organometallics or that at least incorporate some aspects of this field and include ditungsten hexalkoxides (templates for organometallic chemistry and catalysis),2 hydrogenation and hydrogenolysis of thiophenic molecules catalysed by soluble metal complexes,3 the versatile chemistry of arenemanganese carbonyl complexes,4 synthesis structures and reactivity of homo- and hetero-polynuclear complexes of platinum bearing C–– – CR groups as unique bridging ligands5 and finally arene cluster compounds.6 A feature article on heterogeneous catalysis of carbon–carbon bond formation that also focused on the reactivity of dirhodium –methylene complexes as homogeneous models appeared early in 1996.7 Other relevant review articles include the organometallic chemistry of CO 2 ,8 openshell organometallics as a bridge between Werner and low-valent organometallic complexes–the e§ect of spin state on stability reactivity and structure,9 new bonding modes fluxional behaviour and reactivity in dinuclear complexes bridged by fourelectron donor unsaturated hydrocarbons10 and the synthesis of ruthenium and osmium carbonyl clusters with unsaturated rings.11 2 Titanium zirconium and hafnium Insertion of carbon dioxide into the Ti–H bond of [MTi(k-H)CpN2 (k-g5 g5-C 10 H 8 )] gave [MTi[k-g1-(O)CHO]CpN2 (k-g5 g5-C 10 H 8 )] containing two monodentate bridging formate ligands.12 The bis(alkynyl) complex [Ti(C–– – CSiMe 3 ) 2 (g5-C 5 H 4 SiMe 3 ) 2 ] reacts with [AuR(SMe 2 )] to form [(g-C 5 H 4 SiMe 3 ) 2 Ti(C–– – CSiMe 3 ) 2 Au(g1-R)] containing a gold atom with an unusual trigonal-planar co-ordination environment.13 The bridging but-2-yne ligand in [(ZrCp 2 ) 2 (k-CH 3 CCCH 3 )(k-C–– – CCH 3 )][BF 4 ] is dis- Royal Society of Chemistry–Annual Reports–Book A 395 placed by alkylisocyanides to a§ord the novel cationic acetylide–isocyanide bridged dimer [(Cp 2 Zr) 2 (k-g1 g2-C–– – CCH 3 )(k-g1 g2-C––NR)][BF 4 ].14 The zirconium complex [ZrMe 2MC 5 Me 4 SiMe 2 (NBu5)N] containing a monocyclopentadienyl–amido ligand reacts with two equivalents of CO 2 to give [MZr(g2-O 2 CMe)(k- O 2 CMe)[(C 5 Me 4 )SiMe 2 (NBu5)]N2 ] which exists as a dimer in the solid state but predominantly as a monomer with chelating acetates in solution.Reaction with a third equivalent of CO 2 results in elimination of Bu5NCO to give [MZr(g2-O 2 CMe)(k- O 2 CMe)[(C 5 Me 4 )SiMe 2 O]N2 ] containing a novel bridging ansa-tetramethylcyclopentadienyl –silyloxy ligand.15 Fluorination of [Zr(CH 2 Ph) 3 Cp*] by 1.5 equivalents of trimethylpyridine·bis(hydrofluoride) [tmpy·(HF) 2 ] gave [MF 3 Cp*] in quantitative yield whereas excess tmpy·(HF) 2 gave [(MF 2 Cp*) 2 (k-F) 3 ] [Htmpy] (M\Zr or Hf).16 A new approach to the synthesis of constrained-geometry catalyst precursors involves reaction of the Si–Cl bond of chloromethylsilyl-substituted cyclopentadienyl compounds.In one instance LiNHCH(Me)Ph reacted with [ZrCl 3 (g5- C 5 H 4 SiMe 2 Cl)] in the presence of NEt 3 to give [ZrCl 3 (NEt 3 )Mg5- C 5 H 4 SiMe 2 NH(CHMe)PhN]. In the presence of water the binuclear silyloxidebridged zirconium derivative [MZr[g5 g1-C 5 H 4 SiMe 2 (k-O)]Cl 2 [H 2 N(CHMe)Ph]N2 ] was isolated.17 Several new binuclear ansa-metallocene complexes of zirconium and hafnium have been prepared and their catalytic activity for the polymerisation of ethene and propene evaluated.For instance Li 2 [Me 2 C(C 5 H 4 )C 9 H 6 )] reacts with two equivalents of [ZrCl 3 (dme)Cp] to give the ansa-bridged binuclear compounds [ZrCl 2 CpMk-C 5 H 4 (CMe 2 )C 9 H 6NZrCl 2 Cp]. Heterobinuclear ansa-metallocenes of the type [MCl 2 CpMk-C 5 H 4 (CMe 2 )C 9 H 6NMCl 2 Cp] have also been prepared from mononuclear [MClCpM(C 5 H 9 )(CMe 2 )(g5-C 9 H 6 )N] (M\Zr or Hf) and [M*Cl 3 (dme)Cp] (M*\Zr or Hf).18 The terminal phosphide [Zr(Me)Cp 2 (PHC 6 H 2 Me 3 -2,4,6)] generated in situ from the reaction of two equivalents of [ZrMe 2 Cp 2 ] with PH 2 (C 6 H 2 Me 3 -2,4,6) reacted with additional [ZrMe 2 Cp 2 ] to give the binuclear complex [(ZrMeCp 2 ) 2 (k- PC 6 H 2 Me 3 -2,4,6)]. The corresponding reaction of [ZrMe 2 Cp 2 ] with one equivalent of PH 2 (C 6 H 2 Me 3 -2,4,6) gave [(ZrCp 2 )MCpZr(PHC 6 H 2 Me 3 -2,4,6)N(k-PC 6 H 2 Me 3 - 2,4,6)(k-g1 g2-C 5 H 4 )] which arises from activation of a cyclopentadienyl C–H bond.19 3 Vanadium niobium and tantalum The 1,3-dimetallacyclobutanes [(Me 3 SiCH 2 ) 2 MMk-C(SiMe 3 )N2 M(CH 2 SiMe 3 ) 2 ] (M\Nb or Ta) react with carbazoles [N@H\carbazole (Hcb) tetrahydrocarbazole and 3-tert-butylcarbazole to give [(N@) 2 MMk-CSiMe 3 )N2 M(N@) 2 ].Addition of 2,6- dimethylphenylisocyanide(XyNC) to [cb 2 MMk-C(SiMe 3 )N2 Mcb 2 ] resulted in isocyanide –alkylidyne coupling to a§ord [cb 2 MMk-C(SiMe 3 )N(k-XyNCCSiMe 3 )Mcb 2 ] which contains a novel bridging amido–alkyne ligand.20 Solutions of [NbH 2 (SiMe 3 ) 2 Cp] isolated from the reaction between an excess of HSiMe 3 and [NbH 3 Cp 2 ] are unstable and slowly decompose at room temperature to give [MNbHCp(p g5-C 5 H 4 )N2 ] via reductive elimination of trimethylsilane.21 396 S.Doherty Mo Mo OC OC CO CO C C C C H H H H Mo Mo OC OC CO CO C C C C H H H H Nu Mo Mo OC OC CO CO C C C C H H H H Nu Nu 3 2 1 Nu– Nu– + + + Scheme 1 4 Chromium molybdenum and tungsten The crystal structures of [Cr 2 Cl(ind) 3 ] and [Cr(ind) 2 ] have been determined. Both are dimeric in the solid state containing g5- and g3-indenyl groups; toluene solutions of [Cr(ind) 2 ] catalyse the polymerisation of ethylene.22 An improved synthesis of [Cr 2 (CO) 6 fv] has been reported and its structure determined by single-crystal X-ray crystallography. Reduction of [Cr 2 (CO) 6 fv] with Na–Hg amalgam or LiBEt 3 H produced [Cr 2 (CO) 6 fv]2~ which when treated with CF 3 CO 2 H gave [Cr 2 H 2 (CO) 6 fv] capable of dihydrogenation of conjugated dienes.23 The reactions of the pentamethylcyclopentadienylchromium( III) complex [MCrBr(k-Br)Cp*N2 ] with alkyl and aryl amides LiNHR depend markedly on the reactants and reaction conditions.For instance R\C 6 H 3 Pr* 2 -2,6 gave [Cp*Cr(k-NR)(k-Br)CrCp*] and [MCr(k-NR)Cp*N2 ] whereas R\C 6 H 11 gave [MCrBr(k-NC 6 H 11 )Cp*N2 ] and R\C 6 H 2 Bu5 3 -2,4,6 gave [MCr(k- NR)Cp*N2 ]. In the presence of PhC–– – CPh as a trapping agent reaction of [MCrBr(k- Br)Cp*N2 ] with LiNHR (R\C 6 H 3 Pr* 2 -2,6) gave [Cr(NR)(PhC–– – CPh)Cp*] while 2,6- xylyl isocyanide gave a monomer containing an unusual bis(amido)-substituted 2,3- dixylyl aminoquinoline ligand via the coupling of two isocyanides and an amido group with the loss of one C 6 H 3 Pr* 2 moiety.24 Photolysis of [NbI(CO) 2 (PMe 2 Ph) 2 (g2-dppa)] prepared from [NbI(CO) 3 (PMe 2 Ph) 3 ] and dppa with [Mo(CO) 6 ] gave [MMo(CO) 4N2 (k- Ph 2 PC–– CHCH–– CPPh 2 )] in which two [Mo(CO) 4 ] units are linked by a buta-1,3- diene backbone.The butadiene ligand was suggested to form via P–C bond cleavage to give an intermediate p-alkenyl complex containing CH––C(PPh 2 ) 2 .25 The dicarbene fulvene complex [MMo 2 (CO) 4N(k-g3 g3-CH 2 C–– – CCH 2 )fv][BF 4 ] 2 1 has been prepared and its reactivity examined. Mild nucleophiles (Nu~) such as MeOH H 2 O and PhOMe react to give the monocarbenium complexes [MMo 2 (CO) 4N(k-g2 g3- NuCH 2 C–– – CCH 2 )fv][BF 4 ] 2 via a single addition whereas double addition of stronger nucleophiles such as pyridine and triphenylphosphine gave [MMo 2 (CO) 4N(k-g2 g2- NuCH 2 C–– – CCH 2 Nu)fv] 3 (Scheme 1).26 The anion [Mo(CO) 3 (2,4-C 7 H 11 )]~ is a convenient precursor to other pentadienyl complexes such as [HgMMo(2,4- C 7 H 11 )(CO) 3N2 ] and [MoI(CO) 3 (2,4-C 7 H 11 )].27 Deprotonation of [Mo 2 (CO) 3 (PPh 2 H)(k-g2-P 2 )Cp 2 ] and addition of excess CS 2 a§ords [Mo 2 (CO) 3 (k- g3-Ph 2 PC(H)SP 2 SNCp 2 ] 4 containing an unusual four-electron donor CSP 2 S ring.28 One-electron oxidation of [Mo 2 (k-C 8 Me 8 )Cp 2 ] with [FeCp 2 ][PF 6 ] gave [Mo 2 (k- C 8 Me 8 )Cp 2 ][PF 6 ].Further reaction with [FeCp 2 ][PF 6 ] or the trityl radical ·CPh 3 397 Organometallic chemistry of bi- and poly-nuclear complexes P Mo Mo P CO CO C S P S Ph CO Ph H 4 gave [Mo 2 (k-C 8 Me 7 CH 2 )Cp 2 ]` most likely via EEC and EC mechanisms respectively although in the latter case an EEC process was not discounted.A comparison of the structures of [Mo 2 (k-C 8 Me 8 )Cp 2 ] and [Mo 2 (k-C 8 Me 8 )Cp 2 ][PF 6 ] revealed that the Mo–C bond lengths of the g2-alkene in [Mo 2 (k-C 8 Me 8 )Cp 2 ][PF 6 ] are significantly longer than those in [Mo 2 (k-C 8 Me 8 )Cp 2 ].EHMOcalculations suggest this structural change to be associated with depopulation of an orbital involved in Mo–alkene n*-back donation namely theHOMOof aA symmetry.29 The cyclic voltammogram of the bis(metallacyclopentadiene) complex [W 2 (k-C 4 Me 4 ) 2 Cp 2 ] shows two one-electron oxidations.Reaction of [W 2 (k-C 4 Me 4 ) 2 Cp 2 ] with one and two equivalents of [FeCp 2 ][PF 6 ] gave [W 2 (k-C 4 Me 4 ) 2 Cp 2 ]` and [W 2 (k-C 4 Me 4 ) 2 Cp 2 ]2` 5 respectively both of which have been crystallographically characterised.Structural studies and EHMO calculations confirm that the HOMO hasW–Wd* character. Thermolysis of 5 in refluxing MeCN resulted in carbon–carbon coupling to give [W 2 (k- C 8 Me 7 CH 2 )Cp 2 ]` 6 in which the hydrocarbyl ligand is co-ordinated through one g3-allyl and two k-allylidene functionalities. Reaction of [W 2 (k-C 8 Me 7 CH 2 )Cp 2 ]` with NaBH 4 gave [W 2 (k-C 8 Me 8 )Cp 2 ] 7 formulated as a metallacyclononatetraene complex.30 W W W W W H H NaBH4 heat 7 6 5 + 2+ W New mono- and di-bridged bis(cyclopentadienyl) imido and oxo complexes of molybdenum and tungsten have been prepared. Addition of NBu5H 2 to [(MCl 4 ) 2 (k- CpnCp)] (M\Mo or W) gave the MV imido complexes [MMCl 2 (NBu5)N2 (k-CpnCp)] which are easily oxidised by PCl 5 to give [MMCl 3 (NBu5)N2 (k-CpnCp)].Reduction of [MMoCl 2 (NBu5)N2 (k-CpnCp)] and [MMoCl 3 (NBu5) 2N(k-CpnCp)] with Na–Hg amalgam gave [MMoCl(NBu5)N2 (k-CpnCp)] from which [MMo(O)(k-NBu5)N2 (k-CpnCp)] was isolated after treatment with HgO [Cpn n\1 Cp1\(g5-C 5 H 4 ) 2 SiMe 2 ; n\2 Cp2\(g5-C 5 H 3 ) 2 (SiMe 2 ) 2 ].31 The dicarbomethoxydihydrofulvalene complex [HgMo 2 (CO) 6Mg5 g5-(C 5 H 3 CO 2 Me) 2N] has been prepared by the LiBEt 3 Hreduction of [Mo 2 (CO) 6Mg5 g5-(C 5 H 3 CO 2 Me) 2N] to give Li 2 [Mo 2 (CO) 6Mg5 g5- (C 5 H 3 CO 2 Me) 2N] followed by insertion of Hg into the Mo–Mo bond. The elec- 398 S. Doherty trochemistry of these compounds was investigated.32 The octahedralW 6 cluster [W 6 (k-H) 4 (H)(CPr*)(OPr*) 7 (OPr*) 5 ] is labile to exchange with D 2 reversibly inserts ethene and in the presence of H 2 is a catalyst for the hydrogenation of ethene.The chemical inertness of [W 6 (k- H) 4 (H)(CPr*)(OPr*) 7 (OPr*) 5 ] enabled Chisholm and Kramer33 to demonstrate that these processes occur exclusively at the terminalW–H site. The solid-state structure of [W 2 (NMe 2 ) 4 (cot)] has been reported to bear a close resemblance to that previously described for the bis(allyl) complex [W 2 (k-allyl) 2 (NMe 2 ) 4 ]. Variable-temperature 1H NMR spectroscopic studies revealed two isomers in rapid equilibrium at room temperature. 34 Cleavage of the N–N bond of hydrazine by [Mo 2 (k-Cl)(k-SMe) 3 Cp 2 ] gave the mixed amido–sulfido bridged complex [Mo 2 (k-SMe) 3 (k-NH 2 )Cp 2 ]. EHMO calculations confirm a p2d*2d2 electron configuration for the quadruply bridged MoIII 2 d3–d3 centre consistent with a Mo–Mo single bond.35 Condensation of [Mo 2 (k-R1C–– – CR2)(CO) 4 Cp 2 ] (R1\R2\H Me or CO 2 Me; R1\H R2\Me Ph or CO 2 Me) with [Co 2 (CO) 8 ] gave the butterfly clusters [Co 2 Mo 2 (k4 -R1CCR2)(k-CO) 4 (CO) 4 Cp 2 ] with the two molybdenum atoms at the wingtips and the two cobalt atoms forming the hinge.36 5 Manganese technetium and rhenium The dianionic cluster [Mn 3 (CO) 6 (k-NO 2 ) 4 (k-ONO)]2~ isolated from the reaction between [NEt 4 ]cis-[MnCl 2 (CO) 4 ] and [N(PPh 3 ) 2 ][NO 2 ] contains both bridging nitro and nitrito groups.Similarly [Mn 2 (k-Cl) 2 (CO) 8 ] [MnCl 3 (CO) 3 ]2~ and [Mn 2 (k-Cl) 3 (CO) 6 ]~ all react with [N(PPh 3 ) 2 ][NO 2 ] to a§ord [Mn 3 (CO) 6 (k- NO 2 ) 4 (k-ONO)]2~.Surprisingly until now metal carbonyl compounds have only reacted with [N(PPh 3 ) 2 ][NO 2 ] to give nitrosyl-containing products via oxygen atom transfer.37 Addition of AlMe 3 to [MnMN(SiMe 3 ) 2N2 (thf)] gave [MMn(k- Me)[N(SiMe 3 ) 2 AlMe 3 ]N2 ] a methyl-bridged dimer stabilised by Mn–Me interactions involving a methyl group from AlMe 3 .38 Reaction of [Re 4 H 4 (CO) 12 ] with dmf yields the unsaturated 44-electron cluster [Re 3 H 4 (CO) 9 ]~ containing an isosceles Re 3 triangle with two single Re–Re bonds and one double Re––Re bond. This cluster reacts rapidly with donor ligands (CO py PPh 3 or MeCN) to form [Re 3 H 4 (CO) 9 L]~. In the solid state two of the hydrides bridge the two longer Re–Re bonds while the remaining two hydrides bridge the Re––Re double bond. In solution NMR spectroscopic studies suggest an alternative structure [Re 3 (k3 -H)(k-H) 3 (CO) 9 ]~ in which two double Re––Re bonds are delocalised.39 Several methods for synthesising the novel open cluster [Re 2 (CO) 9 (k- H)MReH(CO) 4N]~ have been described. Solution 1H and 13C NMR spectroscopic studies show conformational freedom about both Re–Re bonds and a dynamic process that exchanges the hydride ligands and the carbonyls trans to these. The solution exchange processes in [Re 2 (k-H)(CO) 9MReH(CO) 4N]~ were compared with that of the closely related complex [Re 2 (k-H)(CO) 9MRe(CO) 5N] in which the terminal hydride is replaced by a CO.40 Dynamic laser light scattering was used to demonstrate that the newly formed complexes [Re 2 (CO) 5 (k-OMe) 2 (k-L–L)] (L–L\dppm dppe or dppp) aggregate in solution to form clusters with an average radius of 370 nm.41 Time-resolved infrared spectroscopy was used to probe the photochemical trans- 399 Organometallic chemistry of bi- and poly-nuclear complexes formation of the 3,4-dirhenacyclobutene complex [Re 2 (CO) 4 (g2- CO 2 MeC 2 CO 2 Me)Cp* 2 ] generated from the reactive Re––Re double-bonded compound [MRe(CO) 2 Cp*N2 ] and dmad into the 2,4-dirhena[1,1,0]bicyclobutane [Re 2 (CO) 4 (k-g2 g2-O 2 MeCC–– – CCO 2 Me)Cp* 2 ].Conrotatory ring-opening of the 3,4- dirhenacyclobutane was suggested to give a short-lived bis(metallacarbene) intermediate prior to Re–C bond formation to give [Re 2 (CO) 4 Cp* 2 (k-g2 g2-O 2 MeC–– – CO 2 - Me)].42 Photoextrusion of CO from [Re 2 (CO) 4 (k-g2 g2-MeO 2 CC–– – CCO 2 Me)Cp* 2 ] gave the metallatetrahedrane [Re 2 (CO) 2 (k-CO)(k-g2 g2-MeO 2 CC–– – CCO 2 Me)Cp* 2 ] in high yield.43 Ligand addition reactions of the d2 32-electron dimer [MRe(CO) 2 Cp*N2 ] have been examined.Both CO and MeCN gave stable adducts while those formed with PMe 3 and CH 2 CH 2 at low temperature fragment above [20 °C to give [Re(CO) 2 (thf)Cp*] and [Re(CO) 2 (PMe 3 )Cp*] or [Re(CO) 2 (C 2 H 4 )Cp*] respectively. Reaction with HC–– – CH gave [Cp*(CO) 2 Re(k- g1 g3-CH––CHCO)Re(CO)Cp*] containing a dimetallacyclopentenone whereas CH 3 C–– – CCH 3 initially gave the 1 1 adduct [Cp*(CO) 2 Re(k-CO)(g2- CH 3 C–– – CCH 3 )Re(CO)Cp*] which slowly converts into a mixture of the dimetallcyclopentenone [Cp*(CO) 2 ReMk-g1 g3-(CH 3 )C––C(CH 3 )CONRe(CO)Cp*] and the fragmentation products [Re(CO) 3 Cp*] and [Re(CO)(CH 3 C–– – CCH 3 )Cp*].44 The ring metallated complex [Re(CO) 3 (g5-C 5 H 4 Li)] reacts with [Re(CO) 3 Cp] to give the dirhenium acyl anion Li[Cp(CO) 2 ReMC––O[C 5 H 4 Re(CO) 3 ]N] which was protonated and methylated to give the hydroxy and methoxy carbene complexes [Cp(CO) 2 Re––C(OH)MC 5 H 4 Re(CO) 3N] and [Cp(CO) 2 Re––C(OMe)MC 5 H 4 Re(CO) 3N] respectively.Attempted metallation of the unsubstituted ring in [Cp(CO) 2 Re–– C(OMe)MC 5 H 4 Re(CO) 3N] gave the butylcarbene complex [Cp(CO) 2 Re–– CBuMC 5 H 4 )Re(CO) 3N].45 The 3-methylthietane substituted complexes [Re 2 (CO) 9 (SCH 2 CHMeCH 2 )] and [W(CO) 5 (SCH 2 CHMeCH 2 )] catalyse the ring-opening cyclooligomerisation of 3- methylthietane to give the polythioether macrocycle 3,7,11-trimethyl-1,5,9-trithiacyclododecane.Di§erent orientations of the methyl substituents on the ring give rise to two isomers both of which react with [Re 2 (CO) 9 (NCMe)] to give [Re 2 (CO) 9 (cis,trans,trans-SCH 2 CHMe(CH 2 SCH 2 CHMe) 2 CH 2 ] and [Re 2 (CO) 9 - (cis,cis,cis-SCH 2 CHMe(CH 2 SCH 2 CHMe) 2 CH 2 ].46 Adams et al.47 have also described the catalytic cyclooligomerisation of b-propiothiolactone by [Re 2 (CO) 9 (NCMe)]. The known polymer (SCH 2 CH 2 O)n and the oligomers 1,5,9,13- tetrathiacyclohexadecane-2,6,10,14-tetrone and 1,5,19,13,17,21-hexathiacyclotetracosane- 2,6,10,14,18,22-hexone have been isolated and fully characterised.47 Ring opening via S–S bond cleavage and insertion of 5-(ethoxycarbonyl)amino- 1,2,4-dithiazole-3-thione into the Re–Re bond of [Re 2 (CO) 9 (NCMe)] gave [Re 2 (CO) 9Mk-S 2 CNC[N(H)CO 2 Et]SN] [Re 2 (CO) 8Mk-S 2 CNC(NHCO 2 Et)SN] and [Re 2 (CO) 8Mk-S 2 CNH(CNCO 2 Et)SN].Minor amounts of the mononuclear complexes [Re(CO) 4MS 2 CNHCSNH(CO 2 Et)N] and [Re(CO) 4MSCN(NHCO 2 Et)SCSNN] were also isolated.48 Several new compounds have been isolated from the reaction of [Re 2 (CO) 9 (NCMe)] with EtO 2 CN––C––S including [Re 2 (CO) 9 (Z-trans-k-C,SEtO 2 CN––CS)] [Re 2 (CO) 8 (k-C,S,N-EtO 2 CN–– CS)] [Re 2 (CO) 8Mk-C,N,S 2 - (EtO 2 C) 2 NC–– NCS 2N] [Re 2 (CO) 7 (NCMe)Mk-C,N,S 2 -(EtO 2 C) 2 NC––NCS 2N] and [Re 2 (CO) 8 (k-C,N,S 2 -(EtO 2 C)(H)(NC–– NCS 2N]; the first two contain a dimetallated thioimidate whereas the last three arise from new coupling and rearrangements 400 S. Doherty involving two isothiocyanates. Similarly [Re 2 (CO) 7 (NCMe)(PMe 2 Ph)] reacted with EtO 2 CN––C––S to a§ord the dimetallated product [Re 2 (CO) 8 (PMe 2 Ph)(k- EtO 2 CNCS)] as well as trans-[Re 2 (CO) 8 (PMe 2 Ph)Mk-(EtO 2 C)N––CN(CO 2 Et)(S 2 )N] and cis-[Re 2 (CO) 8 (PMe 2 Ph)Mk-(EtO 2 C)N––CN(CO 2 Et)(S 2 )N] both of which are formed by the coupling and rearrangement of two isothiocyanate molecules.These last two isomers convert into [Re 2 (CO) 7 (PMe 2 Ph)Mk-N,C,S 2 -(EtO 2 C) 2 NC––NCS 2N] via loss of carbon monoxide.49 Thermolysis of [Re 2 (CO) 9 (NCMe)] with MeO 2 CHC–– C––C(H)CO 2 Me in the presence of adventitious water gave [Re(CO) 4MC(CH 2 CO 2 Me)–– C(H)CO 2 MeN] which reacts with PMe 2 Ph to give fac- [Re(CO) 3 (PMe 2 Ph)MC(CH 2 CO 2 Me)––C(H)(CO 2 Me)N]. A similar reaction with [Re 2 (CO) 8 (PMe 2 Ph)(NCMe)] gave mer-[Re 2 (CO) 6 (PMe 2 Ph)Mk-g3 g1-MeO 2 C(H)- CCC(H)CO 2 MeN] and fac-[Re 2 (CO) 6 (PMe 2 Ph)Mk-g3 g1-MeO 2 C(H)-CCC(H)- CO 2 MeN] two isomers both containing an g3 g1-allene ligand.50 The reactions of [Re 2 (CO) 9 (NCMe)] and [Re 3 (k-H) 3 (CO) 10 (NCMe) 2 ] with S––C(NEt 2 )N(H)(C 6 H 4 Me-p) gave [Re 2 (CO) 9MS––C(NEt 2 )N(H)(p-C 6 H 4 Me)N] and [Re 3 (k-H) 3 (CO) 10Mk-S––C(NEt 2 )N(H)(p-C 6 H 4 Me)N] repectively.The first of these new compounds contains an S–– C(NEt 2 )N(H)(p-C 6 H 4 Me) ligand S-co-ordinated in an equatorial site while the second product contains an S-bridged thiourea. When heated at reflux in heptane [Re 2 (CO) 9MS––C(NEt 2 )N(H)(C 6 H 4 Me-p)N] loses H 2 to give the non-metal–metal bonded complex [Re 2 (CO) 6Mk-SCS(N-C 6 H 4 Me-p)(NEt 2 )N].51 Insertion of EtO 2 CN–– C––S into the metal–metal bond of [Re 2 (CO) 9 (NCMe)] gave [Re 2 (CO) 9 (S-trans-k-C,S-EtO 2 CN––SC)] [Re 2 (CO) 8 (S-trans-k-C,S,N-EtO 2 CN–– CS)] and [Re 2 (CO) 8Mk-C,N,S 2 -(EtO 2 C) 2 N––CNCS 2N].52 Reduction of [TcO 4 ]~ in the presence of BH 3 ·thf and carbon monoxide in thf gave [Tc 3 (k-H) 3 (CO) 12 ] the first example of a hydrido-bridged technetium(I) compound.A minor by-product of this reaction was tentatively suggested to be [Tc 3 (k3 -H)(k- H) 3 (CO) 9 ]~ based on IR and 1H NMR spectroscopic data alone.53 6 Iron ruthenium and osmium The room-temperature reaction of [Fe 2 (CO) 9 ] with 2-butyne-1,4-diols gave the butatriene hexacarbonyldiiron complex [Fe 2 (CO) 6 (k-g3 g3-R1R2CCCCR3R4N]. Little mechanistic information was provided but [Fe 2 (CO) 9 ] was suggested to act as a reducing agent as well as a complexing agent.54 The anion [Fe 2 (CO) 6 (k-SN)]~ reacts with the carbon-rich carboranes 6-bromopentamethyl- and 1,6-dibromo-2,5-dibutyl- 3,4-diethyl-2,3,4,5-tetracarba-nido-hexaboranes(6) to give N-M6-[2,3,4,5-tetracarbanido- hexaborane(6)-yl]Nhexacarbonyldiferraazathiatetrahedrane complexes in which the hexacarbonyldiferraazathiatetrahedrane and carbon cage are linked through a B–N bond.55 Oxidation of the vinyl complex [FeL1L2MC(R)––CH 2NCp*] (R\OMe L1\CO L2\PPh 3 ; R\OMe L1\CO L2\PMe 3 ; R\OMe L1\L2\dppe; R\H L1\L2\dppe) with [FeCp 2 ][PF 6 ] at[80 °C gave the unstable 17-electron radical [FeL1L2MC(R)––CH 2NCp*][PF 6 ] which undergoes carbon–carbon coupling in the solid state to give the bis(carbene) complexes [(FeL1L2Cp*) 2 (k- –– CRCH 2 CH 2 RC–– )][PF 6 ] 2 .56 The 1,3-diferrio-1,3-diphosphetane-2,4-diones [MCp@(CO) 2 FeN- PC(O)PMFe(CO) 2 Cp@NC(O)] have been isolated from the reaction between 401 Organometallic chemistry of bi- and poly-nuclear complexes Fe C + Fe CS OC CO Fe C Fe + C OC CO X SMe Fe C Fe CS OC CO X SMe 8 9 10 S Me S Me [NBu4]X MeSO3CF3 Fe C Fe + C OC X S Fe C Fe CS OC X S 11 12 S Me MeSO3CF3 Me Me Fe C Fe C OC X S 13 S Me Me CN – CN – CO Scheme 2 (dme) 2 LiOC–– – P and [FeBr(CO) 2 Cp@].Further reaction of these diones with [Cr(CO) 5 R] (R\Z-cyclooctene)] gave [MCp@(CO) 2 FeN- PMCr(CO) 5NC(O)PMFe(CO) 2 Cp@NC(O)] (Cp@\C 5 Me 5 C 5 H 3 Bu5 2 -1,3 or C 5 H 2 Pr* 3 - 1,2,4).57 The cationic complexes [(Fp)Ph 2 PC–– – CPPh 2 (Fp)]2` and [(Fp)Ph 2 PC–– – CPPh 2 ]` [Fp\Fe(CO) 2 Cp] have been prepared by oxidising [Fe 2 (CO) 4 Cp 2 ] with one and two equivalents of [FeCp 2 ]` in the presence of Ph 2 PC–– – CPPh 2 .Addition of one and two mol equivalents of the diphosphine to [PPh 4 ][Fe 3 (k-H)(CO) 9 (k3 -g2-C––CH 2 )] gave the anionic clusters [PPh 4 ] [Fe 3 (CO) 9 (k3 -CCH 3 )(Ph 2 PC–– – CPPh 2 )] and [PPh 4 ][MFe 3 (CO) 9 (k3 -CCH 3 )N2 (Ph 2 PC–– – CPPh 2 )] respectively. The zwitterionic complex [MFe 3 (CO) 9 (k3 - CCH 3 )N(Ph 2 PC–– – CPPh 2 )(Fp)] has also been described.58 Oxidative addition of phenylsilane to [Fe 2 (CO) 9 ] gave the triply-bridged complexes [MFe(CO) 3N2 (k-SiPhH) 2 (k-CO)] while SiPh 2 H 2 gave the singly-bridged complex [M(CO) 4 FeFe(CO) 3 (SiHPh 2 )N(k-g2-HSiPh 2 )]. Themolysis of a toluene solution of the latter product resulted in the formation of [MFe(CO) 3N2 (k-g2-HSiPh 2 ) 2 ].Both [M(CO) 4 FeFe(CO) 3 (SiHPh 2 )N(k-g2-HSiPh 2 )] and [MFe(CO) 3N2 (k-g2-HSiPh 2 ) 2 ] contain agostic Fe–H–Si interactions.59 Reaction of [Fe 2 (k-CS)(k-CSMe)(CO) 2 Cp 2 ][SO 3 CF 3 ] 8 with [NBu 4 ]X (X\H or CN) occurs with nucleophilic attack at the k-CSMe thiocarbyne carbon to give 402 S. Doherty P Ph2 Fe(CO)3 (OC)3Fe Ca Cb Cg 14 P Ph2 Fe (OC)3Fe C 15 H H H C O NRH H H C H H NRH2 P Ph2 Fe(CO)4 Fe CO C C H H C H Ph2 P H2C P Ph2 OC P Ph2 Fe(CO)3 Fe CO CH3 C C H Ph2 P C P Ph2 CO H 16 17 dppm hn – CO CO CO Scheme 3 [Fe 2 (k-CS)Mk-C(SMe)XN(CO) 2 Cp 2 ] 9. Reaction of 9 with MeSO 3 CF 3 gave [Fe 2 (k- CSMe)Mk-C(SMe)XN(CO) 2 Cp 2 ][SO 3 CF 3 ] 10 via selective methylation of the bridging thiocarbonyl ligand. Photolysis of 9 results in loss of carbon monoxide and coordination of the thiocarbene sulfur atom to give [Fe 2 (k-CS)Mk-g1(S)- C(SMe)XN(CO)Cp 2 ] 11.Treatment of this latter complex with MeSO 3 CF 3 resulted in methylation at the thiocarbonyl carbon to give the mixed carbyne–carbene complex [Fe 2 (k-CSMe)Mk-g(S)-C(SMe)XN(CO)Cp 2 ]` 12 which upon addition of CN~ gave [Fe 2Mk-C(CN)SMeNMk-g(S)-C(SMe)XN(CO)Cp 2 ] 13 the first bis(k-thiocarbene) complex (X\H or CN) (Scheme 2).60 The phosphido-bridged diiron allenyl complex [Fe 2 (CO) 6 (k-PPh 2 )Mk-g1 g2a,b- Ca(H)–– Cb ––CcH 2N] 14 reacts with primary amines to a§ord the amino-functionalised alkenyl complexes [Fe 2 (CO) 5 (k-PPh 2 )Mk-g1 g1 g2-MO–– C(NHR)CH 2 C––CH 2N] 15 (R\Bu5 or Ph) via a novel carbonyl–allenyl–amine coupling sequence and the dimetallacyclopentadienes [Fe 2 (CO) 6 (k-PPh 2 )Mk-g1 g1-CH 2 C(NHR@)CH 2N] (R@\ Cy or Bu5) via nucleophilic attack at Cb and hydrogen transfer to Ca.61 The same allenyl complex reacted with dppm to give [Fe 2 (CO) 6 (k-PPh 2 )Mg1(P) g2(C)- Ph 2 PCH 2 PPh 2 C(H)–– C––CH 2N] 16 containing a dppm-functionalised allene.Photolysis of a toluene solution of 16 resulted in activation of a dppm methylene C–H bond hydrogen migration to the allene and formation of [Fe 2 (CO) 5 (k-PPh 2 )Mk- g1(P) g2(C) g1(C)-Ph 2 PCHPPh 2 C(H)–– CCH 3N] 17 which contains a metal- and carbon- co-ordinated bis(diphenylphosphino)methanide (Scheme 3). In contrast reaction of [Fe 2 (CO) 6 (k-SBu5)(k-g1 g2-CH––C–– CH 2 )] with dppm gave the isomeric alkenyl complexes [Fe 2 (CO) 5 (k-SBu5)Mk-g1(P) g1(C) g2(C)-Ph 2 PCHPPh 2 C(H)––CCH 3N] and [Fe 2 (CO) 5 (k-SBu5)Mk-g1(P) g1(C) g2(C)-Ph 2 PCHPPh 2 CH 2 C––CH 2N] via a similar C–H activation–hydrogen migration pathway.62 The binuclear allenyl complex 403 Organometallic chemistry of bi- and poly-nuclear complexes Fe Fe CO OC P P CO CO C C C O H H CO P H2C C P C Fe Fe(CO)3 C H H P C CCH2Ph Ph2P (OC)3Fe Fe(CO)3 (OC)2 O heat – + H2 Ph2 – + Ph2 Ph2 Ph2 Ph2 heat 18 19 20 Scheme 4 [Fe 2 (CO) 6 (k-PPh 2 )Mk-g1 g2a,b-Ca(H)––b CcH 2N] reacts with one (R\Ph) and 0.5 (R\H) equivalents of PPhHR to give [Fe 2 (CO) 6 (k-PPh 2 )Mk-g1 g2- (Me)C––CH(PPh 2 )N] and [MFe 2 (CO) 6 (k-PPh 2 )[k-g1 g2-(Me)C––CH]N2 (k-PPh)] both of which arise from an unprecedented nucleophilic attack of the phophine at Ca of the k-g1 g2a,b-allenyl ligand.63 The dimetallacyclopentenone complex [Fe 2 (CO) 5Mk-p g3- C(O)CHCHN(k-dppm)] 18 isomerises to the k-vinyl complex [Fe 2 (CO) 6Mk- CH 2 ––CP(Ph) 2 CH 2 PPh 2N] 19 via intramolecular nucleophilic attack of phosphorus on a carbon of the metallacycle with an associated 1,2-hydrogen migration and C–C bond cleavage.On further heating transfer of a phenyl group from phosphorus to carbon gave the isomer [Fe 2 (CO) 6Mk-C(CH 2 Ph)P(Ph) 2 CH 2 PPhN] 20 together with [Fe 2 (CO) 5Mk-p g3-C(O)CRCHNMk-PPh 2 CH 2 P(Ph)C 6 H 4 C(CH 3 )N] the result of a more complicated rearrangement (Scheme 4).64 The redox behaviour of [Fe 3 (CO) 9 (k3 -S)]2~ has been investigated. The anion [Fe 5 (CO) 14 S 6 ]2~ was prepared by oxidative condensation of [Fe 3 (CO) 9 S]2~ and [Fe 6 (CO) 12 S 6 ]2~ was isolated as a by-product. Alternative procedures for the synthesis of these clusters have been described and their crystal structures reported.65 The hexanuclear carbonyl metallate [NEt 4 ] 2 [Fe 6 C(CO) 16 ] reacts with [(AuCl) 2 (k-L)] (L\dppm or dppe) to give [Fe 4 Au 2 C(CO) 12 (dppm))] and [MFe 6 AuC(CO) 16N2 (k- dppe)].66 The binuclear k-oxo complex [(RuCl 2 Cp*) 2 (k-O)] decomposes in chloroform solution by activation of a methyl C–H bond to give the dinuclear tetramethylfulvene complex [MRuCl 2 (g6-C 5 Me 4 CH 2 )N2 ] and water.The chloride bridges are cleaved upon addition of donor ligands (pyridine and dmso) to give [RuCl 2 L(g6-C 5 Me 4 CH 2 )]. Notably though the bromide complex [(RuBr 2 Cp*) 2 (k-O)] is significantly less reactive toward C–H activation although in the presence of donor ligands both the chloride and bromide complexes react under ambient conditions.67 Oxidative addition of the dichalcogenide REER (E\S Se or Te; R\ferrocenyl) to [MRuCp*(k3 - Cl)N4 ] gave the ferrocenylchalcogenate-bridged complex [MRuClCp*(k-ER)N2 ].Reduction of these complexes with Na–Hg amalgam in the presence of buta-1,3-diene gave [MRu(k-ER)Cp*N2 (k-s-trans-g2 g2-CH 2 ––CHCH 2 )] while reaction with AgSO 3 CF 3 gave the co-ordinatively unsaturated complex [Ru(k-ER)Cp*][SO 3 CF 3 ] containing one 16-electron RuII and one 18-electron metal centre.68 Reaction of [MRuCl(k-Cl)Cp*N2 ] with [MS 4 ]2~ gave the mixed disulfide–sulfide complexes [(RuCp*) 2 (k2 -S 2 )(k3 -S)(k2 -S) 2 MS] (M\Mo or W) in which the MS 4 fragment is bonded to the two ruthenium atoms by three sulfur atoms two of which bridge the Ru–M bonds while the third caps all three metal atoms.69 The reaction of [(RuCl 2 Cp*) 2 ] with Li(Bu5NSPh) gave two products [RuCl(g2-Bu5NSPh)(g4- C 5 Me 4 CH 2 )] and [Cp*ClRu(k-NBu5)(k-SPh)RuCp*]; the former has a tetramethyl- 404 S.Doherty fulvene ligand the latter is a sulfido–imido bridged dimer formed via S–N bond cleavage of the sulfenamido species. Similarly S–N bond cleavage in the reaction of [MCrBr 2 Cp*N2 ] gave [Cr(NBu5)(SPh) 2 Cp*].70 Various binuclear mixed k-methoxide –phenolate and bis-k-phenolate complexes of ruthenium have been prepared. In one instance the bridging phenoxide ligand of [(Cp*Ru) 2 (k-OC 6 H 2 R1R2R3) 2 ] rearranged to give the oxocyclohexadienyl complex [RuCp*(g5-C 5 H 2 R1R2R3C–– O)].71 Ru H Tol SPri H Tol Ru SPri SPri Ru SPri Ru 22 21 Tol The co-ordinatively unsaturated dimer [MRu(k-SPr*)Cp*N2 ] reacts with excess alkyne HC–– – CR [R\Tol or C––CH(CH 2 ) 3 CH 2 ] to give the ruthenacyclopentenyl complexes 21 and 22.These complexes both react with Bu5NC to give the k-p-n-alkenyl complexes [Cp*(Bu5NC)Ru(k-SPr*)Mk-g1 g2-C(Tol)–– CHCMC(Tol)––CHSPr*N–– CH(Tol)NRuCp*] and [Cp*(Bu5NC)Ru(k-SPr*)Mk-g1 g2-k-CMC(C–– CH(CH 2 ) 3 CH 2 )–– CHSPr*N––CH(C–– CH(CH 2 ) 3 CH 2 )NRuCp*] respectively via ring-opening of the ruthenacyclopentenyl fragment.72 Thermolysis of [MRuCl(k-SH)Cp*N2 ] isolated from the reaction of [MRu(k3 -Cl)Cp*N4 ] or [MRuCl(k-Cl)Cp*N2 ] with H 2 S gave the cubane cluster [MRu(k3 -S)Cp*N4 ]Cl 2 . Reaction of [RhCl(PPh 3 ) 3 ] with [MRuCl(k-SH) 2 Cp*N2 ] gave the trinuclear mixed-metal cluster [(Cp*Ru) 2 (k-H)Rh(PPh 3 )Cl 2 (k3 -S) 2 ].73 The disulfide–thiolate bridged complexes [Ru 2 (k-S 2 )(k-SR) 2 Cp* 2 ] (R\Pr* or PhCH 2 ) have been prepared and structurally characterised and their redox behaviour examined using cyclic voltammetry.74 The trinuclear ruthenium pentahydride [(RuCp*) 3 (k-H) 3 (k3 -H) 2 ] reacts with buta- 1,3-diene to give [(RuCp*) 3 (H) 4Mk3 -g3-C(Me)CHCHN] which contains a 1-methyl-1,3- dimetalloallyl ligand.An intermediate k-g2 g2-s-cis-isoprene complex was detected by 1H NMR spectroscopy confirming that all three ruthenium atoms are required to activate the 1,3-diene; two act as co-ordination sites the other as an activation site for the C–H bond.75 Insertion of nitriles RCN (R\Me or Et) into the Ru–H bond of [MRu(k-H) 2 Cp*N2 ] in the presence of an arene gave the novel hydrido k- alkylideneamido complexes [(RuCp*) 2 (k-arene)(k-H)(k-N––CHR)] 23 containing an g2 g2-co-ordinated arene.Addition of ethylene to [(RuCp*) 2 (k-arene)(k-H)(k- N––CHR)] gave the bis(ethylene) complex [MRuCp*(g2-C 2 H 4 )N2 (k-H)(k-N––CHR)] 24 which upon thermolysis at 80 °C converted into the k-g2 g2-s-cis-butadiene complex [(RuCp*) 2 (k-g2 g2-CH 2 ––CHCH––CH 2 )(k-H)(k-N–– CHR)] 25 via dehydrogenative coupling of the co-ordinated ethylene molecules (Scheme 5). Possible pathways for this coupling were discussed.76 Thermolysis of a toluene solution of [Ru 2 (k-CO) 2 (CO) 2 Cp 2 ] and HnSiR@4~n (n\2 or 3; R@\Et or Ph) gave a mixture of the mono- and di-k-methylene bridged 405 Organometallic chemistry of bi- and poly-nuclear complexes Ru H Ru C H R N Ru H Ru C H R N Ru H Ru C H R N heat C2H4 25 24 23 Scheme 5 complexes [Ru 2 (k-CH 2 )(k-CO)(CO) 2 Cp 2 ] and [Ru 2 (k-CH 2 ) 2 (CO) 2 Cp 2 ] via deoxygenative reduction of a bridging carbonyl with the silane.Labelling experiments with 13CO and DnSiR@4~n confirm that the carbon and hydrogen atoms of the product k-CH 2 moiety arise from CO and the hydrosilane.77 Thermolysis of [Ru 2 (k-CH 2 )(k- CO)(CO) 2 Cp 2 ] in the presence of HSiMe 3 (170 °C 3 d) generates methane SiMe 4 [Ru(H)(SiMe 3 ) 2 (CO)Cp] and [Ru(CO) 2 (SiMe 3 )Cp]. Reaction of the labile complex [Ru 2 (k-CH 2 )(k-CO)(CO)(NCMe)Cp 2 ] with HSiR 3 gave the hydridosilyl-k-methylene complex [Ru 2 (k-CH 2 )(H)(SiR 3 )(CO) 2 Cp 2 ] and [Ru 2 (k-CH 2 )(SiR 3 ) 2 (CO) 2 Cp 2 ] both of which liberate CH 4 upon further treatment with HSiR 3 . Hydrostannanes HSnR 3 also react with [Ru 2 (k-CH 2 )(k-CO)(CO) 2 Cp 2 ] firstly to give [Ru 2 (k- CH 2 )(H)(SnR 3 )(CO) 2 Cp 2 ] and then [Ru 2 (k-CH 2 )(SnR 3 ) 2 (CO) 2 Cp 2 ].However HSnPh 3 reacts with [Ru 2 (k-CH 2 )(k-CO)(CO) 2 Cp 2 ] to a§ord [Ru 2 (k- CH 2 )(SnPh 3 )(H)(CO) 2 Cp 2 ] which upon standing converts into a mixture of [Ru 2 (k- SnPh 2 ) 2 (CO) 2 Cp 2 ] and [Ru 2 (k-Ph)MSn(CH 3 )Ph 2N(CO) 2 Cp 2 ]. A possible pathway for generation of CH 4 from [Ru 2 (k-CO) 2 (CO) 2 Cp 2 ] and hydrosilanes has been formulated. 78 While addition of CO or PPh 3 to the silylated k-methylene complex [Ru 2 (k-CH 2 )(H)(SiR 3 )(CO) 2 Cp 2 ] gave [Ru 2 (k-CH 2 )(k-CO)(CO)LCp 2 ] via reductive elimination of SiR 3 H reaction with [Ru 2 (k-CH 2 )(SiR 3 ) 2 (CO) 2 Cp 2 ] gave the k-silylmethylene complex [Ru 2 (k-CHSiR 3 )(k-CO)(CO)LCp 2 ] with elimination of SiR 3 H.Presumably the latter occurs via reductive elimination of k-CH 2 and SiR 3 to give g1-CH 2 SiR 3 followed by oxidative addition of a C–H of the new silylmethyl group. Addition of HSiR 3 and H 2 to [Ru 2 (k-CHSiR 3 )(k-CO)(CO)(NCMe)Cp 2 ] gave [Ru(k- CH 2 )(SiR 3 ) 2 (CO) 2 Cp 2 ] supporting the involvement of oxidative addition of CH 2 –SiR 3 in this reaction.79 The binuclear allenyl complex [Ru 2 (CO) 6 (k-PPh 2 )Mk-g1 g2b,c-C(Ph)––C––CH 2N] 26 reacts with diphenyl acetylene to give [Ru 2 (CO) 5 (k-PPh 2 )Mk-g5-C 5 MePh 2 - 406 S. Doherty Ru OC OC P Ph R Ph O Ru CO CO CO PhC CPh PhC CPh Cg H H Cb Ca Ph Ru(CO)3 (OC)3Ru P Ph2 C C Ph Ru(CO)3 (OC)3Ru P Ph2 28 27 R = Me; 29 R = H 26 Ph2 Scheme 6 (C 6 H 4 )(O)N] 27.Under the same conditions [Ru 2 (CO) 6 (k-PPh 2 )(k-g1 g2-C–– – CPh)] 28 gave [Ru 2 (CO) 5 (k-PPh 2 )Mk-g5-C 5 HPh 2 (C 6 H 4 )(O)N] 29 (Scheme 6). Reaction of the allenyl dimer with PhC–– – CH gave two isomers of composition [Ru 2 (CO) 5 (k-PPh 2 )Mk- g5-C 5 MeHPh(C 6 H 4 )(O)N] while similar treatment of the acetylide dimer also gave two isomers of [Ru 2 (CO) 5 (k-PPh 2 )Mk-g5-C 5 H 2 Ph(C 6 H 4 )(O)N].80 The k-g1 g2a,b-butadiynyl complexes [Ru 2 (CO) 6 (k-PPh 2 )(k-g1 g2a,b-C–– – CC–– – CR)] 30 (R\Bu5 Ph or SiMe 3 ) react with the carbene precursors R1 2 CN 2 (R1\H or Ph) to give the 1- ynyl–allenyl complexes [Ru 2 (CO) 6 (k-PPh 2 )Mk-g1 g2-C(C–– – CR)–– C––CR1 2N] (R\Bu5 R1\Ph). In the case of Ph 2 CN 2 attack of the carbene group at Cb gave the g1-indenyl P Ph2 Ru(CO)3 (OC)3Ru C C C C R P Ph2 Ru(CO)3 (OC)3Ru C C C C R Ph H C C Ph H (OC)3Ru P Ph2 Ru(CO)3 C C R + 30 31 R = But 32 R = Ph Ph2CN2 products [Ru 2 (CO) 6 (k-PPh 2 )Mg1 g2-CH(C 6 H 4 )C(Ph)–– CC–– – CBu5N] 31 and [Ru 2 - (CO) 6 (k-PPh 2 )Mk-g1 g2-C––C(Ph)C––C(Ph)(C 6 H 4 )CHN] 32.81 Thermolysis of the binuclear acetylide complex [Ru 2 (CO) 6 (k-PPh 2 )(k-g1 g2-C–– – CBu5)] 33 gave [Ru 4 (CO) 9 (k-PPh 2 ) 2 (k-g1 g2-CCBu5) 2 ] 34 a 64-electron butterfly cluster containing two bridging k-PPh 2 one k-g1 g2-C–– – CBu5 and one k3 -g1 g2 g2-C–– – CBu5.Continued thermolysis of 33 led to carbon–carbon coupling to give [Ru 4 (CO) 8 (k- PPh 2 ) 2 (Bu5C 4 Bu5)] 35 and [Ru 3 (CO) 7 (k-PPh 2 ) 2 (Bu5C 4 Bu5)] 36 containing k4 - and k3 -butadiyne ligands respectively (Scheme 7).82 Removal of the chloride from [N(PPh 3 ) 2 ][Ru 3 (k-Cl)(CO) 9 (k-PhC–– – CPh)] in the 407 Organometallic chemistry of bi- and poly-nuclear complexes C C But Ru(CO)3 (OC)3Ru P Ph2 Ru Ru Ru Ru C C C C But But P Ph2 P Ph2 Ru Ru Ru Ru C C C C But But P Ph2 P Ph2 Ru Ru Ru C P Ph2 P Ph2 C C C But But 36 35 -2CO 34 33 Scheme 7 Carbonyls omitted for clarity Ru Ru Ru C C Ph Ph Ph2 P PPh2 Ru Ru Ru C C Ph Ph Ph2 P PPh2 C C Ph C O C Ru Ru P Ph2 PPh2 Ru C Ph H C C C C Ru Ru Ph Ph H Ph Ph2 P P Ph2 + 40 PhCCH 39 +CO 38 37 Scheme 8 presence of dppm a§ords the unsaturated 46-electron cluster [Ru 3 (CO) 7 (k3-g2-r- PhC–– – CPh)(k-dppm)] 37 in high yield.Reaction of 37 with CO gives its 48-electron counterpart [Ru 3 (CO) 8 (k3 -g2-E-PhC–– – CPh)(k-dppm)] 38 with conversion of the alkyne to the parallel mode. Cluster 37 reacts with dppm to give [Ru 3 (CO) 6 (k- PhC–– – CPh)(dppm) 2 ] and with H 2 to yield the dihydride [Ru 3 (k-H) 2 (CO) 7 (k- PhC–– – CPh)(k-dppm)] which exists as a mixture of two isomers.Addition of one equivalent of phenylacetylene to 37 gave the fly-over cluster [Ru 3 (CO) 6Mk- 408 S. Doherty HCC(Ph)C(O)-C(Ph)CPhN(k-dppm)] 39 containing the dialkenyl ketone ligand HCC(Ph)C(O)-C(Ph)CPh together with [Ru 2 (CO) 4Mk-HCC(Ph)C(Ph)CPhN(k- dppm)] 40 a binuclear ruthenacylopentadiene resulting from alkyne coupling and cluster fragmentation. Cluster 38 readily converts into the vinylidene alkenyl ketone derivative [Ru 3 (k-H)(CO) 6Mk-C(CPh)C(O)C(Ph)CPhN(k-dppm)] via facile C–H bond activation at 35 °C (Scheme 8).83 The k3 -alkyne cluster [Ru 3 (CO) 3 (k3 -CO)(k3 - CF 3 C–– – CCF 3 )Cp 2 ] and its labile acetonitrile derivative [Ru 3 (CO) 2 (k3 - CO)(NCMe)(k3 -CF 3 C–– – CCF 3 )Cp 2 ] react with alkynes.The latter reacts with CF 3 C–– – CCF 3 to a§ord [Ru 3 (CO) 2 (k-CO)Mk3 -g3-C 3 (CF 3 ) 3N(k3 -CF 3 )Cp 2 ] whereas PhC–– – CPh methylbut-2-ynoate and but-2-yne a§ord [Ru 3 (CO) 2 (k-CO) 2Mk3 - C 4 (CF 3 ) 2 R(R@)NCp 2 ] 41 (R\R@\Ph or Me; R\Me R@\CO 2 Me); in which the hydrocarbyl fragment is g3-allyl-k-alkylidene co-ordinated. In contrast refluxing toluene solutions of [Ru 3 (CO) 3 (k-CO)(k3 -CF 3 C–– – CCF 3 )Cp 2 ] react with methyl-2- ynoate PhC–– – CPh and MeC–– – CMe to give the novel closo-pentagonal bipyramidal (OC)2Ru Ru C C C C R¢ R CF3 CF3 Ru Ru Ru C C C R CF3 CF3 C R¢ 42 41 (CO)3 Ru Ru 3 C 4 cluster [Ru 3 (CO) 3Mk3 -C 4 (CF 3 ) 2 R(R@)NCp 2 ] 42 which exists in two isomeric forms (R\Ph or Me) resulting from insertion of the alkyne into the C–– – C triple bond of hexafluorobut-2-yne or from straightforward linking of the alkyne with hexa- fluorobut-2-yne.The room-temperature products [Ru 3 (CO) 2 (k-CO) 2Mk-C 4 (CF 3 ) 2 - R(R@)NCp 2 ] were shown to be intermediates in the formation of [Ru 3 (CO) 2 (k-CO)Mk- C 3 (CF 3 ) 3N(k3 -CCF 3 )Cp 2 ] and [Ru 3 (CO) 3Mk3 -C 4 (CF 3 ) 2 R(R@)NCp 2 ] the former requiring C–C bond cleavage the latter C–C cleavage and C–C regeneration.84 Safarowie and Keister85 have determined the kinetics of isomerisation of [Ru 3 (k- H) 3 (CO) 9 (k3 -CSEt)] to [Ru 3 (k-H)(CO) 9 (k3 -g2-CH 2 SEt)] which involves a double C–H reductive elimination. An inverse dependence of the rate on carbon monoxide concentration is consistent with reversible CO dissociation prior to the rate-determining step but following an intramolecular rearrangement proposed as either hydride migration to give an agostic Ru–H–C bond or a change from k3 -CSEt to k2 -CSEt co-ordination.There is a distinct change in the mechanism of reductive elimination of C–Hbonds in clusters of the type [Ru 3 (k-H) 3 (CO) 9 (k3 -CX)] (X\Ph CO 2 Me or SEt) which depends highly upon the nature of the alkylidyne substituent from a CO associative (Ph) to CO independent (CO 2 Me) to CO dissociative pathway (SEt). The cationic cluster [Ru 3 (k-H)(CO) 8 (k3 -ampy)(k-g1 g2-PhC–– CHPh)]` 43 catalyses the hydrogenation of diphenylacetylene to cis- and trans-stilbene without hydrogenation of stilbene to 1,2-diphenylethane. The catalytic cycle was proposed to involve dissociation of the olefinic moiety of the alkenyl ligand oxidative addition of H 2 to form a trihydride cluster rapid reductive elimination of stilbene and insertion of 409 Organometallic chemistry of bi- and poly-nuclear complexes Ru Ru Ru N H H N H Ph Ph Ru Ru Ru N H H N H Ph Ph Ru Ru Ru N H H N H Ph Ph H H Ru Ru Ru N H H N H PhC CPh Ph2C2 Ph2C2H2 H2 43 + + + + Scheme 9 diphenylacetylene (Scheme 9).86 The triruthenium p-n-vinyl clusters [Ru 3 (k- H)(CO) 9 (k3 -Ph 2 PC 4 H 2 S)] and its diphenyl-2-thienylphosphine substituted counterpart [Ru 3 (k-H)(CO) 8 (k3 -Ph 2 PC 4 H 2 S)(Ph 2 PC 4 H 3 S)] were isolated from the reaction between [Ru 3 (CO) 12 ] and Ph 2 PC 4 H 3 S.Prolonged reflux of a toluene solution of [Ru 3 (k-H)(CO) 9 (k3 -Ph 2 PC 4 H 2 S)] gave two tetranuclear clusters [Ru 4 (CO) 11 (k4 - PPh)(k4 -C 4 H 2 S)] 44 and [Ru 4 (CO) 11 (k4 -PPh)(k4 -C 6 H 4 )] 45 by elimination of benzene and thiophene respectively.87 The P-co-ordinated diphenylvinylphosphine ligand in [M 3 (CO) 11 (Ph 2 PCH––CH 2 )] (M\Ru or Os) readily b-eliminates to give the p-n-alkenyl complexes [M 3 (k-H)(CO) 9 (k-g1 g2-Ph 2 PCH––CH)] containing a phosphino- substituted vinyl ligand.88 Reaction of the phosphazene chain Ph 2 SPN––PH 2 PPPh 2 ––NP(E)Ph 2 with [Ru 3 (CO) 12 ] under oxidative decarbonylation conditions gave [Ru 3 (CO) 6 (k3 -Se) 2 (k-PPh 2 )MPh 2 PNP(Ph) 2 NPPh 2N].89 Ru(CO)2 Ru (OC)3Ru Ru P Ph2 S Ru(CO)2 Ru P Ph2 Ru (OC)3Ru (OC)3 (OC)2 (OC)2 (OC)2 45 44 410 S.Doherty Both [Fe 2 Ru(CO) 12 ] and [FeRu 2 (CO) 12 ] undergo a phase change from a noncentrosymmetric ordered structure at low temperature (\223 and 173K respectively) to a disordered centrosymmetric phase at high temperature.90 The unusual g1-1-azavinylidene cluster [Ru 3 (k-H)(CO) 10 (k-N––CPh 2 )] 46 has been prepared by reacting [Ru 3 (CO) 12 ] with LiN––CPh 2 followed by protonation with trifluoroacetic acid.Notably this compound could not be prepared directly from benzophenone imine and [Ru 3 (CO) 12 ]. Reaction of the azavinylidene cluster with dppm gave [Ru 3 (k-H)(CO) 8 (k-g1-N–– CPh 2 )(k-dppm)] and [Ru 3 (k-H)(CO) 7 (k- N––CPh 2 )(k-dppm)(g1-dppm)]; the former loses CO to give [Ru 3 (k-H)(CO) 7 (k3 -g2- (OC)4Ru Ru Ru(CO)3 N H H C Ph Ph (OC)3Ru Ru Ru(CO)2 N H C Ph Ph P Ph2 PPh2 47 46 (CO)2 (CO)3 N––CPh 2 )(k-dppm)] 47 which contains a k3 -g2-1-azavinylidene ligand.Similarly [Ru 3 (k-H)(CO) 9 (PPh 3 )(k-N––CPh 2 )] loses CO to give [Ru 3 (k-H)(CO) 7 (PPh 3 )(k3 -g2- N––CPh 2 )] also containing a k3 -g2-1-azavinylidene ligand. Further substitution of carbon monoxide with PPh 3 in [Ru 3 (k-H)(CO) 9 (PPh 3 )(k-N––CPh 2 )] gave [Ru 3 (k- H)(CO) 8 (PPh 3 ) 2 (k-N––CPh 2 )] which is thermally unstable and reversibly orthometallates at a azavinylidene phenyl ring to give [Ru 3 (k-H) 2 (CO) 7 (PPh 3 ) 2Mk- N––CPh(C 6 H 4 )N].91 The reaction of [Ru 3 (CO) 12 ] with 3,N-diphenylprop-2-enimine gave [Ru 3 (k-H)(CO) 9 (k3 -g2-PhCH 2 CH 2 C––NPh)] as the major product together with [Ru 2 (CO) 6 (PhC–– CHCH 2 Ph)] [Ru 3 (CO) 6 (PhC––CHCH––NPh) 2 ] and [Ru 4 (CO) 10 (PhC–– CHCH–– NPh) 2 ]. Examples of these latter three clusters are well known whereas clusters analogous to [Ru 3 (k-H)(CO) 9 (k3 -g2-PhCH 2 CH 2 C––NPh)] containing a k3 - g2-imine ligand in which the C––C double bond has been hydrogenated have never been isolated from the reaction of [Ru 3 (CO) 12 ] with azadiene.92 Dodecacarbonyltriruthenium reacts with SnR 2 (R\C 6 H 2 Pr* 3 -2,4,6) to a§ord [Ru 3 (CO) 10 (k-SnR 2 ) 2 ] and [Ru 3 (CO) 9 (k-SnR 2 ) 3 ].Reaction of the pentanuclear cluster [Ru 3 (CO) 10 (k-SnR 2 ) 2 ] with additional diorganotin reagents SnR@2 [R@\R or CH(SiMe 3 ) 2 ] gave the corresponding hexametallic cluster [Ru 3 (CO) 9 (k-SnR 2 ) 2 (k- SnR@2 )] while [Ru 3 (CO) 10 (k-dppm)] reacted with [(SnR 2 ) 3 ] to give [Ru 3 (CO) 8 (k- SnR 2 ) 2 (k-dppm)].93 Reaction of SnR 2 (R\C 6 H 2 Pr* 3 -2,4,6 C 6 H 3 Et 2 -2,6 or C 6 H 2 Ph 3 -2,4,6) with [Fe 3 (CO) 12 ] each gave [Fe 2 (CO) 8 (k-SnR 2 )] whereas prolonged treatment with SnR@2 (R@\Me 5 C 6 ) ultimately gave the known compound spiro- [MFe 2 (CO) 8N2 (k4 -Sn)].Reaction of the tin or lead reagents [MCH(PPh 3 ) 2 ] (M\Sn or Pb) gave [Fe 2 (CO) 6 (k-CO)(dppm)] in near quantitative yield.94 Facile fragmentation of [Ru 3 (k-H)(CO) 9 (k3 -g2-SCNHPhNPh)] in the presence of excess diphenylthiourea gave [Ru(CO) 2 (g2-SCNHPhNPh) 2 ] containing two bidentate diphenylthioureato ligands. The room-temperature reaction of [Ru 3 (k- H)(CO) 9 (k3 -g2-SCNHPhNPh)] with PPh 3 results in CO substitution whereas two equivalents of PPh 3 leads to facile P–C bond cleavage to give the sulfido compound 411 Organometallic chemistry of bi- and poly-nuclear complexes [Ru 3 (CO) 7 (PPh 3 )(k-g2-C 6 H 5 )(k-PPh 2 )(k3 -S)] suggesting that the thioureato ligand is in fact ine§ective as a cluster stabilising ligand.95 Substitution of PPh 3 for CO in the hexanuclear cluster [Ru 6 (k-H)(CO) 16 (k3 -S)(k3 -g2-SCNHPhNPh)] occurs at the apical k-S-bonded ruthenium atom to give [Ru 6 (k-H)(CO) 15 (PPh 3 )(k-S)(k-g2- SCNHPhNPh)].While the diphenylthioureato ligand remains intact the ruthenium cluster framework experiences substantial bond length elongations. In contrast other two-electron donors such as Bu5NC P(OMe) 3 P(OPh) 3 PBu/ 3 and Me 2 S substitute for CO at the nitrogen-bonded ruthenium atom leaving the metal framework essentially unchanged.96 Reaction of Cl 2 PNPr* 2 with [Ru 4 (CO) 13 ]2~ and the reaction product of Na 2 [Os(CO) 4 ] and [Os 3 (CO) 12 ] provides a convenient route to the phosphinidenestabilised clusters [M 4 (CO) 13 (k3 -PNPr* 2 )] (M\Ru or Os).Thermolysis of [M 3 (CO) 13 (k3 -PNPr* 2 )] e§ected the loss of carbon monoxide to give the closo-five vertexM 4 P polyhedral cluster [M 4 (CO) 12 (k3 -PNPr* 2 )] 48. Hydrolysis of this product led to facile P–N bond cleavage with formation of [NH 2 Pr* 2 ][M 4 (CO) 12 (PO)] 49 containing a rare example of a triply-bridging phosphine monoxide ligand.97 M (OC)3M M(CO)3 P M NPri2 M (OC)3M M(CO)3 P O M (CO)3 (OC)3 (CO)3 – [NH2Pri 2] + H2O – M = Ru or Os 49 M = Ru or Os 48 (OC)3 Thermolysis of [Ru 3 (CO) 12 ] with cycloocta-1,4-diene gave [Ru 4 (CO) 12 (k4 -g2- C 8 H 10 )] and an isomer [Ru 4 (CO) 12 (k4 -g2 g2-C 8 H 10 )] butterfly clusters that contain 60 and 62 electrons respectively.TheHOMO–LUMOgap of the 60-electron cluster is large enough for it to be thermodynamically stable even though a closed butterfly structure contains a more stable [Ru 4 (CO) 12 ] fragment.98 Thermolysis of [Ru 3 (CO) 12 ] and 9-anthraacylphosphine gave several products including [Ru 3 (k- H)(CO) 8 (k3 -C 14 H 7 PPh 2 )] and [Ru 4 (CO) 11 (k4 -C 14 H 7 PPh 2 )] anthracyne complexes arising from double metallation of one of the unsubstituted rings and [Ru 5 (CO) 13 (k5 - g1 g2 g3 g3-C 14 H 9 -g1-PPh 2 )] 50 a bowtie cluster with an anthracene unit co-ordinated to the Ru 5 cluster framework via a g2-C–– C double bond two k3 -allyl groups and a g1-p-interaction.99 Reaction of [Ru 3 (CO) 12 ] with 1,3,5-triisopropenyl benzene in refluxing octane gave two isomeric clusters of the formula [Ru 4 (CO) 12 (C 15 H 20 )].Both contain a tetrahedral arrangement of metal atoms but di§er in their hydrocarbyl bonding modes due to hydrogenation of di§erent carbon atoms in the C 15 framework. In one isomer the two hydrogen atoms add across two of the unsaturated side arms to give g3-co-ordination of the C 15 H 20 hydrocarbon while addition across only one double bond a§ords the other isomer with an g2-co-ordinated C 15 H 20 hydrocarbon. 100 Johnson and co-workers101 isolated [Ru 3 (k-H)(CO) 9 (k3 -g1 g2 g1-C 7 H 8 )] 51 and [Ru 4 (CO) 11 (k4 -g1 g1 g2 g2 g2-C 7 H 6 )] 52 from the reaction of [Ru 3 (CO) 12 ] with 412 S. Doherty (OC)3Ru Ru(CO)3 Ru (OC)2Ru (OC)3Ru Ru Ru(CO)3 H H Ru Ru Ru Ph2P Ru Ru (CO)3 (CO)3 51 52 50 norbornene and norbornadiene respectively.The first contains a triangular ruthenium core with norbornadiene bonding through its alkenic bond and an agostic C–H· · ·Ru interaction. This mode of co-ordination was proposed to parallel that of norbornadiene absorbed on a Pt(111) surface. The other a tetranuclear butterfly framework of ruthenium atoms contains norbornadiene co-ordinated through both double bonds.101 Clusters ranging in nuclearity from five to seven were isolated from the reaction between 1,4-diisopropenylbenzene and [Ru 3 (CO) 12 ]. Of those isolated [Ru 6 H(CO) 15 (C 12 H 11 )] and [Ru 6 H(CO) 15 (C 12 H 13 )] are particularly noteworthy because their cluster atom frameworks have not previously been reported. Both consist of a distorted edge-bridged tetrahedron of ruthenium atoms with a ruthenium spike connected to the edge bridging ruthenium.The unsaturated organic ligand is g6- bonded through the arene ring to the spike ruthenium atom g2-co-ordinated through one isopropenyl unit and p-bonded through two carbon atoms and is overall an eleven-electron donor.102 The two cluster allyl compounds [Ru 3 (k-H)(CO) 9 (k3 - g1 g1 g2-C 3 H 2 Ph)] and [Ru 5 (k3 -H)(CO) 14 (k3 -g1 g1 g3 g3-C 3 H 2 Ph)] have been isolated from the reaction between [Ru 3 (CO) 12 ] and isopropenylbenzene. Thermolysis of an octane solution of the former cluster with [Ru 3 (CO) 12 ] gave the hexanuclear alkyne cluster [Ru 6 C(CO) 15 (k3 -g1 g1 g2-C 2 HPh)].103 Addition ofM@(CO) 2 (M@\Ru or Fe) to nido-[Ru 3 (CO) 9Mk3 -PC(CO)Bu5N2 ] gave the phosphinidene clusters [Ru 3 M@(CO) 10 (k-CO)Mk4 -PC(CO)Bu5N2 ] (M\Ru or Fe) via a cluster expansion reaction.104 Gaseous nitric oxide reacts with various high nuclearity ruthenium clusters. The carbide cluster [N(PPh 3 ) 2 ] 2 [Ru 6 C(CO) 16 ] gave [N(PPh 3 ) 2 ][Ru 6 C(CO) 15 (NO)] which reacted further with NO to give [Ru 5 C(CO) 14 (NO)(NO 2 )]. Similarly the allyl derivative [N(PPh 3 ) 2 ][Ru 6 C(CO) 15 (C 3 H 5 )] reacts with NO to give [Ru 6 C(CO) 14 (C 3 H 5 )(NO)] and [Ru 5 C(CO) 11 (C 3 H 5 )(NO) 2 (NO 2 )] clusters containing NO and NO 2 . The NO 2 was thought to form by disproportionation of two NO ligands co-ordinated to an unstable electron-rich intermediate a process accompanied by reduction in cluster nuclearity.105 A range of k-nitrene clusters has been prepared by thermolysis or pyrolysis of nitrosyl clusters. Methylation of [Ru 3 (CO) 10 (NO)]~ gave [Ru 6 (k-H)(CO) 16 (k-CO)(k4 -NH)Mk3 -g2-C(O)OMeN] and [Ru 3 (CO) 10 (NOMe)].Hydrogenation of the latter cluster gave [Ru 3 (k-H) 2 (CO) 9 (k3 -NH)] and [Ru 3 (k- H) 2 (CO) 9 (NOMe)] while in the presence of [Ru 3 (CO) 12 ] [Ru 6 (k-H)(CO) 16 (k- CO) 2 (k4 -NH)(k-OMe)] and [Ru 5 (k-H) 3 (CO) 13 (k4 -NH)(k3 -OMe)] were isolated. Thermolysis of [Ru 3 (CO) 10 (NOMe)] at 90 °C gave [Ru 4 (CO) 12 (k4 -N)(k-OMe)] and [Ru 6 (CO) 16 (k-CO) 2 (k4 -NH)(k-OMe) 2 ] and pyrolysis of [Ru 3 (k-H) 2 (CO) 9 (NOMe)] at 140 °C gave [Ru 6 (k-H) 2 (CO) 14 (k-CO) 2Mk5 -g2-NC(O)OMeN] [Ru 6 (CO) 15 (k- 413 Organometallic chemistry of bi- and poly-nuclear complexes CO) 2 (k4 -NH)(k-OMe)Mk-g2-N(H)C(O)OMeN] and [Ru 6 (CO) 16 (k-CO) 2 (k4 -NH)(k- OMe)(k-NCO)]. The crystal structures of these compounds were reported.106 The C 2 carbide fragment in [Ru 5 (k5 -C 2 )(k-PPh 2 ) 2 (k-SMe) 2 (CO) 11 ] couples with Me 3 SiC–– – CSiMe 3 via a vinylidene intermediate to give [Ru 5Mk4 -CCCCH(SiMe 3 )N(k- SMe)(k3 -SMe)(k-PPh 2 ) 2 (CO) 10 ] which has been converted into [Ru 5 (k4 - CCCCH 2 )(k3 -SMe)(k-SMe)(k-PPh 2 ) 2 (CO) 10 ] 53 via alkali hydrolysis to a§ord the first structurally characterised butatrienylidene cluster.The Ru 5 open envelope conformation of 53 converts into a spiked rhomboidal Ru 5 core upon carbonylation to [Ru 5 (k5 -CCCCH 2 )(k-SMe) 2 (k-PPh 2 ) 2 (CO) 11 ] 54.107 Addition of PhC–– – CR (R\Hor Ph) to [Ru 5 (CO) 11 (k5 -C 2 )(k-SMe) 2 (k-PPh 2 ) 2 ] gave [Ru 5 (CO) 10Mk5 - Ru(CO)2 Ru Ru (OC)2Ru C C C C H H P Ru(CO)2 PPh2 MeS S C C Ru Ru Ru(CO)2 S C PPh2 Ru(CO)3 SMe P (OC)2Ru C H H (CO)2 (OC)2 Ph2 Me (OC)2 (OC)2 Ph2 + CO Me 53 54 CCC(Ph)C(R)N(k-SMe) 2 (k-PPh 2 ) 2 ] via facile carbon–carbon coupling between the exposed dicarbide and PhC–– – CR.These two clusters (R\H or Ph) di§er in the mode of co-ordination of the unsaturated fragment. In one (R\H) the metal atoms retain a pentagonal skeletal framework while the other (R\Ph) contains an extra Ru–Ru bond to give an envelope-type ruthenium atom framework.108 Addition of CNBu5 to [Ru 5 (CO) 11 (k5 -C 2 )(k-SMe) 2 (k-PPh 2 ) 2 ] gave two products [Ru 5 (CO) 11 (CNBu5)(k5 - C 2 )(k-SMe) 2 (k-PPh 2 ) 2 ] and [Ru 5 (CO) 10 (CNBu5)(k5 -C 2 )(k-SMe) 2 (k-PPh 2 ) 2 ] which can be interconverted by loss or addition of carbon monoxide. Overall the addition of CNBu5 results in a flattening and expansion of the Ru 5 metal atom framework and movement of the C 2 unit into the plane of the Ru 5 skeleton.109 Thermolysis of [Ru 3 (CO) 12 ] in ethanol gave the hydridoruthenium cluster anion [Ru 10 H 2 (CO) 25 ]2~ while thermolysis in a methanol–water mixture gave [Ru 11 H(CO) 27 ]3~.Both complexes have been crystallographically characterised. Treatment of [Ru 6 H(CO) 18 ]~ with [Ru 3 (CO) 12 ] in refluxing diglyme also gave [Ru 10 H 2 (CO) 25 ]2~ in good yield. These transformations support a proposal for the formation of high nuclearity ruthenium clusters via a build-up series involving [Ru 6 H(CO) 18 ]~ [Ru 8 H 2 (CO) 21 ]2~ [Ru 10 H 2 (CO) 25 ]2~ and [Ru 11 H(CO) 27 ]3~.110 Reaction of the mixed-metal cluster [Cu 2 Ru 6 (CO) 16 (k6 -C)(NCMe) 2 ] with 1,5,9- trithiacyclododecane resulted in abstraction of the copper atoms to give [Cu(g3-[12] aneS 3 )(g1-[12]aneS 3 )] 2 [Ru 6 (CO) 16 (k6 -C)].111 The osmium(III) complex [Os 2 Br 4 Cp* 2 ] is a convenient starting material for the preparation of mono(pentamethylcyclopentadienyl)osmium complexes in the]2 oxidation state.For instance reaction with PR 3 (R\Me or Ph) or cod gave [OsBr(PR 3 ) 2 Cp*] and [OsBr(cod)Cp*] respectively which upon treatment with NaBH 4 gave the hydride [OsHL 2 Cp*] (L\PPh 3 ). Surprisingly reaction of 414 S. Doherty [OsBr(PMe 3 ) 2 Cp*] with NaBH 4 gave [OsH 2 (PMe 3 ) 2 Cp*]` which deprotonated with MeLi to give [OsH(PMe 3 ) 2 Cp*].112 The primary secondary and tertiary alkane thiolate osmium(VI) nitrido complexes [MOs(N)(CH 2 SiMe 3 ) 2 (k-SR) 2N2 ] have been prepared from [MOsCl(N)(CH 2 SiMe 3 ) 2N2 ] and the corresponding alkali-metal thiolate (R\CH 2 CH 3 CMe 3 CHME 2 CH 2 CHMe 2 or CH 2 Ph).113 Three novel osmium clusters were isolated from the reaction of [MOs(CO) 4 (SnMe 2 )N2 ] under vastly di§erent conditions.Firstly pyrolysis at 170 °C gave [MOs(CO) 3 (SnMe 2 )N3 ] containing a Os 3 Sn 3 triangulated raft-like metal cluster framework whereas UV irradiation gave [Os 4 (CO) 14 (SnMe 2 ) 4 ] characterised by a central Os 2 Sn 2 rhomboidal framework linked to two outer Os 2 Sn triangles via the osmium atoms. Finally treatment of a solution of [MOs(CO) 4 (SnMe 2 )N2 ] with Me 3 NO gave [Os 4 (CO) 14 (k3 -O) 2 (SnMe 2 ) 4 ] containing a central six-membered (OsOSn) 2 ring.114 Several mixed osmium–germanium clusters have been prepared and structurally characterised including [MOs(CO) 3 (GeMe 2 )N3 ] [Os 3 (CO) 11 (GeMe 2 ) 2 ] [Os 4 (CO) 12 (GeMe 2 ) 4 ] and [Os 2 (CO) 6 (GeMe 2 ) 2 ].The majority of these clusters have structural analogues among the binary carbonyls of osmium.115 Three clusters were isolated from the reaction between [Os 3 (CO) 10 (NCMe) 2 ] and C 6 F 5 N–– NNHC 6 F 5 two isomers of [Os 6 (k-H)(CO) 20 (NCMe) 2 (k-g2- C 6 F 5 NNNC 6 F 5 )] and [Os 6 (k-H)(CO) 19 (g2-C 6 F 5 NNNC 6 F 5 )]. The former clusters are thermodynamically unstable and readily convert into the latter via Os–Os bond formation.116 Reaction of RN––NNHR with [Ru 3 (CO) 12 ] at 80 °C gave [Ru 3 (k- H)(CO) 10 (k-RNNNR)] (R\p-C 6 H 4 X; X\F Cl Br I or H) whereas the linear triosmium cluster [Os 3 Cl(CO) 11 (g2-RNNNR)] was isolated from the reaction of RN–– NNHR with [Os 3 (CO) 11 (NCMe)] in CH 2 Cl 2 .Both clusters contain the triazenide ligand the first bridging a Ru–Ru bond in a closed Ru 3 cluster the latter chelating the terminal osmium atom in a linear Os 3 array.117 The Schi§ base NC 5 H 4 CH––NC 6 H 4 OC 16 H 30 reacts with [Os 3 (k-H)(CO) 9 (k3 -CCl)] in the presence of DBU to give [Os 3 (k-H) 2 (CO) 9 (k3 -CCl)MNC 5 H 4 CH––NC 6 H 4 OC 16 H 30N] and with [Os 3 (CO) 10 (NCMe) 2 ] to give the orthometallated cluster [Os 3 (k-H)(CO) 10 (k2 - NC 5 H 4 CH––NC 6 H 4 OC 16 H 30 )]. The low-energy visible absorptions of the former cluster display a marked negative solvatochromism while the absorptions of the latter are relatively insensitive to the nature of the solvent.118 Os(CO)3 Os(CO)4 (OC)3Os H N N Os Os(CO)3 (OC)3Os H H N Os Os(CO)3 (OC)3Os H H – CO 55 56 (CO)3 (CO)3 56 H 415 Organometallic chemistry of bi- and poly-nuclear complexes The transformations of bicyclic trinitrogen heterocycles on triosmium clusters have been investigated.Indoline reacts with [Os 3 (CO) 10 (NCMe) 2 ] to give [Os 3 (k- H)(CO) 10 (k-g2-C 8 H 7 NH)] 55 which decarbonylates to give a tautomeric mixture of [Os 3 (k-H) 2 (CO) 9 (k3 -g2-C 8 H 7 N)] 56; the first tautomer has a k-alkylidene–imino bonding mode the other a k-amido–aryl bonding mode. Continued thermolysis of these tautomers gave the dehydrogenated clusters [Os 3 (k-H) 2 (CO) 9 (k3 -g2- C 8 H 4 NH)]. Direct analogues of 55 and 56 have also been isolated from the reaction between tetrahydroquinoline (thq) and [Os 3 (CO) 10 (NCMe) 2 ] in addition to [Os 3 (k- H)(CO) 10Mk-g1-C 9 H 10 (CH 3 )CNN] the product of nucleophilic attack of thq on MeCN.The dehydrogenation product [Os 3 (k-H)(CO) 9 (k-g2-C 9 H 8 N)] was obtained by thermolysis of [Os 3 (k-H) 2 (CO) 9 (k-g2-C 9 H 9 N)].119 Treatment of a dichloromethane solution of [Os 3 (CO) 10 (NCMe) 2 ] with [N(PPh 3 ) 2 ][NO 2 ] at 40 °C gave [N(PPh 3 ) 2 ][Os 3 (CO) 10 (k-g2-NO 2 )] which was subsequently protonated to give neutral [Os 3 (k-H)(CO) 10 (k-g2-NO 2 )] in high yield.120 Reaction of [Os 3 (CO) 10 (NCMe) 2 ] or [Os 3 (CO) 11 (NCMe)] with HD or a mixture of H 2 –D 2 gave all three possible isotopomers. Similarly all three isotopomers were generated by mixing [Os 3 H 2 (CO) 10 ] with D 2 . This behaviour is consistent with the formation of a highly fluxional 48-electron tetrahydrido–deuterido complex [Os 3 H 2 D 2 (CO) 10 ] followed by rapid reductive elimination to a§ord the observed isotopomer distribution.121 The red unsaturated cluster [Os 3 (CO) 9 (k-dppm)(k3 -g2-r-PhC–– – CPh)] reacts with phosphorus donor ligands L [L\PBu 3 PPh 3 PMe 2 Ph or P(OMe) 3 ] to a§ord the saturated 48-electron clusters [Os 3 (CO) 7 (k-dppm)L(k3 -g1 g2-E-PhC–– – CPh)] which exist in several isomeric forms.The P(OMe) 3 derivative [Os 3 (CO) 7MP(OMe) 3N(k- dppm)(k3 -g2-E-PhC–– – CPh)] undergoes facile decarbonylation to give the unsaturated cluster [Os 3 (CO) 6MP(OMe) 3N(k3 -g1 g2-r-PhC–– – CPh)] containing a k3 -g2-r-alkyne. 122 Triosmium clusters have been introduced into bovine serum albumin (BSA) by acylation of the free amino functionality in [Os 3 (CO) 10 (k3 -g2-L] (L\succinimido- 4-pentynoate).123 S N Me (OC)3Os Os(CO)4 Os(CO)3 H S N Me (OC)3Os Os Os(CO)3 H Me S N (CO)2 H N S (OC)3Os Os Os(CO)3 H Me S N (CO)2 H Me 4-methylthiazole + 59 58 57 416 S.Doherty Several triosmium and triruthenium clusters of 4-methylthiazole have been prepared and their reaction with 4-methylthiazole and PPh 3 examined. The lightly stabilised cluster [Os 3 (CO) 10 (NCMe) 2 ] reacts with 4-methylthiazole to give [Os 3 (k- H)(CO) 10Mk-C––NCMe––CHSN] 57 which reacts with PPh 3 to give the mono- and bis-phosphine substituted products [Os 3 (k-H)(CO) 9 (k-C––NCMe––CHS)(PPh 3 )] and [Os 3 (k-H)(CO) 8 (k-C––NCMe–– CHS)(PPh 3 ) 2 ] which exist in a number of isomeric forms. Further reaction of 57 with 4-methylthiazole gave [Os 3 (k-H) 2 (CO) 8 (k- C––NCMe––CHS)(k-C––CMeN–– CHS)] 58 and [Os 3 (k-H) 2 (CO) 8 (k-C––NCMe––CHS) 2 ] 59 each containing two methylthiazole ligands; the first has one C,N- and one C,S-bound thiazole while the latter contains both C,N-co-ordinated.The reaction of [Ru 3 (CO) 12 ] with one equivalent of 4-methylthiazole in the presence of a catalytic amount of benzophenone–sodium promoter gave [Ru 3 (k-H)(CO) 10 (2,3-g2- CNCMe––CHS)] while two equivalents of thiazole gave [Ru 3 (k-H) 2 (CO) 8 (2,3-g2- CNCMe––CHS) 2 ].124 Several products have been isolated from the reaction of 1- hydroxypyridine-2-thione with [Os 3 (CO) 11 (NCMe)] including [Os 3 (k-H)(CO) 10Mk- g1-SC 5 H 4 N(O)N] [Os 3 (k-H)(CO) 10Mg2-SC 5 H 4 N(O)N] and [Os 3 (k-H)(CO) 9Mk-g1 g2- SC 5 H 4 N(O)N]. Thermolysis of [Os 3 (k-H)(CO) 10Mk-g1-SC 5 H 4 N(O)N] results in N–O bond cleavage to give [Os 3 (k-H)(CO) 9 (k3 -pyS)] and [Os 3 (CO) 9 (k-OH)(k3 -pyS)].The complex [Os 3 (k-H)(CO) 9Mk-g1 g2-SC 5 H 4 N(O)N] was shown to be an intermediate in this transformation.125 Diphenylmercury reacts with [Os 3 (k-H)Br(CO) 9Mk-g2- C––N(CH 2 ) 3N] to give [Os 3 (CO) 10 (k-g1-C 6 H 5 )Mk-g2-C––N(CH 2 ) 3N] which readily converts into [Os 3 (CO) 8Mk-g2-C–– N(CH 2 ) 3N(k-g1 g6-C 6 H 5 )] when heated at reflux in n-octane.126 Os Os Os CO CO CO Ph3P CO CO H OC OC CO Os Os Os CO CO CO OC CO CO OC OC CO H PPh3 CO 60 61 The solid-state structures of both isomers of the p-n-vinyl cluster [Os 3 (k- H)(CO) 9 (PPh 3 )(k-g1 g2-CH––CH 2 )] have been determined by single-crystal X-ray crystallography. Proton NMR spectroscopic studies revealed slow equilibration of these isomers.Variable-temperature 13C NMR spectroscopic studies were used to probe the basis of the non-degenerate p–n interchange of the vinyl group in 60; by analogy the line shape broadening associated with isomer 61 was tentatively proposed to be the result of a similar p–n exchange albeit with a higher energy barrier.127 Thermolysis of the activated tetraosmium hydride cluster [Os 4 (k- H) 4 (CO) 10 (NCMe) 2 ] with cyclohexa-1,3-diene produced the known clusters [Os 4 (k- H) 2 (CO) 12 (g2-C 6 H 8 )] [Os 4 (k-H) 3 (CO) 11 (k-g1 g2-C 6 H 9 )] [Os 4 (k-H) 2 (CO) 11 (g4- C 6 H 8 )] [Os 4 (k-H) 2 (CO) 10 (g6-C 6 H 6 )] and [Os 4 (CO) 9 (g4-C 6 H 8 )(g6-C 6 H 6 )] together with the previously uncharacterised compounds [Os 4 (k-H)(CO) 10 (k3 -g1 g2 g1- C 6 H 8 )(g3-C 6 H 9 )] [Os 4 (k-H) 2 (CO) 10 (g6-C 6 H 5 C 6 H 9 )] and [Os 5 (k-H) 2 (CO) 13 (g4- C 6 H 8 )].Dehydrogenation of the cyclohexyl moiety in [Os 4 (k-H) 3 (CO) 11 (k-g1 g2- C 6 H 9 )] gave low yields of a compound formulated as the new cyclohexyne cluster [Os 4 (k-H) 2 (CO) 11 (k3 -g 1 g2 g1-C 6 H 8 )]. This work serves to demonstrate that the 417 Organometallic chemistry of bi- and poly-nuclear complexes cyclohexa-1,3-diene moiety can undergo hydrogenation to form cyclohexyl and allylic complexes and isomerisation to yield cyclohexyne rings or dehydrogenation via C–H activation to a§ord g6-benzene derivatives.128 Treatment of the unsaturated cluster [Os 3 H 2 (CO) 10 ] with [Hg(C–– – CPh) 2 ] gave two new Os–Hg containing clusters [Os(CO) 4MHgOs 3 (CO) 10 (k-g1 g2-CH––CHPh)N2 ] and [MOs 3 (CO) 10 (k-g1 g2-CH––CHPh)N2 (k4 -Hg)]; the latter contains two Os 3 Hg butterfly clusters linked through a wingtip atom.In refluxing thf [MOs 3 (CO) 10 (k-g1 g2- CH–– CHPh)N2 (k4 -Hg)] undergoes a redistribution reaction with [HgMMo(CO) 3 CpN2 ] to a§ord [MOs 3 (CO) 10 (k-g2-CH––CHPhN(k3 -Hg)MM(CO) 3 CpN]. The reaction of [Os 3 H 2 (CO) 10 ] with RHgC–– – CHgR (R\Me Et or Ph) also involved facile Hg–C bond cleavage and gave [MOs 3 (CO) 10 (k-g1 g2-CH–– CH 2 )N(k4 -Hg)MOs 3 (CO) 10 (k-H)N] and [MOs 3 (CO) 10 (k-g1 g2-CH–– CH 2 )N2 (k4 -Hg)].129 Thermolysis of a solution of [Os 3 (CO) 11 (PH 3 )] and [Os 3 (CO) 11 (NCMe)] results in the formation of [Os 6 (k-H)(CO) 22 (k-PH 2 )] which contains two Os clusters linked through a phosphido bridge together with the by-product phosphinidene cluster [Os 6 (k-H) 2 (CO) 21 (k-PH)].Similarly reaction of [Os 3 (CO) 11 (PH 3 )] with [Os 3 (CO) 10 (NCMe) 2 ] gave [Os 6 (k-H)(CO) 21 (NCMe)(k-PH 2 )]. Deprotonation of [Os 6 (k-H)(CO) 22 (k-PH 2 )] in the presence of [N(PPh 3 ) 2 ]Cl led to the formation of a mixture of anions [Os 6 (k-H)(CO) 21 (k-PH)]~ and [Os 6 (CO) 22 (k-PH 2 )]~ which when heated at reflux in xylene decarbonylate to give [Os 6 (CO) 18 (k6 -P)]~. The same interstitial phosphido cluster was obtained by heating [Os 6 (k-H)(CO) 22 (k-PH 2 )] in xylene while thermolysis of [Os 6 (k-H)(NCMe)(CO) 21 (k2 -PH 2 )] gave [Os 6 (k-H)(k6 - P)(CO) 18 ].130 Braga et al.131 have examined the crystal structures of several organometallic complexes and found that a number contain M–H· · ·O hydrogen-bonding interactions that involve the metal hydride and a carbonyl oxygen atom.The donor capacity of theM–H group appears to be similar to that of C–H andM–H· · ·CO bonds are of comparable strength to those of C–H· · ·O hydrogen bonds.131 An alkoxide–alcohol mobile phase has been used to obtain electrospray mass spectra of various neutral metal carbonyl complexes including; [M 2 (CO) 10 ] (M\Mn or Re) [Ru 3 (CO) 12 ] [Ru 6 (C)(CO) 17 ] [Ir 4 (CO) 12 ] [Ru 6 (C)(CO) 14 (g6-C 6 H 5 Me)] and [Os 4 (CO) 10 -(g6- C 6 H 6 )].132 7 Cobalt rhodium and iridium Kerr and co-workers133 have developed a Me 3 NO-promoted Pauson–Khand reaction that uses gaseous ethylene both under atmospheric pressure and autoclave conditions; in the latter case reduced yields were obtained as the pressure approached 50 atm.This methodology has been used to perform a key transformation in the synthesis of (])-taylorione. A range of alkynepentacarbonylcobalt complexes of (R)- (])-Glyphos has been prepared both under thermal conditions and at room temperature using N-methylamine-N-oxide as promoter.134 Thermolysis of the diyne-bridged complexes [MCo 2 (CO) 6N2 (PhC 4 Ph)] with bma gave [Co 2 (CO) 4 (bma)(PhC 4 Ph)Co 2 (CO) 6 ] a thermally unstable product that readily loses [Co 2 (CO) 8 ] to give [Co 2 (CO) 4Mk-g2 g2 g1 g1-Z-Ph 2 P(Ph)C––C(PhC 2 )C–– 418 S. Doherty C(Ph 2 P)C(O)OC(O)N] via P–C bond cleavage and functionalisation of the diyne with the transient bma and phosphido moieties. Thermolysis of [Co 2 (CO) 4 (bma)(PhC 4 Ph)- Co 2 (CO) 6 ] with excess bma gave additional binuclear complexes [Co 2 (CO) 2 (bma) 2 ] and [Co 2 (CO) 2 (bma)Mk-C––CPPh 2 C(O)O(CO)N(k-PPh 2 )] the former containing two intact bma ligands the latter a phosphido bridge and a p-co-ordinated bma.Cyclic voltammetry data are consistent with low-potential redox couples associated with low lying n* orbitals of the bma ligand.135 Thermolysis of [Co 4 (CO) 9 (C 6 H 3 Me 3 -1,3,5)] with bma gave four products; [Co 2 (CO) 2 (bma) 2 ] [Co 2 (CO) 2 (bma)Mk- C––C(PPh 2 )C(O)OC(O)N(k-PPh 2 )] [Co 3 (CO) 7Mk-g2 g1-P(Ph)C–– C(PPh 2 )C(O)OC- (O)N] and [Co 3 (CO) 6 (PPh 3 )Mk-g2 g1-P(Ph)C––C(PPh 2 )C(O)OC(O)N]. The PPh 3 ligand in the last cluster has been suggested to arise from P–C (phenyl) bond cleavage and transfer to a bridging k-PPh 2 itself generated via P–C (maleic anhydride) cleavage. Both trinuclear clusters are redox-active and show an irreversible one-electron oxidation and two one-electron reductions.The second one-electron reduction is associated with dissociation of the C––C bond of maleic anhydride from the cluster. EHMOcalculations suggest that the instability associated with this reduction is due to unfavourable maleic anhydride n*–Co 3 interactions present in the LUMO.136 Protonation of [Co 3 (k3 -g2 g2-arene)Cp 3 ] (arene\isopropylbenzene 1,4-diethylbenzene 1,2-diphenylethane or 1,1-diphenylethane) yields the hydrido clusters [Co 3 (k3 -H)(k-g2 g2 g2-arene)Cp 3 ]` whereas derivatives containing unsaturated groups (a-methylstyrene p-methylstyrene or p-methoxystyrene) attached to the arene ring protonate at the b-carbon of the side chain to give [Co 3Mk3 -a,1-g2 2-4-g3 4-6- g3(R3)C 6 H 4 -1-C(CH 2 R1)(R2)NCp 3 ]` (R1\R2\H R3\Me or OMe; R1\ R2\Me).The HOMO of the former cluster is localised largely on the cobalt atoms while the latter has a large LCAO amplitude on the b-carbon atom of the styryl group which also carries a substantial negative charge. It appears that both charge and overlap control the preferred site of protonation.137 Reaction of LiMe with [M(acac)Cp*] gave [M 3 (k-H)(k3 -CH)Cp* 3 ] (M\Ni or Co) the crystal structures of which are isomorphous. Both compounds contain metal atoms in oxidation state 7/3 with identical co-ordination numbers but di§erent electron counts. The complex [Co 3 (k3 -CH)(k-H)Cp* 3 ] reacts with dihydrogen to give diamagnetic [Co 3 (k3 - CH)(k[H) 3 Cp* 3 ].138 The trinuclear cluster [Co 3 (CO) 9 (k3 -CR)] reacts with 1,3,5-trithiane 2-methyl-2,4- dimethyl-2,4,6-trimethyl- 2-benzyl- and 2,4,6-tribenzyl-1,3,5-trithiane to give the trisubstituted products [Co 3 (CO) 6 (k3 -CR)(k3 -SCHR1SCHR2SCHR3)].In all cases the trithiane ligand adopts a chair conformation and caps a triangular Co 3 face using all three sulfur atoms. In contrast the nine-membered crown thioether 1,4,7- trithiacyclononane a§ords [Co 3 (k-CO)(CO) 5 (k3 -CR)MS(CH 2 CH 2 ) 3N] in which all three sulfur atoms of the ligand are co-ordinated to a single cobalt atom.139 The diphenylvinylphosphine-substituted clusters [Co 3 (CO) 9~n(Ph 2 PCH––CH 2 )n(k3 -CR)] (R\Me or CO 2 Me n\1 or 2) have been prepared; in the case of [Co 3 (CO) 8 (Ph 2 PCH––CH 2 )(k3 -CR)] loss of carbon monoxide results in co-ordination of the vinyl moiety of the Ph 2 PCH–– CH 2 ligand to give [Co 3 (CO) 7 (Ph 2 PCH––CH 2 )(k3 - CR)].140 Condensation of [Co 2 (CO) 6 (k-PPh 2 ) 2 ] with PhC–– – CPh gave [Co(CO) 2MPPh 2 CPh––CPhCOC(O)CPhCPhN] containing a diphenylphosphine vinyl substituted lactonyl ring g3-g1(P)-co-ordinated to cobalt.Insertion of CO into the lactonyl ring gave [Co(CO) 2MPh 2 PCPh–– CPhCC(O)OC(O)CPhCPhN] in which the 419 Organometallic chemistry of bi- and poly-nuclear complexes lactonyl group has been converted into a cyclic anhydride also g3-co-ordinated to cobalt.141 Methylation of the heterobinuclear complex [IrRh(CO) 3 (k-dppm) 2 ] gave [IrRh(CH 3 )(CO) 3 (k-dppm) 2 ][CF 3 SO 3 ] with the methyl and two terminal carbonyls co-ordinated to the iridium. At ambient temperature [IrRh(CH 3 )(CO) 3 (k-dppm) 2 ] [CF 3 SO 3 ] reacts with H 2 to liberate methane and give [IrRh(H)(k-H) 2 (CO) 2 (k- dppm) 2 ][CF 3 SO 3 ] via the intermediate dihydride [IrRh(H) 2 (CH 3 )(CO) 2 (k-dppm) 2 ] [CF 3 SO 3 ].Reaction of [IrRh(CH 3 )(CO) 3 (k-dppm) 2 ][CF 3 SO 3 ] with SO 2 promoted metal-to-metal methyl migration to give [IrRhMC(O)CH 3N(CO) 2 (k-SO 2 )(k-dppm) 2 ] [CF 3 SO 3 ] presumably via an alkyl-bridged complex; reaction with Bu5NC gave the iminoacyl [IrRh(CO) 2 (k-Bu5N––CMe)(k-dppm) 2 ][CF 3 SO 3 ] via migratory insertion. 142 The low-temperature reaction between [Ir 2 H(CO) 3 (k-CH 2 )(k-dppm) 2 ] [CF 3 SO 3 ] and acetylene and phenylacetylene yields the alkyne- and vinylidenebridged complexes [Ir 2 (CH 3 )(CO) 3 (k-HC–– – CH)(k-dppm) 2 ][CF 3 SO 3 ] and [Ir 2 (CH 3 )(CO) 3Mk-C––C(H)PhN(k-dppm) 2 ][CF 3 SO 3 ] respectively the former via the acetylide–hydride [Ir 2 (H)(CH 3 )(CO) 3 (k-C–– – CH)(k-dppm) 2 ][CF 3 SO 3 ].Under similar conditions the mixed-metal dimer [IrRh(CH 3 )(CO) 3 (k-dppm) 2 ][CF 3 SO 3 ] reacts with acetylene to a§ord [IrRh(CH 3 )(CO) 3 (k-HC–– – CH)(k-dppm) 2 ][CF 3 SO 3 ] and then [IrRh(CH 3 )Mk-g1 g2 g1-HCC(R)PPh 2 CH 2 PPh 2N(k-dppm) 2 ][CF 3 SO 3 ] via P–C bond formation between the bridging dppm and the rhodium end of the alkyne. Phenylacetylene reacts with [IrRh(CH 3 )(CO) 3 (k-dppm) 2 ][CF 3 SO 3 ] to give the unstable methyl–hydrido–acetylide [IrRh(H)(CH 3 )(CO) 3 (k-C–– – CPh)(k-dppm) 2 ] [CF 3 SO 3 ] which reacts with excess phenylacetylene to give [IrRh(H)(CO) 2 (C–– – CPh)(k-C–– – CPh)(k-dppm) 2 ][CF 3 SO 3 ].143 Addition of excess [MgCl(allyl)] to rac-[Rh 2 (nba) 2 L][BF 4 ] 2 gave [Rh 2 (g3-allyl) 2 L] [L\(Et 2 PCH 2 CH 2 )PhPCH 2 Ph(CH 2 CH 2 PEt 2 )] which exhibits slow non-selective hydroformylation activity contrasting with the dication [Rh 2 (nba) 2 L][BF 4 ] 2 which is a highly active and regioselective hydroformylation catalyst for alk-1-enes.In the presence of carbon monoxide the bis(acyl) product [Rh 2 (COC 3 H 5 ) 2 (CO) 4 L] is formed presumably via an g1-allyl intermediate. Further reaction with H 2 –CO under pressure eliminated the unsaturated aldehyde to give the unsymmetrical binuclear complex [Rh 2 (k-CO)(CO) 3 (g3 g1-L)].144 Addition of secondary silanes SiH 2 RR@ to [MRh(dippe)N2 (k-H) 2 ] gave [Rh 2 -(k–H)(k-g2-HSiRR@)(dippe) 2 ] containing an agostic Rh–Si–H interaction. Addition of one equivalent of carbon monoxide then led to loss of H 2 and the formation of [Rh 2 (k-SiRR@)(k-CO)(dippe) 2 ].The hydride complex [Rh 2 (k-H) 2 (dippe) 2 ] is an e§ective catalyst precursor for the hydrosilylation of olefins by diphenylsilane to give Ph 2 SiEt 2 and Ph 2 SiBuH at ambient temperature and pressure and a catalytic cycle has been proposed.145 Addition of one equivalent of HBF 4 to the binuclear acetylide complexes [Rh 2 (CO) 2 (PCy 3 ) 2 (k-O 2 CMe)Mk-g1 g2-C 2 C(OH)R 2N] (R\Ph CO 2 Me or SiMe 3 ) a§ords the k-p,p-allenylidene complexes [Rh 2 (CO) 2 (PCy 3 ) 2 (k-O 2 CMe)(k-p,p- C––C––CR 2 )][BF 4 ] formally unsaturated and containing 30 valence electrons and a single Rh–Rh bond. A study of the bonding in the model k-p p-allenylidene complex [Rh 2 (CO) 2 (PH 3 ) 2 (k-O 2 CMe)(k-p p-C––C–– CH 2 )]` successfully rationalises the 30 valence electron count and reveals a net acceptor behaviour for the unsaturated g1- hydrocarbyl ligand.146 Treatment of [NBu 4 ][Ir 4 Br(CO) 11 ] with [Fe(g5-P 3 C 2 Bu5 2 )(g5-P 2 C 3 Bu5 3 )] gave 420 S.Doherty [Ir 4 (CO) 11MFe(g5-P 3 C 2 Bu5 2 )(g5-P 2 C 3 Bu5 3 )N] via co-ordination through one phosphorus atom of the g5-P 3 C 2 Bu5 2 ring. Similarly [NBu 4 ][Ir 4 Br(CO) 11 ] reacts with [FeCpMg5-P 3 C 2 Bu5 2N] to give [Ir 4 (CO) 11MFeCp(g5-P 3 C 2 Bu5 2 )N]. Further treatment of [Ir 4 (CO) 11MFeCp(g5-P 3 C 2 Bu5 2 )N] with [NBu 4 ][Ir 4 Br(CO) 11 ] in the presence of AgSbF 6 gave [Ir 4 H(CO) 10MFeCp(g5-P 3 CH 2 (CMe 2 )(CBu5)NIr 4 (CO) 11 ] via metallation of one of the C 2 P 3 ring Bu5 groups.147 The monodentate 2-(diphenylphosphino) pyridine ligands in [Ir 4 (CO) 10 (PPh 2 py) 2 ] are both co-ordinated at basal iridium atoms one axial and the other equatorial.Reaction with [Cu(NCMe) 4 ][BF 4 ] and AgPF 6 gave [Ir 4 M(CO) 10 (PPh 2 py) 2 ]Y (M\Cu Y\BF 4 ; M\Ag Y\PF 6 ) with Moccupying an apical site of a trigonal bipyramidal IrM 4 cluster.148 Reaction of the dinuclear IrII compound [MIr(k-SPr*)Cp*N2 ] with S 8 gave the novel k-S 9 nonasulfido-bridged complex [MIr(k-SPr*)Cp*N2 (k-S 9 )] which reacts with NaBPh 4 to give the paramagnetic IrIII–IrIV complex [MIr(k-SPr*)Cp*N2 (k-S 2 )]- [BPh 4 ].149 8 Nickel Palladium and Platinum The new cyclopentadienylnickelamido complexes [MNi(k-NHR)(g5-C 5 Me 4 R@)N2 ] (R\Ph p-tolyl 2,6-xylyl or Bu5; R@\Me or Et) are dimeric in both solution and the solid state and undergo cis–trans isomerisation via Ni–N bond cleavage rotation of the amido group and reco-ordination of the amido ligand.Reaction of [MNi(k-NH(p- Tol)]Cp* 5 )N2 ] with CO and CNBu5 gave the insertion products [Ni(CO)MC(O)NH(p- Tol)NCp*] and [Ni(CNBu5)MCN(Bu5)NH(p-Tol)NCp*] respectively.150 The halide- and pseudo-halide-substituted vinylidene-bridged binuclear A-frame complexes [Ni 2 X 2Mk-C––CH 2N(PR 2 CH 2 PR 2 )] (X\Cl Br or I R\Me; X\Cl Br NCS or OCN R\Ph) have been prepared and characterised. The bonding between the [Ni 2 Cl 2 (PH 2 CH 2 PH 2 ) 2 ]2` framework and the vinylidene fragment (C––CH 2 )2~ was examined using EHMO calculations. The HOMO was found to be primarily metal based with significant dp* character and b 2 symmetry with a minor contribution from a vinylidene n orbital.TheLUMOhas substantial vinylidene n* character and b 1 symmetry. This MO picture was used to account for the trends in the electronic absorption spectra of these compounds.151 Addition of TePPr/ 3 to a solution of [Ni 2 (dppm) 3 ] gave [Ni 3 (k-Te) 2 (k-dppm) 3 ] a highly redox-active metal cluster with three reversible electrochemical couples. Treatment of [Ni 3 (k-Te) 2 (k-dppm) 3 ] with one and two equivalents of [FeCp 2 ][PF 6 ] gave [Ni 3 (k-Te) 2 (k-dppm) 3 ]` and [Ni 3 (k- Te) 2 (k-dppm) 3 ]2` containing 49 and 48 electrons respectively; the monocation has been crystallographically characterised.152 The diplatinum and platinum–palladium complexes [RPt(k-H)(k-dppm)MR1] [PF 6 ] (R R1\Me Et or Ph;M\Pt or Pd) react with HCl with metal–carbon bond cleavage.Reaction of the dipalladium complexes is more rapid than with the diplatinum counterparts while the reactivity of the mixed platinum–palladium complexes is complicated yielding products of Pd–C and Pt–C bond cleavage. In the latter case Pt–C bond cleavage is followed by migration of the remaining R group from palladium to platinum.153 The phosphine ligands in [Pd 2 (k-PBu5 2 )(PR 3 ) 4 ]` (PR 3 \PCy 2 H or PMe 3 ) can be substituted with CO or isoprene to give [Pd 2 (k-PBu5 2 )(PR 3 ) 3 (CO)]` and [Pd 2 (k- 421 Organometallic chemistry of bi- and poly-nuclear complexes PBu5 2 )(PR 3 ) 2Mk-g2 g2-H 2 C––CHC(Me)–– CH 2N]` respectively. In the case of [Pd 2 (PBu5 2 )(PCy 2 H) 3 (CO)]` bridge–terminal exchange of PCy 2 H for PBu5 2 gave a mixture of isomers with that containing a k2 -PCy 2 group and a terminal PBu5 2 H ligand the most stable.154 In refluxing toluene the dinuclear PtII complex [MPt(H)(PBu5 2 H)(k-PBu5)N2 ] reacts with CO to give the 44-electron PtI 2 PtII triangular complex [Pt 3 (H)(CO) 2 (k-PBu5) 3 ] via reductive elimination of PBu5 2 H.155 Oxidatively induced reductive elimination of PPh 2 and the terminal phenyl group in [Pt 3 Ph(PPh 3 ) 2 (k-PPh 2 ) 3 ] in the presence of I 2 gave the cationic cluster [Pt 3 (k- I)(PPh 3 ) 3 (k-PPh 2 ) 2 ]`.While the conversion of tertiary phosphine ligands into phosphido bridges via thermal P–C bond cleavage is known this is the first example of the cluster-mediated conversion of a M–C bond into a P–C bond of a tertiary phosphine. 156 A series of k-allyl PdIPdI complexes has been prepared from [MPd(k-Cl)[k- CH 2 C(R)CH 2 ]N2 ] and [Pt(C 2 H 4 )(PPh 3 ) 2 ](R\H 1-Me 1-Ph 1-CO 2 Me 1-Cl 2-Cl 2-CO 2 Me 2-CN or 2-SO 2 Ph).Allyl ligands containing withdrawing substituents were found to co-ordinate to the PdIPdI dimer more strongly than those with less electronwithdrawing substituents. A distinct preference for an anti configuration of the 1- substituted allyl ligand was also noted. Ab initio MO/MP2 calculations performed on [Pd 2 (k-Br)(k-CH 2 CHCH 2 )(PH 3 ) 2 ] revealed donation from the allyl non-bonding n-orbital to the dp* orbital of the Pd 2 (k-Br)(PH 3 ) 2 fragment together with back donation from occupied dp–dp and dn–dn bonding combinations to the allyl n* orbital.157 A range of neutral anionic and cationic butadiene PdIPdI dimers has been prepared by the addition of buta-1,3-diene to a mixture of PdII halide and Pd0 complexes.Mixtures of [Pd 2 X 4 (PPh 3 ) 2 ] (X\Cl or Br) and [Pd 2 (dba) 3 ] gave [Pd 2 X(k-X)(PPh 3 )(k-H 2 C–– CHCH–– CH 2 )] of [Pd 2 (k-Cl) 2 (PPh 3 ) 4 ][PF 6 ] 2 and [Pd 2 (dba) 3 ] gave [Pd 2 (k-Cl)(PPh 3 ) 2 (k-H 2 C––CHCH––CH 2 )][PF 6 ] and of [PdCl 2 (NCPh) 2 ] [PPh 4 ]Cl and [Pd 2 (dba) 3 ] gave [PPh 4 ][Pd 2 (k-Cl)Cl 2 (k2 - H 2 C––CHCH––CH 2 )].158 The reaction of cis-[Pt(C 6 F 5 ) 2 (C–– –CR) 2 ]2~ or [Pt(C–– – CR) 4 ]2~ with [MPdCl(g3-allyl) N2 ] is a convenient route to the zwitterionic complexes Qcis-[(C 6 F 5 ) 2 Pt(k-g1 g2- C–– – CR) 2 Pd(g3-allyl)] (Q\PMePh 3 R\Ph; Q\NBu 4 R\Bu5 or SiMe 3 ) and [NBu 4 ][(RC–– – C) 2 Pt(k-g1 g2-C–– – CR) 2 Pd(g3-allyl)] (R\Ph Bu5 or SiMe 3 ). Addition of one equivalent of [Pt(C–– –CR) 4 ]2~ to [MPdCl(g3-allyl)N2 ] gave the trinuclear acetylide [MPt(k-g1 g2-C–– – CR) 4NMPd(g3-allyl)N2 ].159 Addition of cis-[Pt(C 6 F 5 ) 2 (thf) 2 ] to a dichloromethane solution of [NBu 4 ] 2 [MPt(C 6 F 5 ) 2 (k-PPh 2 )N2 ] gave [NBu 4 ] [Pt 3 (C 6 F 5 ) 5 (k-PPh 2 ) 2 ] 62 which contains a PPh 2 ligand that donates six-electrons through three Pt–P interactions and an g2-co-ordinated phenyl group.160 Phenylvinylsulfide (PhSCH–– CH 2 ) reacts with [PdX(C 6 F 5 )(NCMe) 2 ] (X\Cl Br or I) to give [MPd(k-X)(PhSCHCH 2 C 6 F 5 )N2 ] a three-membered dimeric (phenylthio)alkylpalladacycle which slowly isomerises to the heteroatom complex [MPd(k-Cl)(k-p-i- PhSCHCH 2 C 6 F 5 )N4 ] containing a bridging k-p-i (phenylthio)alkyl fragment.Decomposition of [MPd(k-Cl)(PhSCHCH 2 C 6 F 5 )N2 ] in refluxing toluene gave vinylpenta- fluorobenzene consistent with a 1,2-hydrogen shift and Pd–SR b-elimination.At room temperature in the presence of tetrahydrothiophene hydrolysis of the C–S bond gave C 6 F 5 CH 2 CHO and PhSPdBr.161 Double directed lithiation of the diarylplatinum complex cis- [Pt(PEt 3 ) 2MC 6 H 3 (CH 2 NMe 2 ) 2 -3,5N2 ] followed by transmetallation with [PtCl 2 - 422 S. Doherty P Pt Pt Pt Ph P F5C6 C6F5 C6F5 C6F5 Ph2 62 – C6F5 (SEt 2 ) 2 ] gave the trinuclear platinum complex cis-[Pt(PEt 3 ) 2MC 6 H 3 (CH 2 NMe 2 ) 2 -3,5- PtClN2 ] which reductively eliminates [PtClM2,6-(Me 2 NCH 2 ) 2 C 6 H 2 - C 6 H 2 (CH 2 NMe 2 ) 2 -2,6NPtCl] containing the formally anionic biphenyl bridging ligand [2,6-(Me 2 NCH 2 ) 2 C 6 H 2 C 6 H 2 (CH 2 NMe 2 ) 2 -2,6]2~.162 Palladium complexes of nitrogen-donor ligands have been prepared with both symmetrical terminal and asymmetrical bridging isocyanide ligands.The palladium dimer [Pd 2 Cl 2 (CNR) 4 ] reacts with bipy phen and dmphen to give [Pd 2 (CNR) 2 L 2 ] [PF 6 ] 2 which contain square-planar palladium atoms with terminal isocyanides and chelating N–N ligands (L). In contrast reaction with bquin and napy a§ords [Pd 2 (k- CNR) 2 (bquin) 2 ][PF 6 ] 2 and [Pd(k-CNR) 2 (napy) 4 ][PF 6 ] 2 co-ordinated by bi- and mono-dentate nitrogen-donor ligands respectively; the former contains a symmetrically bridging isocyanide while the isocyanide in the latter asymmetrically bridges the two palladium atoms.163 9 Heterometallics Addition of phenylazide and diazoacetate to [Cp 2 Zr(k-NBu5)IrCp*] gave [Cp 2 Zr(k- NBu5)(k-N 2 Ph)IrCp*] and [Cp 2 Zr(k-NBu5)Mk-N 2 C(H)CO 2 EtNIrCp*] respectively.Thermolysis of the former at 75 °C resulted in loss of N 2 and formation of the mixed amido complex [Cp 2 Zr(k-NBu5)(k-NPh)IrCp*]. Cross-over experiments showed that loss of N 2 occurred without fragmentation a mechanism has been proposed for the transformation of an organoazide complex into a bridging imido complex.164 Ti S S Mo CO OC OC CO P Ph Mo CO CO CO CO P Ph Ph Ph R R 63 The titanium complexes [Ti(SR) 2Mg5-C 5 H 4 PPh 2N2 ] have been used as metallo biand tri-dentate ligands to prepare a variety of heterometallic complexes. Reaction with one equivalent of [Mo(CO) 4 (nda)] gave [(CO) 4 Mo(k-Ph 2 PC 5 H 4 ) 2 Ti(SR) 2 ] (R\Et or Ph) while three equivalents gave [M(CO) 4 MoN2 (k-Ph 2 PC 5 H 4 ) 2 Ti(k-SR) 2 ] 63.Coordination of the thiolate ligands in [(CO) 4 Mo(k-Ph 2 PC 5 H 4 ) 2 Ti(SR) 2 ] with [M(C 6 F 5 ) 2 (thf) 2 ] (M\Pd or Pt) gave the trinuclear complexes [(CO) 4 Mo(k- Ph 2 PC 6 H 4 ) 2 Ti(k-SR) 2 Pt(C 6 F 5 ) 2 ].165 Terminal acetylenes react with the hetero- 423 Organometallic chemistry of bi- and poly-nuclear complexes bimetallic complex [Cp*NiM(CO) 3 CpA] (M\Mo or W CpA\C 5 H 5 or C 5 H 4 Me) to give [Cp*NiMk-g3-g1-C(H)C(Ph)CONM(CO) 2 CpA] containing a five-membered metallacycle. Upon protonation or methylation these metallacycles rearrange to give the four-membered ring systems [Cp*NiMk-g3 g2-C(H)C(Ph)CORNM(CO) 2 CpA]` (R\H or Me).166 Reaction of the tripodal amido-supported complex [MClMMeSi[SiMe 2 N(C 6 H 4 Me- 4)] 3N] (M\Ti Zr or Hf) with K[M@(CO) 2 Cp] (M@\Fe or Ru) gave [MMeSi[Si- MeN(C 6 H 4 Me-4)] 3NMM@(CO) 2 Cp] containing a highly polar unsupported metal–metal bond.Reaction with MeNC led to rapid insertion into the metal–metal bond to give [MMeSi[SiMe 2 N(C 6 H 4 Me-4)] 3NM(k-g2-C––NMe)M@(CO) 2 Cp] 64 demonstrating the mixed electrophilic–nucleophilic character of their metal centres.167 Insertion of a heteroallene X––C–– Y into the unsupported early–late heterobimetallic [HC(SiMe 2 NC 6 H 4 F-2) 3 ZrM(CO) 2 Cp] (M\Fe or Ru) gave [MHC(SiMe 2 NC 6 H 4 F- 2) 3NZr(XCY)M(CO) 2 Cp] (X\O or S Y\O S or NR).168 Si Si N M Si Me M¢ N Me CO R CO 64 Si N N R R Photochemical loss of CO from [CpACpTa(CO)(k-PMe 2 )W(CO) 5 ] generated from racemic [Ta(CO)(PMe 2 )CpACp] and [W(CO) 5 (thf)] yields [CpACpTa(k-CO)(k- PMe 2 )W(CO) 4 ] (CpA\1-Bu5-3,4-Me 2 C 5 H 2 ).Addition of the optically active phosphine neomenthyldiphenylphosphine gave a pair of diastereoisomers which could be separated by fractional crystallisation.169 The chiral anionic ligand [Mo(CO) 5 (PPhH)]~ reacts with [PtCl 2 (L–L)] to give the neutral trimetallic monophosphido- bridged complexes [Pt(k-PPhH) 2MMo(CO) 5N2 (L–L)] (L–L\dppe dpae dppee) the first examples of heterometallic complexes that contain two chiral primary phosphido bridges.170 The mechanism of palladium-catalysed metal–carbon bond formation was investigated by reacting [MI(CO) 3 (g5-1-Ph 2 P-2,4-Ph 2 C 6 H 2 )] (M\Mo or W) with stoichiometric amounts of Pd0 to give [M(CO) 3 (g5-1-Ph 2 P-2,4-Ph 2 C 5 H 2 )PdI(PPh 3 )] (oxidative addition step). Reaction with representative ethynyl tin derivatives then gave [M(CO) 3 (g5-1-Ph 2 P-2,4-Ph 2 C 5 H 2 )Pd(PPh 3 )(C–– – CC 6 H 4 NO 2 -p)] a model for the transmetallation step in Pd-mediated C–C bond formation.171 The trinuclear oxo–acetylide cluster [Cp*W(O)Re 2 (CO) 8 (k-C–– – CPh)] reacts with thiophenol to give the fragmentation product [Cp*W(O)ReH(CO) 4 (k-C–– – CPh)].Oxidative decarbonylation of this complex in acetonitrile first gave [Cp*W(O)ReH(CO) 3 (k-C–– – CPh)] and then the head-to-tail dimer [MCp*W(O)ReH- (CO) 3 (k-C–– – CPh)N2 ].172 Refluxing toluene solutions of [Cp 2 W 2 Ru 3 (CO) 13 ] slowly lose CO to give the oxo–carbido cluster [Cp* 2 W 2 (O)Ru 3 C(CO) 11 ] 65 which adopts a wingtip bridged butterfly arrangement of metal atoms.173 Reaction of the cationic carbyne complex [Mn(CO) 2 (–– – CPh)Cp][BBr 4 ] with 424 S.Doherty [NEt 4 ] 2 [Fe 2 (CO) 8 ] gave the heteronuclear carbene-bridged complex [Mn(CO) 2 CpMk-C(COEt)PhNFe(CO) 3 ] the tetranuclear dicarbene [M(CO) 2 CpMn(––CPh)N2 Fe 2 (CO) 8 ] and [Mn(CO) 3 Cp]. Possible mechanisms for the formation of these compounds have been described.174 Deprotonation of the spiked triangular cluster [Re 2 Pt(k-H) 2 (CO) 9MHRe(CO) 5N] with [NEt 4 ]OH a§ords the anion [Re 2 Pt(k-H) 2 (CO) 9MRe(CO) 5N]~ which is thermally unstable in solution readily converting to [Re 2 Pt(k-H) 2 (CO) 9MHRe 2 (CO) 6N]~. Alternative synthetic routes to these clusters have been described.175 Ru W Ru Ru C CO OC OC OC OC CO CO CO O W OC CO Cp* 65 CO A number of reactions of silylated dinuclear Fe–Pd acyl complexes have been reported. Addition of CNR to the heterobimetallic complex [(CO) 3 FeMk- Si(OMe) 2 (OMe)N(k-dppm)PtMC(O)MeN] results in cleavage of the Pt–OMe bond to give [(CO) 3MSi(OMe) 3NFe(k-dppm)PtMC(O)MeN(CNR)] whereas dmad inserts into the Pd–acyl bond to give the alkenyl complex [(CO) 3 FeMk-Si(OMe) 2 (OMe)N(k- dppm)PtM(MeO 2 C)C––C(CO 2 Me)C(O)MeN] isolated as its isocyanide adduct [(CO) 3MSi(OMe) 3NFe(k-dppm)PtM(MeO 2 C)C––C(CO 2 Me)C(O)MeN(CNR)].Excess PhC–– – CH and Bu5C–– – CH react with [(CO) 3 FeMk-Si(OMe) 2 (OMe)N(k- dppm)PtMC(O)MeN] to give the vinylidene-bridged complex [(CO) 3 Fe(k-C––CHR)(k- dppm)Pt(CO)] (R––Ph or Bu5). The additional carbonyl ligand was suggested to arise from the acyl ligand while the fate of the silyl group was less certain. The k-Si–Obridge in [(CO) 3 FeMk-Si(OMe) 2 (OMe)N(k-dppm)PtPh] opens under a purge of carbon monoxide to give [(CO) 3MSi(OMe) 3NFe(k-dppm)PtPh(CO)] which rather surprisingly does not undergo insertion into the Pt–phenyl bond but loses CO reversibly under reduced pressure.176 Facile methoxy–dimethylamino exchange during the reaction of [Fe(CO) 4MP(OMe) 3N] with HSi(NMe 2 ) 3 gave the amine-stabilised iron–silylene complex [Fe(CO) 3MP(OMe)(NMe 2 ) 2NM––Si(OMe) 2 NHMe 2N].Deprotonation with excess KH and reaction with [CuCl(PPh 3 )] gave the heterobimetallic [(CO) 3MP(OMe)(NMe 2 ) 2NFeMk-Si(OMe) 2 (NMe 2 )NCu(PPh 3 )].177 Di§erences in the reactivity of mer-[FeH(CO) 3MSi(OMe) 3NMPh 2 PCH 2 C(O)PhN] and mer- [FeH(CO) 3MSi(OMe) 3NMPh 2 PCH 2 C(O)NPh 2N] have been observed. The former containing a g1-(P)-co-ordinated diphenylphosphino ketone reacts with SnX 2 Bu 2 (X\Cl Br or O 2 CMe) in the presence of NEt 3 to a§ord [(CO) 3M(MeO) 3 SiNFeMk- Ph 2 PCH––C(O)PhNSnBu 2 ] via spontaneous deprotonation of the functional phosphine to give an enolate ligand.In contrast the N,N-diphenyl-2-diphenylphosphinoacetaminde complex mer-[FeH(CO) 3MSi(OMe) 3NMPh 2 PCH 2 C(O)NPh 2N] reacts with SnX 2 Bu 2 (X\Cl or B) but not with [Sn(O 2 CMe) 2 Bu 2 ].178 425 Organometallic chemistry of bi- and poly-nuclear complexes Substitution of the bridging carbonyl in [Fe 2 (CO) 3 (k-CO)(k-Ph 2 PXPPh 2 )Pt(PR 3 )] (R\Ph or Tol X\CH 2 or NH) by the cyclic amides ENBu5SiMe 2 NBu5 (E\Ge or Sn) gave [Fe(CO) 3 (k-ENBu5SiMe 2 NBu5)(k-Ph 2 XPPh 2 )Pt(PR 3 )]. In the presence of excess GeNBu5SiMe 2 NBu5 [Fe(CO) 3 (k-GeNBu5SiMe 2 NBu5)(k-Ph 2 XPPh 2 )Pt(PR 3 )] and the tetranuclear complex [Fe(CO) 3 (k-GeNBu5SiMe 2 NBu5)(k-Ph 2 XPPh 2 )Pt(k- GeNBu5SiMe 2 NBu5)] are in equilibrium.In contrast reaction of [Fe(CO) 3Mk- Si(OMe) 2 (OMe)N(k-dppm)M(Me)] (M\Pd or Pt) with ENBu5SiMe 2 NBu5 leads to the terminal base-stabilised complexes mer-[Fe(CO) 3Mk- Si(OMe) 2 (OMe)N(ENBu5SiMe 2 NBu5)(k-dppm)M(Me)].179 Octacarbonyldiosmacyclobutane reacts with [Pt(C 2 H 4 )(PPh 3 ) 2 ] to give [Os 2 Pt(CO) 8 (PPh 3 ) 2 ] which exists in solution as three interconverting isomers. Two distinct mechanisms for their interconversion have been described (i) a low energy restricted trigonal twist motion at a phosphine-substituted osmium centre (*H8\10.1 kcal mol~1) and (ii) an olefin-type rotation of the Os 2 fragment about the platinum centre (*H8\10.7 kcal mol~1).180 The reaction between [BiClMFe(CO) 2 CpN2 ] TlPF 6 and OP(NMe 2 ) 3 gave [BiMOP(NMe 2 ) 3N2MFe(CO) 2 CpN2 ][PF 6 ] which has a Bi centre most aptly described as four-co-ordinate with an equatorially vacant trigonal-bipyramidal geometry; the [Fe(CO) 2 Cp] fragments occupy equatorial sites.The structure was compared with that of [BiCl 2MFe(CO) 2 CpN2 ]~ and those of the aryl bismuth compounds [BiR 2 - L 2 ]`.181 The diethoxycyclopropenylidene complex [M(CO) 5MC 3 (OEt) 2N] (M\Mo or W) reacts with K[Fe(CO) 2 Cp] replacing only one ethoxide to give [M(CO) 5MC 3 (OEt)[Fe(CO) 2 Cp]N]. A zwitterionic cyclopropenium-type structure was favoured based on the available structural data.182 Reaction of [W(CO) 3 (C–– – CC–– – CH)Cp] with one equivalent of [Ru 3 (CO) 10 (NCMe) 2 ] gave the alkyne-bridged cluster [Ru 3 (CO) 9 (k-CO)M(k-g1 g1 g2- HC–– – CC–– – C)W(CO) 3 CpN] which reacts with a further equivalent of the tungsten–acetylide complex to give [Ru 3 (CO) 7 W(CO) 2 CpMk3 -Cp(CO) 3 WC 2 [C 5 (O)] C 2 CH(O)N] via co-dimerisation of two CO ligands and two molecules of the diyne fragment.183 Several complexes and clusters containing the 1,3-diyne unit have been prepared and the unco-ordinated carbon–carbon triple bond used to prepare a range of novel mixed-metal clusters.Such complexes include [Mo 2 (k-g2- Me 3 SiC 2 C–– – CSiMe 3 )(CO) 4 Cp 2 ] [Pt(g2-Me 3 SiC 2 C–– – CSiMe 3 )(PPh 3 ) 2 ] [Ru 3 (k-g2- SiMe 3 C 2 C–– – CSiMe 3 )(k-CO)(CO) 9 ] [MMo 2 (CO) 4 Cp 2NMCo 2 (CO) 6N(k-g2 k-g2- SiMe 3 C 2 C 2 SiMe 3 )] [Re 2 (k-H)M(k-g1 g2;k-g2-C 2 C 2 SiMe 3 )[Co 2 (k-dppm)(CO) 4 ]N (CO) 8 ] [Re 3 (k-H)M(k3 -g1 g2;k-g2-C 2 C 2 SiMe 3 )[Co 2 (CO) 4 )k-dppm)]N(CO) 9 ] and [Co 2Mk-g2-RC 2 C–– – C[W(CO) 3 Cp]N(k-dppm)(CO) 4 ] (R\H or SiMe 3 ).184 An electrochemical analysis of [Os 3 (CO) 10MCpFe(C 5 H 4 C–– – CH)N] and the bis(ferrocenyl)- substituted cluster [Os 3 (CO) 9M(CpFeC 5 H 4 C–– – CH) 2 CON] showed reversible one-electron oxidations associated with the ferrocenyl moiety; the two well separated redox processes of the latter cluster are consistent with inequivalent ferrocenyl sites.185 The activated cluster [Os 3 (CO) 10 (NCMe) 2 ] reacts with [Fe(C 5 H 4 C–– – CSiMe 3 ) 2 ] to yield the k-g2-alkyne bridged cluster [Os 3 (CO) 10Mk3 -g2-Fe[C 5 H 4 (C 2 SiMe 3 )] 2N] which readily decarbonylates to a§ord the butadiendiyl clusters [Os 3 (CO) 9Mk3 -g4- Fe[C 5 H 4 (C 2 SiMe 3 )] 2N] the product of carbon–carbon bond formation and a 1,2- SiMe 3 shift.A cyclic voltammetric study of the last cluster revealed a single reversible one-electron oxidation associated with the ferrocenyl group and an irreversible two- 426 S. Doherty electron reduction assigned to the osmium fragment.186 Reaction of [N(PPh 3 ) 2 ] [Re(CO) 5 ] with [N(PPh 3 ) 2 ][Fe 3 (CO) 9MCCOC(O)CH 3N] gave the k-acetylide cluster [Fe 3 (CO) 9MC–– – CRe(CO) 5N] together with the by-product [N(PPh 3 ) 2 ] 2 [Fe 3 (CO) 9 CCO]. Addition of acyl chloride to the reaction mixture appears to reduce the concentration of the ketenylidene by-product presumably by regenerating the acylketenylidene precursor [Fe 3 (CO) 9MCCOC(O)CH 3N].187 Ionic coupling of [Ru(NCMe) 3 Cp*]` and [Os 5 (CO) 15 ]2~ gave the heptanuclear cluster [Os 5 Ru 2 (CO) 15 Cp* 2 ] best described as either a bicapped trigonal bipyramid with the two RuCp* units at the apical sites or as four Os 3 Ru tetrahedra sharing four common faces.188 Condensation of [Ru 3 (CO) 12 ] with [Mo 2 (CO) 4 (k-HC–– – CR1)Cp 2 ] (R1\H Me Ph or CO 2 Me) a§ords the k-vinylidene clusters [Mo 2 Ru(k3 - C––CHR1)(CO) 7 Cp 2 ] together with low yields of [Mo 2 Ru 6 (k3 -CH)(k3 - CR1)(CO) 12 Cp 2 ].189 Condensation of [Ru 3 (CO) 12 ] with [Mo 2 (k3 -S) 2 (k-SR) 2 Cp 2 ] gave the tetrahedral cluster [Mo 2 Ru 2 (k3 -S) 2 (k-SR) 2 (CO) 4 Cp 2 ].190 The pentaosmium carbide cluster [Os 5 C(CO) 15 ] has been used to prepare new mixed-metal osmium –palladium clusters.Its reaction with [Pd(PPh 3 ) 4 ] in dichloromethane gave [Os 5 PdC(CO) 12 (k-CO) 2 (PPh 3 ) 2 ] whereas its reaction with [PdCl 2 (PPh 3 ) 2 ] in refluxing chloroform gave [Os 5 PdC(CO) 15 (k-Cl) 2 (PPh 3 )].The first of these clusters consists of a square-based pyramid of osmium atoms with the [Pd(PPh 3 ) 2 ] group capping a triangular face; the latter cluster consists of a butterfly arrangement of four osmium atoms with the palladium capping a wingtip itself bonded to the remaining [Os(CO) 3 (PPh 3 )] moiety.191 Reaction of [Mo 2 (CO) 4 Cp 2 ] with [Ru 3 (CO) 12 ] a§ords moderate yields of the carbido–oxo cluster [Mo 2 Ru 2 (k6 -C)(k-O)(CO) 12 Cp 2 ] which crystallises with two distinctly di§erent molecules in the asymmetric unit. These clusters obey the 18-electron rule but contain two electrons less than that predicted for a closo-octahedral skeletal framework.192 Reaction of [Pt 3 Ru 6 (k-H) 2 (CO) 21 ]2~ with [Ir(NCMe) 3 Cp*]2` and HgI 2 gave [Pt 3 Ru 6 (k3 -H) 2 (CO) 21 (k3 -IrCp*)] and [Pt 3 Ru 6 (k3 -H) 2 (CO) 21 (k3 -HgI)] respectively.Both clusters comprise an alternating Ru 3 Pt 3 Ru 3 layered metal-atom framework with the heterometal atom k3 -triply bridging one Ru 2 Pt triangular face. Both complexes comply with the expected electron count for monocapped face shared bioctahedra (132 cluster valence electrons).193 In situ IR and NMR spectroscopic studies showed that the heterobimetallic complex [(g5-C 5 R 5 )Ru(k-CO) 2 (k-L–L)RhX 2 ] (R\H or Me; L–L\dppm dppe or dppee; X\Cl or I) readily and reversibly dissociates to give [Ru(CO) 2 (g1-L–L)Cp]` and [RhX 2 (CO) 2 ]~ in the presence of CO.In situ 31P NMR spectroscopic studies of the reaction provided evidence for an intermediate species [Cp(CO) 2 Ru(k- dppm)RhCl 2 (CO)].194 A mixture of [MIrcl(cod)N2 ] and the chiral oxazolylferrocene–phosphine ligand dipof catalyses the hydrosilylation of simple ketones to give the corresponding sec-alcohols after acid hydrolysis. The first step of the reaction presumably involves exchange of cod with dipof to give a heterobimetallic Fe–Ir complex.195 Both isomers [Ir(g4-2,5-dmt)Cp*] and [Ir(C,S-2,5-dmt)Cp*] react with [Ru 3 (CO) 12 ] to a§ord [MIr(g4-2,4-dmt)Cp*NRu 3 (CO) 11 ] while two products were isolated from the reaction with [Re 2 (CO) 10 ] the first [MIr(g4-2,4-dmt)Cp*NRe 2 (CO) 9 ] containing an S-co-ordinated thiophene in an equatorial position the other [MIr[g4- SC 3 H 2 MeC(––O)Me]Cp*NRe 2 (CO) 9 ] resulting from ring opening to give an S-coordinated acyl–thiolate.A similar compound [MIr[g4-SC 3 H 2 MeC(––O)Me] 427 Organometallic chemistry of bi- and poly-nuclear complexes Cp*NMn 2 (CO) 9 ] was isolated from the reaction with [Mn 2 (CO) 10 ].196 Cobaltocene readily desulfurises the 2,5-dmt ligand in [Ir(g4-2,5-dmt)Cp*] and [Ir(C,S-2,5- dmt)Cp*] to a§ord [Cp*IrC(Me)––CHC(H)C(Me)CoCp] containing a planar iridacyclopentadiene g4-co-ordinated to a CpCo fragment.197 Reaction of the ring-opened iridathiabenzene [Ir(C,S-2,5-dmt)Cp*] with [Co 4 (CO) 12 ] and [Co 2 (CO) 8 ] gave [MIr(g4-2,5-dmt)Cp*NCo 4 (CO) 11 ] in which the 2,5-dmt ligand is co-ordinated through sulfur to Co and g4-co-ordinated to the IrCp* fragment. At high reaction temperatures the desulfurised linear tetranuclear cluster [MCp*Ir[C(Me)–– CHCH–– C(Me)](k- CO) 2 CoN2 ] is formed in high yields.In contrast [Co 4 (CO) 9 (g6-C 6 H 3 Me 3 )] reacts with [Ir(C,S-2,5-dmt)Cp*] to give the g6-iridathiabenzene cluster [Mg6-Cp*Ir(C,S-2,5- dmt)NCo 4 (CO) 9 ] together with low yields of another isomer of [MIr(g4-2,5- dmt)Cp*NCo 4 (CO) 11 ].198 Refluxing [WIr 2 (CO) 10 Cp 2 ] with triphenylamine gave low yields of [W 3 Ir 4 (k- H)(CO) 12 Cp 3 ] with a metal-atom framework based on a seven-vertex bicapped trigonal bipyramid.199 The heterobimetallic complex [WCo(CO) 7 Cp] has been used as a single-source precursor for the preparation of polycrystalline WCoO 4 on Si(100).200 References 1 J. Chem. Soc. Dalton Trans. 1996 issue 5 555–800. 2 M.H. Chisholm J. Chem. Soc. Dalton Trans.1996 1781. 3 C. Bianchini and A. Meil J. Chem. Soc. Dalton Trans. 1996 801. 4 S. Sun. C. A. Dullaghan and D. A. Sweigart J. Chem. Soc. Dalton Trans. 1996 4493. 5 J. 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Braunstein Organometallics 1996 15 3868. 180 J. Cooke R. E. D. McClung J. Takats and R. D. Rogers Organometallics 1996 15 4459. 181 C. J. Carmault L. J. Farrugia and N. C. Norman J. Chem. Soc. Dalton Trans. 1996 443. 182 M. S. Morton J. P. Selegue and A. Carrillo Organometallics 1996 15 4664. 183 M. I. Bruce B. W. Skelton A. H. White and N. N. Zaitseva J. Chem. Soc. Dalton Trans. 1996 3151. 184 M. I. Bruce P. J. Low A. Werth B. W. Skelton and A. H. White J. Chem. Soc. Dalton Trans. 1996 1551. 185 S. L. Ingham B. F. G. Johnson P. R. Raithby K. J. Taylor and L. J. Yellowlees J. Chem. Soc. Dalton Trans. 1996 3521. 186 L. P. Clarke J. E. Davies P.R. Raithby and G. P. Shields J. Chem. Soc. Dalton Trans. 1996 4147. 187 D.M. Norton R. W. Eveland J. C. Hutchinson C. Stern and D. F. Shriver Organometallics 1996 15 3916. 188 J. Lewis C. A. Morewood P. R. Raithby M. Carmen and A. de Arellono J. Chem. Soc. Dalton Trans. 1996 4493. 431 Organometallic chemistry of bi- and poly-nuclear complexes 189 H. Adams L. J. Gill and M. J. Morris Organometallics 1996 15 4182. 190 H. Adams N. A. Bailey S. R. Gay L. J. Gill T. Hamilton and M.J. Morris J. Chem. Soc. Dalton Trans. 1996 2403. 191 J. W.S. Hui and W. T. Wong J. Chem. Soc. Dalton Trans. 1996 2887. 192 H. Adams L. J. Gill and M. J. Morris Organometallics 1996 15 464. 193 R. D. Adams T. S. Barnard J. E. Cortopussi and L. Zhang Organometallics 1996 15 2664. 194 P. S. Bearman A.K. Smith N. C. Tong and R. Whyman Chem. Commun. 1996 2061. 195 Y. Nishibayashi K. Segawa H. Takada K. Ohe and S. Uemura Chem. Commun. 1996 847. 196 J. Chen V. G. Young jun. and R. J. Angelici Organometallics 1996 15 2727. 197 J. Chen L. M. Daniels and R. J. Angelici Organometallics 1996 15 1223. 198 J. Chen V. G. Young jun. and R. J. Angelici Organometallics 1996 15 1414. 199 S. M. Waterman M. G. Humphrey and D. C. R. Hockless Organometallics 1996 15 1745. 200 S. G. Shyu J. S. Wu S. H. Chuang K. M. Chi and Y. S. Sung Chem. Commun. 1996 2239. 432 S. Doherty

ISSN:0260-1818    

DOI:10.1039/ic093395

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

24 Magnetism By S. T. BRAMWELL University College London Department of Chemistry Christopher Ingold Laboratories 20 Gordon Street London WC1H 0AJ UK 1 Introduction Magnetism forms a gigantic field of research of which magnetochemistry is a small part. It is an exceedingly rich subject which does not fit easily into the traditional disciplines of science. This report strives to do justice to the richness and diversity of the subject whilst concentrating on developments of direct relevance to chemistry. In no sense is an attempt made to give an exhaustive account of the literature; this is clearly unfeasible when the 1996 literature contains over twenty thousand references that allude to magnetic properties. The report is divided into four sections. The introductory section will try to identify the major highlights and trends in magnetochemistry and will mention some of the more interesting developments across diverse fields that may give inspiration to the magnetochemist.Some books and general reviews will also be described. Section 2 is devoted to a more detailed summary of the year’s literature in several selected topics. The remaining sections 3 and 4 group work according to the type of material under investigation. Section 3 describes work on ionic covalent and metallic materials particularly oxides and halides which are the traditional interest of solid state chemists. Section 4 describes work on metal complexes and molecular solids more traditionally the domain of coordination chemists. In this section the materials are classified by dimensionality of the complex which from the magnetic point of view seems to provide a natural division.This report follows on from the excellent series by Harrison and colleagues,1 their policy of not reporting on the simple characterisation of magnetic materials has been adopted apart from in cases that this is relevant to a more general problem in magnetism. Perhaps the highlight of the year is the discovery by several groups of dramatic quantum tunnelling e§ects in molecular clusters. These results which are described in detail in Section 2 raise hopes that one day such e§ects can be harnessed to make nanoscale magnetic memory devices. They provide a clear motive for the magnetochemist to synthesize and study such clusters and open up exciting prospects for future research. It is to be hoped that coordination chemists will rise to this challenge as it is clear from this survey of the literature that the huge amount of work on small magnetic clusters although worthy has little scientific motive beyond basic characterization.A rather more focused and equally popular subject of research is ‘molecular Royal Society of Chemistry–Annual Reports–Book A 457 ferromagnetism’ but unfortunately progress in this field is slow with transition temperatures largely rooted to liquid helium levels. In solid state chemistry giant magnetoresistance is the most popular current topic of study and is again motivated by a commercial goal. Progress in this field is much faster than in the rather ailing ‘high-T#’ industry although there are still many chemists involved in studying magnetic oxides related to the superconducting cuprates.From the point of view of pure science frustrated magnets are the main focus of attention and although only a relatively modest number of researchers are involved in this field the collaboration of chemists and physicists ensures steady progress. The research areas described above form the main subject of the forthcoming sections of this report. Also selected are some topics which are really physics but are of general relevance to magnetochemistry. These include new magneto-optic e§ects nuclear magnets and various theoretical studies. It is interesting to put all these subjects into a wider context by examining the magnetic research that has made it into the pages of the semi-popular scientific journals. Both Nature2 and Science3 have devoted editorial space to discussing the quantum mechanical tunnelling phenomena described in section 2 giant magnetoresistance,4,5 and the new magneto-optic e§ects described in section 3.6,7 Science also carried an editorial article on the e§ect of ‘negative viscosity’ in magnetic fluids.8 This topic is so fascinating that it would seem worthy of a brief description.Negative viscosity refers to the reduction in viscosity of a magnetic fluid–a colloidal suspension of single-domain magnetic particles–when subject to changing magnetic fields. Almost magical e§ects can be observed for example if a drop of magnetic fluid is suspended in another fluid in a rotating magnetic field the drop develops starfish-like spiny arms and depending on the frequency weird eel-like structures. Although magnetic fluids are widely used in hard disk drives audio speakers printing and magnetic resonance imaging there is as yet no practical use for these e§ects.Other magnetic headlines in these journals have concentrated on magnetic fields in space. Nature has reported an interesting result from the Galileo space probe that Jupiter’s moon Ganymede is the first moon to be discovered that has a magnetic field possibly arising from convection in a layer of salty water below the surface.9 On a more popular level New Scientist described how Pobell and colleagues at Bayreuth University have made the ‘coldest lump of metal ever’ a 32g piece of Pt cooled to 3 kK by adiabatic demagnetization of the nuclear spins.10 A number of magnetic devices have been reported in engineering magazines. Professional Engineering describes a torque magnetometer that travels along the 17 000km of Britain’s steel gas pipelines detecting flux leakage into corroded regions on the exterior of the pipe.11 Insight magazine noted that Oxford Instruments have announced the successful operation of the world’s first cryogen-free superconducting magnet; the work required to cool the magnet to 5K being provided entirely by the external power supply.12 Electronics Today reported an ingenious new type of magnetometer designed by Givens et al.:13 based on a xylophone resonator it is sensitive to a wide range of fields and lends itself to miniaturization.This introductory section is concluded by noting a number of books and reviews that have appeared this year. Aharoni has published a book on the theory of feromagnetism that covers basic aspects of exchange anisotropy and magnetostatics as well as giving a more comprehensive coverage of the theory of micromagnetics.14 Although 458 S.T.Bramwell mainly intended for physicists and engineers magnetochemists may find this book a useful guide to the more mysterious aspects of ferromagnetism. At a much more popular level Livingstone’s book Driving Force–The Natural Magic of Magnets,15 has been reviewed very favourably by Grant.16 Volume 644 of the American Chemical Society Symposium Series the published proceedings of the 1995 International Chemical Congress of the Pacific Basin Societies at Honolulu is devoted to molecular-based magnetic materials.17 It contains 22 reviews and research papers on theory technique and applications in molecular magnetism.Gatteschi et al.18 have reviewed the magnetism of large iron-oxo clusters. Giant magnetoresistance in layered and granular alloy systems has been reviewed by Levy.19 The giant or colossal magnetoresistance of the lanthanide manganates has been reviewed by Rao et al.20 Several reviews on molecular ferromagnetism have appeared. 21,22 Kirchmayr has reviewed permanent magnets with an emphasis on hard magnetic materials based on rare-earth intermetallics.23 Mydosh has reviewed disordered magnetism and spin glasses and suggested that future avenues of research will concentrate on the changes of magnetic behaviour caused by nanostructuring such materials.24 Govord et al.25 have reviewed the magnetism of intermetallics with non-magnetic constituent elements such as weakly ferromagnetic PdTiAl.There have been several rather less general reviews concentrating on work from particular groups. Khan et al. have reviewed their work on the magnetism of molecular based bimetallic species,26 and Day has reviewed work at the Royal Institution of Great Britain on organic–inorganic layer compounds.27 Broholm et al. have published a short review of the magnetism of transition metal oxides in which there are strong magnetic fluctuations at low temperature.28 They discuss frustration the Haldane e§ect in one-dimensional chains and the unusual case of (V 1~xCrx) 2 O 3 in which orbital fluctuations limit spin correlations to small clusters. Some other reviews are mentioned in the forthcoming test. 2 Selected topics Quantum mechanical tunnelling As mentioned above one of the highlights of the magnetic year is the demonstration by several groups of quantum tunnelling e§ects in molecular magnets.Physicists have for a number of years being trying to make an ensemble of identical nanometer scale magnets. This is of interest both for theoretical reasons as a probe of the ‘grey area’ between quantum and classical mechanics and for practical reasons as a development in the nanomolecular engineering of magnetic memory elements. It is well known that small magnetic clusters behave quantum mechanically with the projection of the magnetic moment on the field direction adopting a series of discrete values labelled by the quantum number M S . Much larger assemblies of magnetic ions however behave as classical monodomain particles that is as tiny ‘compass needles’ with statistical averaging masking all evidence of discreteness in the magnetization.In the presence of uniaxial magnetic anistropy a classical cluster will not be able to ‘flip’ its magnetization if the temperature is much lower than the anisotropy barrier which prevents rotation of the particle’s magnetization; but a quantum mechanical cluster may 459 Magnetism Fig. 1 Structure of Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 showing Mn4` (shaded) and Mn3` (open) support resonant tunnelling of the magnetization through the potential barrier an e§ect that should be largest when ‘spin up’ and ‘spin down’ states are degenerate in energy. In order to measure the tunnelling an assembly of identical nanoparticles is required as all particles will have identical tunnelling rates.Large molecular clusters can be crystallized easily into such an ensemble. Friedman et al.29 used the sample [Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 ] the structure of which is illustrated in Fig. 1. It has a tetrahedral core of fourMn4` ions each with spin 3/2 which are strongly exchange-coupled ferromagnetically to one another. These are surrounded by eight Mn3` ions with their spins (S\2) coupled ferromagnetically to each other but antiferromagnetically to the central core. This gives the complex an overall spin S\10 which behaves at low temperature as a single ‘super spin’. Anisotropy arising from spin-orbit coupling gives the complex an easy axis of magnetization and causes theM S \^10 states to lie lowest in energy withM S \^9 lying next lowest and so on. The anisotropy axis corresponds to the crystalline c-axis; Friedman et al.have exploited this to magnetically align a polycrystalline sample which is then immobilized in epoxy resin obviating the need for large single crystals. Experimentally the sample is found to have a stepped hysteresis loop illustrated in Fig. 2; but more remarkably the rate of approach to equilibrium of the magnetization at a given external field depends sensitively on the magnitude of the field with maxima in the rate at fields corresponding to the steps in the hysteresis curve (Fig. 3). To appreciate just how remarkable this result is consider the following if the sample is initially magnetized in say the M S \]10 state and the field is switched o§ then individual complexes will occasionally flip their spins and the bulk magnetization will decay slowly.However if a weak field is applied opposite to the magnetization direction the rate of decay is suppressed rather than enhanced as would be intuitively supposed. These remarkable observations provide clear evidence for quantum mechanical tunnelling between the di§erentM S states on either side of the potential barrier. 460 S.T. Bramwell Fig. 2 Magnetization-v-field isotherms for Mn 12 O 12 (CH 3 CO 2 ) 4 (H 2 O) 4 Fig. 3 Relaxation of the magnetization in Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 the di§erence between the magnetization (M) and its asymptopic value (M 0 ) as a function of time for two di§erent fields As the applied field is increased states with negativeM S are stabilized with respect to states with positiveM S which become increasingly metastable.At regular intervals of the field accidental degeneracies occur between states of positive and negativeM S and resonant quantum mechanical tunnelling through the potential barrier is greatly enhanced. Thus the magnetization is more prone to ‘flipping’ at every field at which such a degeneracy occurs leading to the steps in the hysteresis loop and the oscillating 461 Magnetism Fig. 4 Schematic representation of tunnelling in Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 tunnelling from the metastable state M S \S to M S \[S]2 followed by thermally assisted decay to the ground stateM S \[S relaxation times. This process is illustrated schematically in Fig. 4 at the field which brings the M S \S state degenerate with M S \[S]2. The magnetization tunnels from M S \S to M S \[S]2 and then decays to the ground state M S \[S by losing quanta of energy to the lattice vibrations.There is evidence that the main tunnelling processes are in fact from excited states on the left hand side of the potential barrier which gives rise to a concise description of the phenomenon as ‘thermally assisted quantum tunnelling’. Similar e§ects have been observed in the closely related complex [Mn 12 (CH 3 CO 2 ) 16 (H 2 O) 4 O 12 ]· 2CH 3 CO 2 H·4H 2 O.30,31 A comprehensive theory of tunnelling e§ects has been published,32 which is relevant to zero temperature. However the theory does not immediately support the experimental findings and the authors discuss possible reasons for this. Giant and colossal magnetoresistance Among solid state chemists giant magnetoresistance in magnetic oxides continues to be a very popular subject of research.In private however some do not see what all the fuss is about as such e§ects have been known for years. It is therefore interesting to identify the factors that motivate the current research. Magnetoresistance is the change in resistivity with magnetic field. Doped ferromagnetic semiconductors such as the chromium chalcogenide spinels (e.g. HgCr 2 S 4 ) have long been known to show particularly large magnetoresistance. This can be thought of as arising from the scattering of conduction electrons by spin deviations in much the same way as normal metallic resistivity arises from the scattering by lattice vibrations or impurities. Just like the scattering of light or neutrons this spin disorder scattering reaches a maximum at the magnetic critical point.The application of a field in this region suppresses the spin disorder and hence reduces the resistivity leading to large negative magnetoresistance. Similar e§ects in doped LaMnO 3 have also been known for many years; for example Searle and Wang reported it in La 1~xPbxMnO 3 as long ago as 1969.33 The origin of the e§ect in these magnets has traditionally been ascribed to the Zener double exchange mechanism of ferromagnetic interaction between Mn3` and Mn4`. This mechanism arises because it is particularly easy for Mn3` and Mn4` to exchange an electron. In reality the ‘excess’ electrons become delocalized over several Mn4` sites forcing the remaining localized spins onMn4` to align parallel by Hund’s rules. When 462 S.T. Bramwell these ‘magnetic polarons’ form a percolating cluster a transition occurs to a ferromagnetic metal.The application of a magnetic field not only reduces spin disorder but also encourages delocalization which is generally competing with antiferromagnetism and hence can precipitate the transition to the metallic state. However the doping of LaMnO 3 also reduces the concentration of the Jahn–Teller ion Mn3` and so leads to structural changes. At the time of the discovery of the giant magnetoresistance in the doped lanthanum manganates the complex magnetic and structural properties of these materials had already been extensively characterized by the pioneering work of among others Wollan and Koehler and Goodenough. Harrison in last year’s Annual Report has given a detailed account of this early work and has emphasized that the ‘rediscovery’ of the lanthanum manganates is due in part to the improvements in experimental di§raction techniques allowing a more detailed analysis of the complex phases that exist in this system.To that can be added several other factors including the widespread availability of commercial magnetometers with magnetoresistance probes the renewed interest in magnetoresistance among the physics community arising largely from the discovery of giant magnetoresistance in metallic multilayers and the modern emphasis of research funding bodies on research with an applied aspect. However perhaps the most important single factor motivating the research is the genuine increase in the importance of magnetoresistance as a commercially exploitable magnetic property.As described by Brug et al.,34 magnetoresistive recording heads have recently been introduced into the magnetic recording industry and there is a direct relationship between the magnitude of the magnetoresistance and the final storage capacity of the disk drive. Magnetoresistance thus clearly has a role to play in the computer age and the rediscovery of magnetoresistance in the manganates is timely. On the other hand only time will tell if the rewards justify the magnitude of the e§ort. A question of current interest is whether the simple double exchange mechanism described above does indeed account for the ‘colossal’ magnetoresistance in Ln 1~xBxMnO 3 (Ln\lanthanide; B\alkaline earth metal). Field-induced antiferromagnetic insulator to ferromagnetic metal transitions have been observed in Nd 1~xCaxMnO 3 ,35 Pr 1~xCaxMnO 3 36 and La 0.96 Mn 0.96 O 3 37 emphasizing the correlation between magnetic ordering and resistivity.Other work has emphasised the importance of Jahn–Teller induced strains leading to charge ordering.38 Millis et al.39 have presented a theory that incorporates the e§ects of both double exchange and the Jahn–Teller e§ect. Experimental work by Ramirez et al. on La 1~xCaxMnO 3 has lent support to this picture.40 It is undoubtedly the case that electron–phonon coupling is as important as double exchange in determining the electronic properties of these systems. Zhou et al. have gone as far as suggesting that the strong coupling between conduction electrons and cooperative local lattice distortions leads to a new type of electronic state.41 Fontcuberta et al.42 have shown that the colossal magnetoresistance in Ln 1~xCaxMnO 3 is correlated with the size of the Ln ion; this is also equivalent to the e§ect of hydrostatic pressure.43 The occurrence of giant magnetoresistance in the mixed valence manganates does not seem to depend too crucially on the crystal structure and there are a growing number of examples of giant magnetoresistance materials that adopt structures di§erent to varying degrees from the cubic perovskite structure of LnMnO 3 .The series Ln 1`xB 2~xMn 2 O 7 (Ln\lanthanide; B\alkaline earth) have crystal structures of 463 Magnetism Fig. 5 Colossal magnetoresistance in La 1.2 Sr 1.8 Mn 2 O 7 (adapted from ref. 43). Inplane resistivity (upper panel) and magnetization (lower panel) as a function of magnetic field the Ruddlesdon–Popper type which consist of layers formed of alternating perovskite and rock-salt-type blocks.Moritomo et al.44 have reported colossal negative magnetoresistance in La 1.2 Sr 1.8 Mn 2 O 7 over a wide range of temperatures above the ferromagnetic Curie temperature of 126 K. These results are illustrated in Fig. 5. The authors suggest that the large spin fluctuations associated with two dimensionality enhance the magnetoresistance in this range of temperature. The closely related La 1.5 Ca 1.5 Mn 2 O 7 is a ferromagnetic metal below 215K and shows large resistivity and magnetoresistance (60%) over a wide temperature range below the Curie point.45 There is a peak in the resistivity at 100 K and the magnetoresistance rises as the temperature falls in this region.Battle et al. have made a detailed study of Sr 2 NdMn 2 O 7 and Sr 1.9 Nd 1.1 Mn 2 O 7 .46–48 These compounds exhibit colossal magnetoresistance of more than 104%at 14T in the temperature range 4.2–100 K. Both have low resistivity and neither are bulk ferromagnets which the authors interpret as casting doubt on the involvement of double exchange. However both Sr 2 NdMn 2 O 7 and Sr 1.9 Nd 1.1 Mn 2 O 7 show a di§erence between the field cooled and zero field cooled susceptibility which could be interpreted as evidence of the competing ferromagnetic and antiferromagnetic interactions that are inherent to the double exchange mechanism. It appears that both compounds are biphasic with each phase adopting the Ruddlesdon–Popper structure and having very similar lattice 464 S.T.Bramwell parameters. Another series of oxides showing giant magnetoresistance are Tl 2~xInxMn 2 O 7 which adopt the cubic pyrochlore structure;49 the pressure behaviour of these materials is di§erent to that of the perovskites.50 Frustrated antiferromagnets Frustrated antiferromagnets continue to provide the major themes in magnetic research. As described in last year’s report most interest currently centres on those systems which as a consequence of frustration have an infinite number of degenerate spin structures at zero temperature. The prototypical examples are the Heisenberg model on the two-dimensional kagome� lattice in which the spins occupy the vertices of an infinite network of corner-linked triangles and on the three-dimensional pyrochlore lattice in which the spins occupy the vertices of an infinite network of cornerlinked tetrahedra.The relationship between these two lattices is illustrated in Fig. 6. In zero magnetic field these systems do not order even at temperatures well below the Curie–Weiss constant (which is a measure of the near-neighbour exchange) but rather adopt a ‘spin liquid’ ground state in which only near neighbours are strongly correlated. Competing with the spin liquid are spin glass and long-range ordered ground states. The ordered ground states may be stabilized by weak terms in the spin Hamiltonian or more remarkably by thermal or quantum fluctuations a phenomenon referred to as ‘order by disorder’. It may sound surprising that entropy can favour ordered arrangements in this way but in magnetism it is certainly a powerful ordering force.SrCrxGa 12~xO 19 (x[4) with the magnetoplumbite structure has for several years been considered to approximate to a kagome� lattice antiferromagnetic and has been the subject of numerous studies. However it is probably fair to say that progress in understanding this interesting material is rather slow owing to the complexity of its structure and the lack of chemically ordered single crystals. The most important property of SrCrxGa 12~xO 19 is its apparent spin glass transition at T\3.3 K a temperature far below the Curie–Weiss temperature of 500 K. Lee et al.51 have characterized the spectrum of magnetic excitations in the low temperature phase and shown that this has more structure than that of a conventional spin glass. In an e§ort to characterize more suitable model materials than SrCrxGa 12~xO 19 attention has turned to the jarosite series with general formula AM 3 (OH) 6 (SO 4 ) 2 where A is a monopositive cation and M\Fe or Cr.In these materials which bear more chemical similarity to hydroxithat to sulfates theM3` ions occupy a kagome� lattice in layers formed by the OH~ and SO 4 2~ ions. A particularly interesting representative of this series is hydronium jarosite an important mineral which has been synthesized by Harrison et al.52 by direct hydrolysis in an autoclave of Fe 2 (SO 4 ) 3 solutions. The full crystal structure of the synthetic material was determined by powder neutron di§raction and showed a surface coverage of Fe3` on the kagome� lattice of 97% in the best samples. Despite a huge Curie–Weiss constant of [1200 K the material does not adopt a long range ordered state down to at least 1.5 K but rather has an apparent spin glass freezing transition at 17.2 K and is thus comparable to SrCrxGa 12~xO 19 .Iron jarosite KFe 3 (OH) 6 (SO 4 ) 2 and chromium jarosite KCr 3 (OH) 6 (SO 4 ) 2 have been carefully investigated by the muon spin rotation technique. 53 The Curie–Weiss constants in these materials are 600K and 67.5K respectively. The iron compound was observed to order magnetically at 55K in agreement 465 Magnetism (a) (b) Fig. 6 Relationship between the two-dimensional kagome� lattice (a) and the threedimensional pyrochlore lattice (b). Figure (b) represents a slice of cubic pyrochlore lattice perpendicular to (1 1 1) showing a kagome� plane (medium circles) decorated by lattice points above and below the plane (large and small circles respectively) to form a network of corner-linked tetrahedra with previous measurements but in the chromium compound spin fluctuations were found to persist even at T\25 mK a temperature well below that at which history dependence of the bulk susceptibility sets in (T\2 K).This result again lends support to the idea that the low-temperature phase of the kagome� lattice antiferromagnet bears a certain resemblance to that of a conventional spin glass but is far more fluctuating. A susceptibility and nuclear magnetic resonance study of the jarosite RFe 3 (OH) 6 (SO 4 ) 2 with R\NH 4 Na and K54 has revealed successive phase transitions to long range order around 60 K which have been ascribed to the e§ect of weak 466 S.T. Bramwell Ising-like anisotropy in the Heisenberg Hamiltonian.Jarosite-type materials currently o§er the best prospects for future experimental study of the kagome� lattice antiferromagnet although a number of alternative systems are available. These include the RCuO 2.66 delafossite-type oxides which have been the subject of a theoretical study this year,55 and fluormica clays intercalated with magnetic Ni2` ions.56 Although an imaginative chemical approach to the problem of preparing kagome� lattice magnets there is no evidence that the Ni2` ions lie on a kagome� lattice rather than a partially filled triangular lattice. The compound is nevertheless interesting on its own right. It exhibits a spin glass-like transition at T' \4.5 K which is close to the Curie–Weiss constant of 8K suggesting little relationship with the kagome� lattice antiferromagnets discussed above.The pyrochlore lattice antiferromagnet has received if anything more attention than its two-dimensional counterpart the kagome� lattice. The pyrochlore lattice is much more common in nature than the kagome� lattice occurring for example in the pyrochlore oxides A 2 B 2 O 7 the B-sublattice of the spinels AB 2 O 4 various fluorides (e.g. FeF 3 and CsNiCrF 6 ) and in intermetallics with the C15 structure (e.g. YMn 2 ). Work on the oxide pyrochlores has been pioneered by Greedan and co-workers in Canada while the fluoride pyrochlores have mainly been studied in Europe. Harris and Zinkin57 have reviewed experimental and theoretical work on these systems. A striking generality is that the oxide pyrochlores A 2 B 2 O 7 are chemically ordered but thought to form conventional spin glasses whilst the fluoride pyrochlores ABCF 6 are chemically disordered yet do not.This is contrary to the received wisdom that a spin glass requires both frustration and disorder. This year Greedan et al. have investigated the pyrochlore oxides Ho 2 Mn 2 O 7 and Yb 2 Mn 2 O 7 by dc and ac susceptibility heat capacity and neutron di§raction and compared the results with those of the previously studied Y 2 Mn 2 O 7 .58 In these materials both the lanthanide and Mn4` ions occupy interpenetrating pyrochlore lattices. In Y 2 Mn 2 O 7 only the Mn4` sublattice with spin S\3/2 is magnetic and the predominant coupling is ferromagnetic. Y 2 Mn 2 O 7 displays conventional spin glass behaviour below 7K preceeded by an apparent ferromagnetic transition at 17 K.It is thus like a ‘re-entrant’ spin glass. The Ho and Yb homologues in which the lanthanide ion is also magnetic display similar behaviour but at a higher temperature. The ferromagnetic transition is at about 40K in both cases but the divergence of the field cooled and zero field cooled susceptibility occurs at about 33K for Ho 2 Mn 2 O 7 and 23K for Yb 2 Mn 2 O 7 . The spin glass transition in the oxide pyrochlores Y 2 Mo 2 O 7 has been studied by non-linear susceptibility measurements59 and the spin dynamics of Y 2 Mo 2 O 7 Y 2 Mo 1.6 Ti 0.4 O 7 and Tb 2 Mo 2 O 7 have been investigated by muon spin relaxation.60,61 In general the results show that the low temperature state of these magnets is like a conventional spin glass but with a large density of states for magnetic excitations near zero energy.Zinkin et al. have reported susceptibility and neutron inelastic scattering measurements on the pyrochlore compoundTm 2 Ti 2 O 7 in which only the rare earth ion Tm3` may be magnetic. However the crystal-field ground state of the Tm3` ion has zero magnetization and it was found that the crystal-field interaction dominates any superexchange between the Tm ions leading to a non-magnetic ground state in the compound.62 As mentioned above in the spinels the B sites occupy a pyrochlore lattice but the magnetism is complicated by the possibility of the A site being 467 Magnetism Fig. 7 Regular triangular lattice with all magnetic bonds equal magnetic as well as by the well-known inversion properties of spinels. In ZnFe 2 O 4 the Zn ions are non-magnetic and the Fe3` ions with spin 5/2 occupy the pyrochlore lattice with negligible inversion making this compound closely analogous to the pyrochlores.ZnFe 2 O 4 has been the subject of a thorough investigation using neutron di§raction muon spin rotation and relaxation and 57Fe Mossbauer spectroscopy.63 The samples were prepared by ceramic techniques involving slow cooling from 1500 °K. The spinel was found to exhibit magnetic order at T N \10.5 K an unusually low temperature scale for magnetic ordering in a spinel. Most intriguingly the short range magnetic order observed at temperatures as high as \10T N was observed to coexist with the long range order below T N . This study may provide a link between pure research on the pyrochlores and applied research on the spinel ferrites which are technologically important magnetic materials.Further reports of frustrated magnetism in spinel oxides are described in Section 3. The intermetallic C15 Laves phase compound Y 0.97 Sc 0.03 Mn 2 in which the Mn ions occupy a pyrochlore lattice has been studied by inelastic neutron-scattering.64 It was found that the spins on individual tetrahedra were strongly correlated as a spin singlet over short timescales but uncorrelated over long timescales. A theoretical study of the e§ect of lattice distortion on the spin structure of YMn 2 has been described,65 and Plumier has calculated magnetic structures as a function of exchange couplings in C15 and spinel structures.66 Another important class of frustrated antiferromagnet is the triangular lattice antiferromagnet (Fig.7). This and its three-dimensional relative the stacked triangular lattice antiferromagnet have long been of interest in connection with unusual modes of ordering unconventional phase transitions and critical exponents and the possibility of formation of a Pauling–Anderson resonating valence bond (RVB) solid. The previous five years’ reports by Harrison and co-authors for a detailed overview of recent work in these systems. The stacked triangular lattice is well-represented by the ABX 3 hexagonal perovskite type halides (e.g. CsMnBr 3 ) but there are relatively few realizations of essentially two-dimensional systems. In particular the only known examples of the interesting S\1/2 triangular lattice antiferromagnet which according to Anderson is a candidate for RVB behaviour are LiNiO 2 and NaTiO 2 .However the latter is a poor model compound and a high resolution powder di§raction study published this year shows that NaxTiO 2 (x[1) has previously 468 S.T. Bramwell Fig. 8 The ‘row’ model for frustrated triangular lattice antiferromagnets with horizontal bonds (double lines) of di§erent strength to other bonds (single lines) unsuspected multiple-phase behaviour which casts doubt on the validity of NaxTiO 2 as a model S\1/2 material.67 A new series of essentially two-dimensional triangular lattice antiferromagnets has been identified in which the spin value can be varied from S\5/2 to S\1/2. They are an extensive series of compounds related to the mineral yavapaiite KFe(SO 4 ) 2 . The structure type is such that magnetic ions (e.g.Fe3`) occupy a regular or very slightly distorted triangular lattice in well separated layers. Bramwell et al.68 reported a susceptibility study of RbFe(SO 4 ) 2 which has an equilateral triangular lattice and KFe(SO 4 ) 2 and KTi(SO 4 ) 2 which both have isosceles triangular lattices. RbFe(SO 4 ) 2 appears to be a good example of an S\5/2 two-dimensional Heisenberg lattice antiferromagnet with a weak cusp in the magnetic susceptibility at T\4.2 K. KFe(SO 4 ) 2 also with S\5/2 has a more rounded susceptibility curve reflecting the relief of frustration which results from the orthorhombic distortion and can be thought of as a realization of the so-called ‘row model’ described in Fig. 8. The susceptibility vs. temperature for these two compounds are plotted in terms of reduced units in Fig.9. KTi SO 4 ) 2 which was made for the first time has S\1/2 and is a realization of the quantum row model. Zhitomirsky69 has published an analysis of the phase diagram of the classical stacked row model as a function of the various magnetic couplings involved; it will be interesting to compare the behaviour of KFe(SO 4 ) 2 and its relatives with these predictions. Inami et al. have extended work on the yavapaiite compounds to the analogous molybdates and have published a high field magnetization study of CsFe(SO 4 ) 2 and CsFe(MoO 4 ) 2 .70 Another frustrated triangular lattice system is 3He adsorbed on graphite as described below. New magneto-optic e§ects Magneto-optical e§ects provide some of the most sensitive methods of investigating magnetic materials as well as being of practical use in the optical industry.It is therefore always of interest when new e§ects are discovered. Rikken and van Tiggelen71 reported the observation of a new magneto-optic e§ect which they had previously predicted theoretically. It is to some extent analogous to the Hall e§ect in which an electrical current when subject to a transverse magnetic field develops a component perpendicular to both the current direction nd the field. In a non-scatter- 469 Magnetism Fig. 9 Reduced susceptibility vs. reduced temperature for RbFe(SO 4 ) 2 (upper curve) and KFe(SO 4 ) 2 (lower curve). The former approximates to the model in Fig. 7 the latter to Fig. 8 ing non-absorbing medium there is no analogous e§ect on a flux of photons. Such ‘beam walk o§’ can be caused by absorption and Rikken and vanTiggelen have now shown that scattering can have the same e§ect.The experiments were performed using fine (2 km) CeF 3 powder suspended in glycerol which has a di§erent refractive index. The e§ect observed may be compared to the well-known Faraday e§ect in which a magnetic field applied longitudinally to a sample causes rotation of the plane of polarization. A perhaps more dramatic magneto-optic e§ect was discovered by Pittini et al.,72 following on from earlier work by Reiem et al.73 When light is reflected from the surface of a ferromagnet its polarization rotates the so-called Kerr e§ect which arises from spin-dependent transitions. Normally the e§ect is rather small typically only a few degrees but Pittini et al. have shown that a crystal of CeSb at 1.5K and in a field of 5T has a giant Kerr e§ect of 90°.This is 6.5 times more than the previous record rotation of 14° and it is the absolute maximum observable rotation in a single reflection. The authors suggest that the e§ect is caused by the proximity of the band edge to an optical transition involving the paramagnetic f electron of Ce. Langmuir–Blodgett films Following the pioneering work of Pomerantz74 the subject of magnetic Langmuir –Blodgett films lay dormant for several years probably as a result of the di¶culty in making magnetic measurements on such systems. In recent years magnetochemists have shown renewed interest in such systems. This year sees an electron paramagnetic resonance study by Seip et al.75 on a monolayer Langmuir–Blodgett film of manganese octadecylphosphonate Mn(O 3 PC 18 H 37 )·H 2 O.The authors show that in-plane antiferromagnetic coupling J\[2.8 K is similar to that observed in bulk samples of manganese octadecylphosphonate. The latter order magnetically at about 15 K but the resonance signal 470 S.T. Bramwell was too weak below 17 K to probe this e§ect in the film. Ando et al.76 reported the formation of Langmuir–Blodgett films of stearates of Mn Fe Co and Ni. The Ni film has ferromagnetic interactions. Nuclear magnets Although nuclear magnetism plays a decisive role in chemistry via nuclear magnetic resonance the role of nuclear spins in bulk magnetism is usually ignored. This is acceptable because the magnitude of the nuclear contribution to the paramagnetic susceptibility is usually only about 10~6 that of the electronic susceptibility and the temperature scale for ordering of the nuclear moments is of the order of mK or even kK.Nevertheless advances in instrumentation have in recent years allowed physicists to observe nuclear magnetism. Gross et al.77 have described a SQUID ac susceptometer which has been tested by measuring the nuclear susceptibility of HoVO 4 in the range 15mK–130mK. In this material the nuclear paramagnetism of Ho is enhanced by hyperfine coupling to the electronic spin system. Nuclear magnetic ordering has been observed in AuIn 2 at 35 kK and has been shown to a§ect the superconductivity of the sample.78 Other nuclear ordering phenomena reported this year include hexagonal close packed solid 3He which is ordered ferromagnetically at 0.6 mK.79 The nuclear magnetism of 3He has been exploited as a method of making truly twodimensional magnets.The results have been reviewed by Godfrin and Rapp,80 and may be compared to the behaviour of other truly two-dimensional systems such as Langmuir–Blodgett films74 and ultrathin metallic films.81 In 3He absorbed on graphite three particle exchange leads to ferromagnetic coupling and two particle exchange to antiferromagnetic coupling the result is a two-dimensional frustrated Heisenberg magnet with a triangular lattice.82 Theoretical work A small selection of theoretical studies that are of relevance to magnetic materials are listed here. The question of quantum disordered ground states in low dimensional frustrated magnets has been of interest to theorists for a number of years. This year Schulz et al.83 report exact diagonalization of clusters of up to 36 spins of the Heisenberg antiferromagnet on the square lattice with competing antiferromagnetic (J 1 ) and ferromagnetic (J 2 ) bonds.They find that for 0.34\J 2 /J 1\0.68 there is a region of no magnetic order. Albrecht and Mila84 have investigated the transition between valence bond order and magnetic order in a two-dimensional frustrated Heisenberg model. Deaven and Rokhsar85 have used variational methods to investigate the ground states of the quantum spin S\1/2 magnet on various two-dimensional lattices. For the square honeycomb and triangular lattice they find evidence of order while for the kagome� lattice they do not. A problem of direct relevance to cuprate superconductors is the annealed positions adopted by ferromagnetic bonds when doped into a two-dimensional antiferromagnet.Salem and Gooding86 have determined the ground state of such a system in both the classical and quantum cases. In the classical case there are an infinite number of degenerate arrangements of the ferromagnetic bonds but quantum fluctuations lift this degeneracy leading to phase separated ground states such as stripes of ferromagnetic bonds. The ferromagnetic order–disorder transition in the classical Heisenberg model in a 471 Magnetism fluid has been simulated87 and it is claimed that the critical exponents di§er by a small but significant amount from the ones for the lattice Heisenberg model. The random field Ising model relevant to CoxZn 1~xF 2 has been investigated by real space renormalization group techniques and all three critical exponents have been calculated in arbitrary dimension for the first time.88 In three dimensions the exponent b has the value 0.02.The spin dynamics of the classical two-dimensional XY model which is of relevance to many ultrathin films81 and layered magnets,89 has been investigated numerically.90 Both spin waves and central peak are found below the Kosterlitz –Thouless transition temperature in agreement with experiment.91 3 Ionic covalent and metallic materials Halides Halides with the layered perovskite K 2 NiF 4 -type crystal structure are classic quasi two-dimensional magnetic model materials. The magnetic ions (e.g. Ni2`) occupy a square lattice in well-separated layers formed by the halide ions. InK 2 CuF 4 the Cu2` moments with S\1/2 order ferromagnetically below T C \6.25 K.K 2 CuF 4 is a prototypical two-dimensional easy-plane ionic ferromagnet but is also of interest as an example of a compound in which a cooperative antiferrodistortive ordering of the partly filled Cu e' orbitals gives rise to ferromagnetism by enforcing orthogonal overlap. The distortion associated with the orbital ordering can be thought of as a cooperative Jahn–Teller e§ect. The question then arises as to why isostructural La 2 CuO 4 the parent compound of the high temperature superconductors has ferrodistortive orbital ordering and concomitant antiferromagnetism? Ishizuka et al.92 have shed light on this question by characterizing the magnetism of K 2 CuF 4 at pressures of up to 13 GPa. They have observed a pressure induced change from ferromagnetism to antiferromagnetism at 8–9 GPa which was interpreted in terms of a change from antiferrodistortive to ferrodistortive orbital order.Sekine et al.93 have studied the magnetism of the related compound [CuCl 4 (p-cyanoanilinium) 2 ] under hydrostatic pressure but have found that in this case the ferromagnetic exchange increases with increasing pressure presumably because of the shrinkage of Cu–Cl–Cu exchange pathway. Ferromagnetic exchange arising from a cooperative Jahn–Teller distortion also occurs in related compounds with the Jahn Teller ions Cr2` and Mn3` of which a well known example is Rb 2 CrCl 4 . Moro� n et al. have recently demonstrated that the series AMnF 4 (A\Na K Rb Cs) has a particularly rich magneto-structural chemistry.94 The structure type can be thought of as derived from K 2 NiF 4 by removing half theK ions and shifting the layers relative to one another.The isolated A ions induce small structural distortions in the MnF 4 layers which have a significant e§ect on the magnetism. Thus there is a gradual crossover in magnetic behaviour between tetragonal CsMnF 4 a collinear ferromagnet which orders at 18.9 K to monoclinic NaMnF 4 a canted antiferromagnet which orders at 13.0 K. The K and Rb compounds are respectively a monoclinic canted antiferromagnet and an orthorhombic collinear antiferromagnet which order at 5.2K and at 3.7K respectively. This year Moro� n et al. report a pressure study of the crystal structures of the series,95 with the main result that increasing the external pressure to the order of GPa has a similar e§ect to decreasing the size of the A ion in that it 472 S.T.Bramwell induces a series of distortions from tetragonal to orthorhombic to monoclinic. It would be of evident interest to study the magnetism of the pressure-induced phases. The hexagonal perovskite halides AMX 3 (A\alkali metal M\transition metal X\halide) form another classic series of model magnets. The crystal structure consists of linear chains of magnetic M ions stacked in a triangular array such that the inter chain interactions in the ab plane are frustrated. They might therefore be regarded as either one-dimensional or as triangular lattice magnets depending on the phenomenon being investigated. CsCoX 3 (X\Cl Br) are Ising-like while CsFeX 3 (X\Cl Br) are so-called ‘singlet ground state’ systems as the ground term of Fe2` in the absence of an applied field has M S \0.Visser et al.96 have described pressureinduced changes in the magnetic ordering of AFeX 3 arising from structural phase transitions. The compounds AMnBr 3 (A\Cs Rb) are XY-like. The anisotropy of their magnetization at high magnetic fields has been ascribed to quantum e§ects but Santini et al.97 have shown that the anisotropy occurs at a classical level provided thermal fluctuations are accounted for. CsMnI 3 and CsNiCl 3 are nearly Heisenberg systems with weak Ising anisotropy. The magnetic phase diagram of the mixed crystals CsNi 1~xFexCl 3 have been investigated by Takeuchi et al.,98 and the results interpreted in terms of a crossover from Ising-like to XY-like anisotropy with substitution of Fe2` ions. KNiCl 3 has the complication of small structural distortions.This year Petrenko et al.99 have reported neutron scattering measurements to reveal an unusual e§ect in a seemingly perfect crystal of KNiCl 3 . It segregates into two phases with di§erent lattice distortions in the basal ab plane at temperatures below T[270 K. These two phases have di§erent magnetic structures with T N \12.5K and 8.6 K. Cs 2 CuCl 4 although of the same stoichiometry as K 2 CuF 4 (mentioned above) forms a di§erent crystal structure with Cu2` spin chains running parallel to the orthorhombic b-axis. Coldea et al.100 have solved the magnetic structure of this compound using elastic neutron scattering. They find that below the ordering temperature T N \0.62 K the Cu2` spins (S\1/2) form an incommensurate structure with a temperature-independent ordering wavevector (0 0.472 0).The proposed magnetic structure at 0.3K is cycloidal with the spins rotating in a plane that contains the b-axis and arises as a result of the frustrated inter-chain interactions. Oxides As well as for their magnetoresistance described in Section 2 oxides with the perovskite structure continue to be of interest for other reasons. Kawasaki et al.101 have investigated the magnetic properties of the cubic perovskite solid solutions SrFe 1~xCoxO 3~d which were prepared by pressure treatment and high temperature oxidation of oxygen deficient samples prepared at ambient pressure. These phases contain Fe and Co in the rather unusual oxidation state of ]4. SrFeO 3 is an antiferromagnet with a Ne� el temperature of 134 K. At x[0.1 a change occurs to a ferromagnetic state with a high Curie temperature which reaches a maximum of 240K at x\0.6; it would seem that the phase Sr 2 FeCoO 6 is responsible for the ferromagnetism.Amow and Greedan102 have investigated the structural and magnetic properties of the orthorhombic (almost tetragonal) perovskite-type phases Nd 1~xTiO 3 with x\0 0.5 and 0.1. With x\0 the Ti ions have spin S\1/2 and the Nd3` ions have 473 Magnetism J\9/2. Antiferromagnetic ordering of the Ti3` sublattice was observed at T N \90K and 75K for the x\0 ol;0.05 samples respectively; and low temperature neutron di§raction on these samples indicated that both the Nd and Ti sublattices were ordered. However for the phase with x\0.1 no antiferromagnetic transition was found above 4K. Eylem et al.103 have investigated the isostructural series LaTi 1~xVxO 3 .The end members LaTiO 3 and LaVO 3 are antiferromagnetic insulators with Ne� el temperatures of 148K and 140K respectively. As in the case of Nd 1~xTiO 3 increasing substitution of Ti decreases the ordering temperature until at x\0.1 a paramagnetic and poorly metallic phase is formed. For x[0.25 antiferromagnetism is restored. The x\0.9 and x\1.0 phases display negative magnetization as previously discussed by Goodenough and Nguyen.104 Ten years after the discovery of ‘High-T#’ cuprate oxides with the layered perovskite structure continue to command a great deal of interest. This section will concentrate on purely magnetic problems. Birgeneau et al.105 have reviewed their extensive studies of the static and dynamic magnetic fluctuations in La 2 CuO 4 -based phases.La 2 CuO 4 with the K 2 NiF 4 structure is a quasi two-dimensional antiferromagnet with a Ne� el temperature of 325 K. Between 340Kand 820Kthe correlation length behaves exactly according to the predictions of the two-dimensional quantum non-linear sigma model (a continuum approximation to the Heisenberg model) in the renormalized classical regime. Substitution of La in La 2 CuO 4 by isovalent ions does not have a strong e§ect on the magnetic or electronic properties. Thus La 1.2 Tb 0.8 CuO 4 reported this year has a similar magnetic structure to La 2 CuO 4 ,106 although short range order of the Tb3` moments is observed at low temperature. The substitution of aliovalent ions on the other hand leads to hole doping with well known and extraordinary consequences.Thus the phase La 1.96 Sr 0.04 CuO 4 behaves as a spin glass and the phase La 1.85 Sr 0.15 CuO 4 as a superconductor with T# \37.3 K. The primary e§ect of hole doping in La 2 CuO 4 is believed to be the formation of ferromagnetic clusters or ferrons.107 Zakharov et al. report a susceptibility study of La 2 CuO 4`d also hole doped which lends support to this picture.108 Substitution of La for Nd in otherwise superconducting lanthanum cuprate compositions causes an anomalous suppression of the superconductivity. Tranquada et al.109 have described a neutron scattering study of the charge and spin order in the CuO 2 planes of La 1.48 Nd 0.4 Sr 0.12 CuO 4 which establishes a remarkable coexistence of two-dimensional magnetic and charge order below 50 K. The dopant-induced holes collect in domain walls which separate antiferromagnetic antiphase domains with the neodymium ions ordering magnetically below 3K.The sensitivity of superconductivity to lanthanide substitution is also well known in the ‘123’ type superconductors LnBa 2 Cu 3 O 7~d (Ln\lanthanide). In particular the compound YBa 2 Cu 3 O 7~d has a T# of \90 K but Y 0.5 Pr 0.5 Ba 2 Cu 3 O 7~d is nonsuperconducting. However only Pr among the lanthanides has this e§ect. Other evidence of the anomalous nature of Pr comes from the fact that the Pr moments in PrBa 2 Cu 3 O 7~d order antiferromagnetically at the relatively high temperature of 17 K while the homologous compounds of the other lanthanides only order at temperatures below 2.5 K. There is no fully accepted explanation of this anomaly but it is interesting to compare the behaviour of the related series LnSr 2 Cu 2 NbO 7`d (Ln\Y Pr Gd) in which the Y compound is a superconductor with a transition at 13 K.110 The Pr and Gd compounds are found to be non-superconducting down to 1.8 K but while the Gd 474 S.T.Bramwell moments order at 2.2 K the Pr moments remain magnetically disordered at this temperature. Other phases which illustrate the sensitivity of electronic properties to lanthanide substitution include the series Ln 3 Ba 2 Mn 2 Cu 2 O 12 .111 With Ln\Gd or Eu the behaviour is ferromagnetic below about 25 K while with Ln\Sm there is a spin glass transition at about 15 K. The magnetic properties of the lanthanide ions themselves in high-T# related oxides are also a popular field of research. Staub and Ritter112 have investigated HoBa 2 Cu 3 O 7 by neutron scattering and have observed two-dimensional short range ordering of the Ho3` moments even at 1.6 K a temperature far above the ordering temperature 0.19 K.Quasi two-dimensional ordering of Gd moments below 4K is observed in CaCdCuO 3 Cl and Ca 4 Gd 2 Cu 3 O 8 Cl 4 by susceptibility measurements. 113 Other cuprates of interest include the ‘spin ladder’ type compound LaCuO 2.5 . The spin ladder model was described in the 1994 Report and consists of parallel strongly antiferromagnetically coupled spin S\1/2 chains. It is a candidate for a non-magnetic ‘spin liquid’ ground state. Matsumoto et al.114 have reported that although the susceptibility of LaCuO 2.5 suggests a spin liquid 63Cu NMR measurements indicate antiferromagnetic ordering at 110 K.However it is interesting to note that the doping of 5% holes by substituting La for Sr suppresses this transition. Another chain-like cuprate Ca 2 CuO 3 has antiferromagnetic interchain coupling. Abdelgadir et al.115 have reported that sintering in oxygen produces a remarkable transition to room temperature ferromagnetism apparently by introducing excess oxygen to form Ca 2 CuO 3`d; one might tentatively suggest a similarity to oxygen excessive La 2 CuO 4 described above. The fact that the high-T# superconductors contain magnetic ions with spin S\1/2 means that any related magnetic oxides with S\1/2 are immediately of interest. James and Attfield have described several new Ni3` oxides with S\1/2 although these are all paramagnetic down to 6K. These include the series MSr 3 NiO 6 (M\Sc In Tm Yb Lu),116 the solid solutions Ln 2~x SrxNi 4Bd (Ln\La Nd Sm Gd; x[1)117 and the compound CeSr 7 Ni 4 O 15 .118 The latter is a metallic compound adopting the K 2 NiF 4 type structure with Ce and Sr disordered over the K sites.The Ni3` moments have a moment of 0.5 kB per atom a value which is much less than the spin only value of 1.73. Oxides with the ‘other’ majority ternary oxide structure type the AB 2 O 4 spinel structure have long been popular subjects of research. The magnetostructural properties of the spinels are very complex with cation ordering competing magnetic exchange and mixed valency all playing a role. Nowadays most attention is focused on spinel solid solutions although results on the site-ordered frustrated antiferromagnetic ZnFe 2 O 4 are described in Section 2.The structure and frustrated magnetism of the mixed spinels Cu 1~xZnxTiO 4 ,119 LiCr 1~xAlxTiO 4 ,120 CoFe 2~xCrxO 4 ,121 Mg 1`tFe 2~2tTitO 4 122 and Li[Mn 2~xLix]O 4~d123 have also been investigated. The latter series is particularly interesting. The compounds are normal spinels with Li on the A-sites and Mn on the B-sites of the spinel structure. The x\0 composition LiMn 2 O 4 has a formal oxidation state of]3.5 for theMnion and exists in both cubic and tetragonal forms. The dominant exchange coupling is antiferromagnetic with Curie–Weiss temperatures of [367K for the tetragonal form and [273K for the cubic form. There is a spin freezing transition at 60K for the tetragonal from and 55K 475 Magnetism for the cubic form reflecting the frustration of the B-site spinel lattice which is identical to the pyrochlore lattice described in Section 2.The substitution of Li on to the B-sites increases the formal oxidation state of manganese and causes a progressive decrease of both the magnitude of the Curie–Weiss temperature and the spin freezing temperature. At x\0.33 which corresponds to an oxidation state of ]4 the Curie–Weiss temperature is ]10 K indicating ferromagnetic coupling and the spin freezing temperature is also 10 K. The Ti and V homologue of LiMn 2 O 4 have been the subject of an investigation by Kirchambare et al.124 These compounds are again both normal spinels form the full range of solid solutions. LiTi 2 O 4 is a superconductor with a transition temperature of 13 K while LiV 2 O 4 is metallic apparently with localized magnetic moments.The compositions LiTi 1~xVxO 4 show a decrease in superconducting transition temperature with increasing x and the superconductivity is destroyed at x\0.05. The V ions are found to have a moment of 1.7 kB indicative of V4`. These localized moments are believed to be responsible for the destruction of superconductivity. Goya et al.125,126 have investigated the magnetic properties of the series of orthorhombic ternary rare earth oxides Ln 2 BaZnO 5 with Ln\Sm Eu Dy Ho and Gd mainly by susceptibility measurements. The Eu compound has a singlet ground state and the Sm compound has negligible magnetic coupling. The Dy and Ho compounds have quite large antiferromagnetic coupling for rare earth compounds with Curie–Weiss temperatures [15 and [11K respectively. The absence of magnetic ordering in these materials down to 1.8K was interpreted in terms of frustrated interactions.Gd 2 BaZnO 5 which also has a large Curie–Weiss temperature of 15.9 K shows an antiferromagnetic transition at 2.3 K; this is raised to 12.0Kin the analogous Cu compound Gd 2 BaCuO 5 . Among other rare earth oxides a new ternary samarium rhenium oxide Sm 3 Re 3 O 7 has been described.127 The Re ions appear to be in the]5 oxidation state with S\1 and the Sm ions show a very large temperature independent paramagnetism. There is however only very small magnetic coupling between the Re ions with a Curie–Weiss temperature of [1.4 K. Hydroxides and other hydroxy-salts Hydroxides often have very interesting magnetic properties but have been rather neglected in the past perhaps because of di¶culty in obtaining good quality crystalline samples.Metal hydroxide layers are a common structural motif in later transition metal hydroxides and hydroxy-salts and altering the interlayer separation can be used as the basis for chemical ‘tuning’ of the magnetic properties. Thus in Co(OH) 2 the layers can be forced up to 25Å apart by exchange of (OH) by organic or inorganic anions.128 The intralayer interactions are ferromagnetic. In the three-dimensional ordered magnetic phase the ferromagnetic planes are stacked antiferromagnetically for small interlayer spacing and ferromagneticaly for large interlayer spacing; for example [Co 2 (OH) 3 (C 12 H 25 )SO 4 ]·2H 2 O has a Curie temperature of 8K. The compounds [Cu 2 (OH) 3 (CnH 2n`1 )CO 2 ] with n\0 and 1 similarly have ferromagnetic intralayer magnetic exchange and show metamagnetic behaviour at 20 K.129 However their homologues with n\7 8 and 9 appear to have an antiferromagnetic intralayer interaction undoubtedly arising from structural changes with the layer on intercalation.476 S.T. Bramwell Ni(OH) 2 is also a layered compound with ferromagnetic intralayer interaction exhibiting metamagnetism below T N \25.8 K.130 The series [Ni(OH) 2`xNO 3 ) 2~x] (x\0.64 0.74 1.14) however show spin glass behaviour below 10 K showing that the substitution of nitrate severely disrupts the magnetic long range order. Another nickel(II) hydroxynitrate [Ni(OH)(NO 3 )]·H 2 O behaves as a double chain system with two ferromagnetic intrachain interactions between neighbouring S\1 spins of magnitude]26.9K and]6.9 K.130 Salts of oxy-anions phosphates perrhenates silicates and carbonates Phosphates of the first row transition metals commonly adopt layered structure which can intercalate long chain alkylammonium cations.Such materials can be of interest magnetically as examples of quasi two-dimensional systems in which the magnetic coupling in the third dimension can be perturbed in a systematic way by intercalation. Goni et al.131 have prepared the new layered compound [NiH(PO 4 )]·H 2 O and its intercalated derivatives [NiH(CnH 2n`1 NH 2 )x(PO 4 )]·H 2 O with n\3–7. [NiH(PO 4 )] ·H 2 Ois antiferromagnetic with a maximum in the susceptibility at 12 K. The magnetic behaviour is not strongly a§ected by intercalation reflecting two-dimensionality [NiH(C 7 H 15 NH 2 ) 0.76 (PO 4 )]·H 2 O is also antiferromagnetic with a maximum in the susceptibility 11 K.Another layered material [Fe(OH)(H 3 NCH 2 CH 2 NH 3 ) 0.5 (PO 4 )] has a weak ferromagnetic transition at about 30 K;132 homologues with di§erent alkyl chain lengths have not been reported. The perrhenates [M(ReO 4 ) 2 ] (M\Ni Fe Co) again have a layered crystal structure and order ferromagnetically at 12.5 K 8.5Kand 4.7K respectively.133 The Ni compound exhibits substantial coercivity (\0.26T at 4.2 K). The pyrosilicate BaCo 2 Si 2 O 7 contain chains of distorted tetrahedral CoO 4 groups interlinked by Si 2 O 7 . The compound has a weak ferromagnetic transition at 21 K. The magnetic critical exponents of FeCO 3 an antiferromagnet ordering at 38 K have been determined by time-of-flight neutron di§raction techniques and have been found to be in good agreement with those of the three-dimensional Ising model.134 Pnictides and pnictide oxides The compounds CePdPn with Pn\As Sb have been found to be quasi two-dimensional metals with metallic conductivity perpendicular to the crystallographic c-axis but semiconducting behaviour parallel to c.135 The magnetic behaviour is similarly anisotropic with CePdAs and CePdSb showing paramagnetic susceptibility perpendicular to c but apparently antiferromagnetic behaviour parallel to c.The Ne� el temperatures are 4K and 17K respectively. Brock et al.136 have reported neutron di§raction results for the pnictide oxides Sr 2 Mn 3 Pn 2 O 2 with Pn\As Sb. The crystal structure of these compounds consists of alternating tetragonal layers of MnO 2 2~ and Mn 2 Pn 2 2~ separated by Sr2` ions.The Mn occupy a centred tetragonal lattice in the pnictide layer and a primitive one in the oxide layer; however the symmetry of the structure is such that the layers are substantially decoupled. Thus in both the Pn\As and Pn\Sb compounds the manganese ions in the oxide layer order antiferromagnetically at ambient temperature (300K Sb 340K As) while ordering e§ects are not observed in the Pn layer until much lower temperature. In Sr 2 Mn 3 Sb 2 O 2 the Mn ions in the pnictide layer order antiferromagnetically at 65 K while in Sr 2 Mn 3 As 2 O 2 they remain disordered until at least 4K despite a build up in two-dimensional correlations. 477 Magnetism Chalcogenides and phosphide chalcogenides BaVS 3 has a hexagonal crystal structure consisting of chains of face-sharing VS 6 octahedra.At low temperatures it behaves like a semiconducting one-dimensional antiferromagnet with spin S\1/2; however at 70K there is a transition to a metallic state. Imai et al. have determined the entropy change for this phase transition by specific heat measurements and shown that is close to Rln2 the expected magnetic entropy for S\1/2 spins of V4` ions.137 Well known for their electronic and intercalation properties are the layered transition metal dichalcogenides of Groups IV V and VI. Miwa et al.138 reported the synthesis and magnetic properties of MnxNbS 2 with 0.19\x\0.52. The compounds show metallic behaviour with paramagnetic Mn2`. However the Curie–Weiss temperature changed from positive to negative at x\0.4. MPS 3 type compounds (M\Ni Mn) similarly consist of van der Waals bonded layers which may intercalate a variety of species.Qin et al.139 report a new intercalation compound Mn 0.86 PS 3 (2,2@-bipy) 2 which is ferromagnetic below the Curie temperature of 40 K. The Chevrel phase type compoundsMxMo 6 Ch 8 (M\metal; x\1; Ch\S Se Te) are also well known for their remarkable physical properties which arise from the electron transfer from the A ion to the molybdenum chalcogenide host clusters. Daoudi et al.140 have reported the magnetic behaviour of UMo 6 S 8 U 0.82 Mo 6 Se 8 and Th 0.81 Mo 6 Se 8 which illustrate the sensitivity of the electronic properties to the A ion. Thus while UMo 6 S 8 is paramagnetic do to 2K U 0.82 Mo 6 Se 8 is weakly ferromagnetic below 25 K and Th 0.81 Mo 6 Se 8 appears to have a superconducting transition at 3K.Zhang et al.141 reported the magnetic properties of amorphous Fe 5 (InTe 4 ) 3 which can be prepared by the metathesis reaction between FeCl 3 and the Zintl phase K 5 (InTe 4 ) in water; the compound has a spin glass-like transition at about 6K. Boron carbides The discovery in 1994 of superconductivity at 12K in tetragonal YNi 2 B 2 C142 led to increased activity in the field of intermetallic superconductors. There is much interest in the magnetism of the members of the series LnNi 2 B 2 C (Ln\lanthanide) with magnetic Ln ions. With Ln\Tm Er Ho and Dy superconductivity and magnetism coexist below typically 10 K but with Ln\Th and Gd the magnetic transitions occur at slightly higher temperatures (15.5K and 19.4K respectively) and there is no superconductivity down to 2K.Among many other papers this year sees magnetic structure determinations of TmNi 2 B 2 C by neutron di§raction,143 and of GdNi 2 B 2 C by resonant and non-resonant X-ray scattering.144 TmNi 2 B 2 C exhibits superconductivity below 11K and magnetic ordering below 1.5 K; the magnetic structure consists of aligned ferromagnetic (1 1 0) planes of Tm moments with the magnitude of the moments sinusoidally modulated along the crystalline (1 1 0) direction. It is suggested that the modulated structure allows the coexistence of magnetism and superconductivity. GdNi 2 B 2 C on the other hand orders magnetically at 19.4K with an incommensurately modulated antiferromagnetic spin structure and there is a further magnetic transition at 13.6 K. Between these two temperatures the modulation wave vector increases with decreasing temperature.The use of X-ray techniques in this magnetic structure determination is noteworthy. Gadolinium has very large neutron absorption which means that neutron investigations of Gd salts require expensive isotopic substitution. 478 S.T. Bramwell Metallic elements and intermetallic compounds Although not of immediate interest to many chemists intermetallic compounds display a huge wealth of magnetic phenomena of which a small selection are illustrated here. Plutonium is at the critical composition between the light actinides which have itinerant 5f electrons and the heavy actinides which have localized 5f electrons. Meotreymond and Fournier have argued that that the high temperature d-Pu phase has far more localisation than the low temperature a-Pu phase.145 There is a 26% volume expansion at the a–d phase transition which the authors argue is indicative of electron localization.They conclude that ‘the delocalization-localization transition of the 5f electrons along the actinide series starts within the plutonium phase diagram’ a conclusion which was previously predicted by Antropov et al.146 on the basis of band structure calculations. The alloys CeFe 1~xRux (x\0.07 0.08) show a metamagnetic transition in which ferromagnetic sheets aligned antiferromagnetically in zero field are aligned ferromagnetically by the application of a field.147,148 In CeFe 1~xRux the transition is accompanied by giant ([20%) magnetoresistance. Other metallic metamagnets include EuPdIn and EuAuIn149 which order at 13K and 21K respectively.Frustration e§ects are as common in intermetallics as they are in ionic and covalent solids. As a result of frustrated interactions the LnNiAl series (Ln\Tb Dy Ho) show successive ferromagnetic and antiferromagnetic transitions below 30 K and the TbNiAl shows sinusoidal ordering.150 FePt 3 is a frustrated antiferromagnet and when alloyed with ferromagnetic CoPt 3 becomes a spin glass.151 Hippert et al.152 have reported an interesting e§ect that occurs in the melting of Al 1~x~yPdxMny quasicrystals such as Al 0.771 Pd 0.207 Mn 0.0702 . TheMn atoms which are diamagnetic in the solid state acquire a local moment of about 3kB in the liquid which can possibly be explained in terms of a di§erent local environment. This agrees with the theoretical prediction of Smirnov and Bratovsky who considered AlMn liquids.153 Micron thickness films of Ho have been used to probe the phenomenon of ‘two length scales’ observed in neutron and X-ray scattering experiments.154 At a critical point one expects critical fluctuations on all length scales up to a ‘cut-o§’ the correlation length m.In quasielastic neutron or X-ray scattering the critical spin fluctuations give rise to a scattering function in reciprocal space with a Lorentzian line shape the width of which is equal to 1/m. However recent improvements in instrumental resolution have revealed that the scattering is often described by two Lorentzian line shapes the second with a width about 1/10 that of the first. In order to prove the possible involvement of surface e§ects Gehring et al.154 have measured the critical scattering at the 131K magnetic phase transition in 1 km Ho films and once again observed the two length scales.However the origin of the e§ect remains unclear. Major improvements in nanotechnology in the last decade have meant that ultrathin metallic films of one to three monolayers thickness can be deposited on nonmagnetic metallic substrates. An example of this is ferromagnetic Fe on the (1,0,0) surface of W.155 This material appears to be an almost ideal realization of a twodimensional XY magnet showing the finite size critical exponent b\3n2/128\0.23 predicted by Bramwell and Holdsworth89 (see last year’s report).1 Shi et al.156 have made submicron ferromagnets in GaAs semiconductors by the ion implantation of 479 Magnetism Mn2` followed by annealing which precipitates GaMn ferromagnetic clusters.The bulk behaves as a room-temperature ferromagnet. Methods of a more chemical nature to nanoparticulate magnets include the reduction of a metal halide using organic solvents in the presence of lithium. Leslie-Pelecky et al.157 have used this technique to make ultrafine Co particles stabilized in a polymer matrix; the materials show ferromagnetic properties at room temperature with additional spin-glass-like transitions at about 10 K. Another reported chemical method to nanoparticulate magnets is the reduction of Fe2` ions to Fe by KBH 4 in aqueous solution.158 4 Coordination complexes and molecular compounds Framework structures; three-dimensional systems The bimetallic complex [CoCu(OH)(H 2 O) 3 (pba)] [pbaOH\2-hydroxy-1,2- propylenebis(oxamato)] is of interest as a three-dimensional ferromagnet which orders at 38 K and has rather a large coercive field of about 6 kOe at 2K.159 The dilute ferromagnetic complexes [Fe 1~xAsxCl 2 S 2 CN(C 2 H 5 ) 2 ] order magnetically below about 2K.They are expected to show critical exponents of the universality class predicted by Kawamura160 which has a large positive specific heat exponent a. According to the Harris criterion the positive a means that critical exponents will depend upon dilution. Careful susceptibility measurements by DeFotis et al.161 show that the susceptibility exponent c does indeed depend on x although only slightly. It increases from c\1.19 at x\0.014 to c\1.22 at x\0.040. An unusual e§ect has been observed in the linear chain antiferromagnet [MnCl 3 (CH 3 NH 3 )·H 2 O.162 A 1% substitution of Mn for Cu or Cd leads to a remnant magnetization when samples are field cooled through the three-dimensional Ne� el temperature of 4.12 K.There is continued interest in developing purely organic ferromagnets. The main problem is not to find a ferromagnet but to find one with a reasonably high Curie temperature. Thus a report of interest is that by Ueda et al.,163 who have found that the purely organic tetracyanoquinodimethane (TCNQ) charge-transfer salts R`(TCNQ) 0.5 (TCNQ~·) [R\Cs` and N(CH 3 ) 4 `] are room temperature ferromagnets with coercive fields of 100 Oe and 300 Oe respectively. However the materials are not fully structurally characterized and the saturation moments are only about 1/1000 that expected for full spin ordering. A metal-containing ro-temperature molecular ferromagnet is the amorphous [V(tetracyanoethene)x]·(solvent) the improved synthesis of which has been reviewed.164 A novel new molecular magnet is the sulfurnitrogen radical p-NC-C 6 F 4 -CNSSN 1 which has a Curie–Weiss constant of [107K and a weak ferromagnetic transition at 36 K.165 The sensitivity of molecular magnetism to chemical substitution crystal structure 480 S.T.Bramwell and pressure e§ects has been demonstrated by several groups. Thus Togashi et al.166 studied 165 organic radicals of the type 4-arylmethyleneamino-2,2,6,6-tetramethylpiperidin- 1-yloxyls (and related compounds) and found that 52 compounds exhibited ferromagnetic coupling but the highest Curie–Weiss temperature among these was only 0.75 K for the compound 2. Interestingly an antiferromagnetic Curie–Weiss temperature was found which was much higher than this:[24K for the compound 3.The prototypical purely organic ferromagnet b-nitrophenyl nitronylnitroxide has a Curie temperature of 0.6Kunder ambient pressure but this is lowered to 0.35Kunder a pressure of 7 kbar.167,168 Di§erent behaviour is observed for the compound [NH 4 ] [Ni(maleonitrile) 2 ]·H 2 O.169 The structure of this charge transfer salt consists of stacked planar metal ligands separated by ammonium cations; the Ni(maleonitrile) 2 anion is 4. Coomber et al.168 found that the ferromagnetic Curie temperature increased from 4.5Kat ambient pressure to 7Kat 6 kbar and ascribed this large change to a switch from dominant metal-sulfur to dominant metal–metal interactions with increasing pressure.At 6.8 kbar the ferromagnetic order abruptly disappeared probably as a result of a structural phase change. Polymorphism is always a possibility with molecular compounds and this can a§ect the magnetic properties. Thus the charge-transfer salt FeMC 5 (CH 3 ) 5N2·TCNQexists as three di§erent structural phases which are respectively paramagnetic metamagnetic and ferromagnetic at low temperature.170 Layered structures; two-dimensional systems The layered bimetallic oxalate complexes AMIIMIII(C 2 O 4 ) 3 (A\monovalent organic cation) show a wealth of interesting magnetic behaviour which depends upon the metals MII andMIII and the nature of the A ion. Mathonie` re et al.171 have characterized an extensive series of compoundsAMIIFe(C 2 O 4 ) 3 [MII\Mn Fe; A\alkyl- and aryl-ammonium phosphonium and arsonium ions].In these compounds the metal ions occupy alternate sites in a two-dimensional honeycomb lattice. The metals are coordinated by oxalate and neighbouring layers are separated by the organic cations. TheMII\Fe series behave as ferrimagnets with Curie temperatures between 30Kand 50 K but several exhibit an unusual crossover to negative magnetization at low temperature when cooled in a field of 10 mT. In simple terms the ferrimagnetic behaviour can be thought of as arising from the imbalance in atomic magnetic moment between antiferromagnetically coupled Fe2` and Fe3` but the origin of the negative 481 Magnetism magnetization is not yet understood. In the MII\Mn series both ions have spin S\5/2 and behave as low dimensional antiferromagnets with a broad susceptibility maximum at about 50 K but an increase in the magnetization characteristic of spin canting at lower temperatures.Bhattacharjee et al.172 have investigated the series N(C 4 H 9 ) 3 FeIIFeIII xCrIII 1~x- (C 2 O 4 ) 3 . The compound N(C 4 H 9 ) 3 FeIICrIII(C 2 O 4 ) 3 has a ferromagnetic transition at 14 K while N(C 4 H 9 ) 3 FeIIFeIII(C 2 O 4 ) 3 has a ferrimagnetic transition at 35 K. For 0\x\1 the transition temperature varies smoothly between these extremes. For x[0.4 a spin glass-like transition is observed at a temperature just below the ferrimagnetic transition. Another series of honeycomb layered magnets are the azidesM 2 (N 3 ) 4 (bipyridinium) (M\CoII and FeII). These compounds exhibit metamagnetic behaviour below 60K and 40K respectively.173 The compound KMn(3-CH 3 O-salen)Mn(CN) 6 174 is also a quasi two-dimensional metamagnet and also exhibits negative magnetization below the Ne� el temperature of 16 K.Layered charge-transfer salts based on bis(ethylenedithio)tetrathiofulvalene (BEDT) are well known as ‘molecular metals’ and also show magnetic coupling with a quite high temperature scale although so far only antiferromagnetic coupling has been observed. For example Kurmoo et al.175 have investigated the compounds MCl 4 (BEDT-TTF) 2 (M\Ga Fe). The magnetic susceptibility of the semiconducting Ga compound is well described by a model containing two antiferromagnetic spin dimers with couplings of 108K and 212 K. That of the Fe compound is modelled by a single antiferromagnetic dimer with coupling 45K in addition to an almost paramagnetic term arising from the Fe3` moments.The exchange coupling between the magnetic moments on the Fe and those on the organic layer is apparently negligible. Chain structures; one-dimensional systems From a theoretical point of view one- and two-dimensional magnets are not expected to order magnetically. However the one-dimensional Ising model and the twodimensional Heisenberg and XY models are all at their lower critical dimension and order is easily restored by minor perturbations. From a practical point of view this means that the only systems that actually show a marked reluctance to order magnetically are one-dimensional magnets that approximate the Heisenberg or XY model. The one-dimensional Heisenberg antiferromagnet also has the unusual property of the magnetic ground state depending strongly on spin value.Thus while in general the grounds state is a singlet only the even-spin chains have an energy gap to the first magnetic excited state which makes them particularly resistant to magnetization. This so-called Haldane gap has been observed in several compounds but mainly with spin S\1. This year Granroth et al. report evidence of the Haldane gap in a spin S\2 chain MnCl 3 (bipyH).176 The compound has exchange coupling of about 30 K but no magnetic order at 30 mK and at that temperature no magnetization in fields of up to 1T. Several quasi one-dimensional Heisenberg ferromagnets have been reported. An example is the organic radical 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl which has ferromagnetic exchange coupling of about 6K and a transition to an ordered state at 0.68K induced by inter-chain exchange of magnitude 0.21 K.177 The low ordering temperatures of ferromagnetic chains have not deterred workers from seeking chain-like molecular magnets.Lang et al.178 have reported a magnetic 482 S.T. Bramwell characterization of the organic radical methyl-1,2,4-triazolenitronyl nitroxide for which the authors claim ‘exceptionally strong’ magnetic interactions although the Curie–Weiss temperature is only about 9K. Bohm et al.179 report a ferromagnetic Curie–Weiss constant of about 90K and a Curie temperature of 15K for the complex [meso-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphinato] manganese(III) tetracyanoethenide. Isolated complexes; ‘zero’ dimensional systems As well as being centre stage this year in terms of quantum mechanical tunnelling there have been a huge number of reports of dimers trimers tetramers and so on.However in most cases the magnetic measurements stop at the characterization stage and so there is little purpose in listing them all here. A handful of examples will su¶ce. Boca et al.180 report spin crossover behaviour in an FeII complex tris(pyridylbenzimidazole) iron(II). The crossover between a low-temperature low spin state to a high-temperature high spin state occurs in the temperature range 90–160 K. Castro et al.181 have described a complex of VIII with a butterfly structure which exhibits magnetic frustration. It has formula [V 4 O 2 (O 2 CEt) 7 (bipy) 2 (ClO 4 )] and contains both ferromagnetic and antiferromagnetic interactions. 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ISSN:0260-1818    

DOI:10.1039/ic093456

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

25 Conducting solids covering ionic and electronic conductors By M. G. FRANCESCONI School of Chemistry University of Birmingham Edgbaston Birmingham B15 2TT UK and P. R. SLATER School of Chemistry University of St Andrews St Andrews Fife KY16 9ST UK 1 Introduction The image of high T# superconductivity research is undergoing important changes. The frenetic activity engendered in the past to break transition temperature records is slowly turning towards obtaining a deeper understanding of the chemical and physical foundation of the phenomenon with some materials moved out of the laboratory into the technological arena. While known experimental evidence such as composition driven structural/physical transitions are still providing powerful tools to gain insight into the basic mechanism of superconductivity important results have been achieved in the more applications-oriented research such as the use of high T# materials in wires and cables.On the other hand the discovery of superconductivity in materials which do not appear to have stoichiometric long-range CuO 2 layers such as the spin ladder compound SrxCa 14~xCu 24 O 41 the high-pressure synthesised phase Sr 2 CuO 3`d and the isostructural ambient-pressure synthesised phase Sr 0.8 Ba 1.2 CuO 3`d raises fundamental questions concerning superconductivity in cuprates. In the area of ionic conductors and intercalation compounds Li batteries and solid oxide fuel cells continue to be the highest profile areas of research. For the former there is still considerable focus on the synthesis of new intercalation materials for use as electrodes.Increasingly in this search low-temperature synthesis routes are being used for the preparation of new materials and this is resulting in a whole host of interesting new compounds which are not possible by conventional high-temperature routes. In solid oxide fuel cells there is interest in cell operation at lower temperatures using electrolytes such as doped LaGaO 3 . The major problem associated with this appears to be a lack of an anode material with su¶ciently good properties at the lower temperature. 2 High temperature superconductors and related phases Mercury-containing superconductors Large-scale production of HgBa 2 Ca 2 Cu 3 O 8`d (HBCCO-1223) and even Royal Society of Chemistry–Annual Reports–Book A 489 HgBa 2 Ca 3 Cu 4 O 10`d (HBCCO-1234) compounds may now be encouraged by newly discovered synthetic routes under ambient-pressure conditions.In fact a nearly single phase HBCCO-1234 compound was prepared at ambient pressure for the first time by a controlled vapour/solid reaction technique using high purity monoxides as starting materials.1 The lower member of the mercury superconductors family HBCCO-1223 was found to play an important role in the intermediate step of this reaction. Samples of HgBa 2 Ca 2 Cu 3 O 8`d were reproducibly obtained in only one hour under ambient-pressure conditions by addition of B2wt% of ReO 2 and 2wt% of HgCl 2 HgF 2 HgBr 2 to the precursor mixture.2 Thermogravimetric analysis on HgBa 2 Can~1 CunO 2n`2`d (n\1–3) superconductors at various oxygen partial pressures has shown that a careful control of the oxygen partial pressure below 300 °C yields samples with di§erent oxygen contents.3 The T# decreases monotonically as the O content increases in the 1201 and the 1212 phases while it does not seem to be a§ected by changes in the oxygen content in the 1223 phase.The e§ect of oxygen defects in Hg-based superconductors has been investigated by means of Raman spectroscopy;4 a systematic evolution of spectra with an increasing number of Cu–O layers was found for the five members of the HgBa 2 Can~1 CunO 2n`2`d (n\1–5) series. The phonon modes associated with interstitial oxygen in the HgOd planes were identified and the spectral evolution related to the variation of interstitial oxygen content. The pressure dependence on the T# of HgBa 2 Ca 2 Cu 3 O 8`d and HgBa 2 Ca 3 - Cu 4 O 10`d has been investigated up to 30 GPa.5 The di§erent behaviour showed by these samples at low and high pressure was explained in terms of the presence of inequivalent Cu–O layers within the structure.The first measurement of the in-plane transport properties of a single crystal of the four-layer superconductor HgBa 2 Ca 3 Cu 4 O 10`d has been reported.6 The electrical resistivity and the Hall coe¶cient in the normal state indicate that this compound is underdoped. Superconducting carrier density and e§ective mass of HgBa 2 CuO 4`d in the over-doped region has been determined.7 An increase in the hole doping induced a decrease of superconducting carrier density but did not induce an increase of e§ective mass this suggested that localisation of superconducting pairs in the overdoped region can be excluded.It is well known that Ba-based mercury cuprates are moisture sensitive; substitutions of higher oxidation state cations on the mercury site stabilise these phases due to extra oxygen introduced into the Hg–O layers. The new (Hg,Re)Ba 2 Can~1 CunO 2n`2`d superconductors showed enhanced chemical stability compared to the undoped members of the HBCCO family without any significant alteration of the superconducting properties.8 Interesting results and confirmations of previous structural evidence emanated from the structural analysis of the first Hg 0.8 Bi 0.2 Ba 2 CuO 4`d single crystals. Extra oxygen was found to be located in the Hg–Bi layer in which linear HgO 2 coexist with BiO 6 octahedra bismuth being nearly pentavalent.9 In the new series of superconductors Hg 0.7 V 0.3 Sr 2~xLaxCuO 4`d La substitution on the Sr site was found to stabilise the Ba-free 1201 phase.10 Resistance measurements show that the T# does not change significantly with the La content.Substituting about 40% of mercury for gold in Hg-1223 led to overdoping of the Cu–O layers and to a progressive decrease of both cell parameters and T# with increasing gold content.11 On the other hand substitution of gold in the Hg-1245 490 M.G. Francesconi and P.R. Slater compound which is normally obtained in an underdoped state and has a T# \100 K led to an increase in the T# to 110 K. Synthesis and neutron powder di§raction studies of Pb-substituted HgBa 2 Ca 2 Cu 3 O 8`d were reported byWuet al.12 The 1223 structure was retained throughout the range of substitutions and the Rietveld refinement of the X-ray powder di§raction data indicated that Hg is partially replaced by Pb.The increase of T# up to 143Kin the Hg 0.7 Pb 0.3 Ba 2 Ca 2 Cu 3 O 8`d sample may be related to an increase of the copper oxidation state. Thallium substitution on the Hg site has been found to extend the Ca solubility on the Y site in the superconducting HgBa 2 (Ca,Y)Cu 2 O 6`d compounds prepared by a high pressure route. This has been associated with a compensation of excess negative charge.13 Measurements of magnetization hysteresis and temperature dependence of ‘zero-field cooled’ and ‘field cooled’ susceptibilities showed that TlxHg 1~x(Ca 0.86 Sr 0.14 ) 2 Cu 3 O 8`d samples with x[Ohave a much larger J#(H,T) and H*33(T) shifted to higher magnetic fields compared with the Tl-free materials.14 Thallium substitution on the Hg site seems therefore to improve the flux pinning.Surface morphological characterizations of Tl doped Hg–Ba–Ca–Cu–O superconducting films deposited by spray pyrolysis on MgO and ZrO 2 (100) crystals showed spiral-like growth features which have never been observed in Hg superconducting cuprates so far.15 The main superconducting families Theoretical and experimental studies oriented towards technological applications of high T# superconductors have shown that the intrinsic problem of thermally activated flux can be reduced in high-temperature superconductors by creating correlated columnar defects in the crystal lattice. These line-like defects are usually created by irradiating samples with heavy ions having energies of the order of a gigavolt.Nanorods of MgO were grown and incorporated into Bi 2 Sr 2 CaCu 2 O 8`d (BSCCO- 2212) by melt-texturing procedures.16 In general the enhancement in J# and the shift of the irreversibility line in the nanorod BSCCO composites are comparable to those observed previously in ion-irradiated samples. The use of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10`d in tapes requires Ag cladding that counterbalances the brittle nature of this superconducting material. The mechanical strength of the Ag sheath can be reinforced by addition of other metals. The use of Ag–Au–Mg alloys as sheath materials improved the uniformity of BSCCO core thickness and the Ag sheath mechanical strength without any deterioration of the superconducting properties.17 Microstructural studies of BSCCO-2223/Ag–Au–Mg revealed a more dense and aligned BSCCO structure particularly along the sheath–core interface.The possibility that the well documented suppression of the peritectic melting temperature of Bi 2 Sr 2 CaCu 2 Ox by Ag could be due to a combination of carbon and Ag has been refuted by the lack of any significant changes in the melting temperature of both BSCCO-2212 and BSCCO-2212]Ag under systematic increase of C content.18 Subsequently a detailed study of the e§ects of Ag and P(O 2 ) on the decomposition pathway of Bi 2 Sr 2 CaCu 2 Ox was performed.19 The decomposition pathway of Bi 2 Sr 2 CaCu 2 Ox with 0 2 and 10wt% Ag was examined at 0.01 0.21 and 1 bar P(O 2 ) by SEM/EDS. In all atmospheres Ag had no dramatic e§ect on the decomposition of 2212 and only modified the temperature and order in which some of the subliquidus solid phases nucleate and decompose.491 Conducting solids covering ionic and electronic conductors The preparation and properties of Pb/V-doped Bi based superconductors have been performed to find cations which favour the formation of the BSCCO-2223 phase.20 The authors found that when V substitutes Bi and the Pb content is kept constant the content of the 2223 phase in the sample approaches 87% while for V/Pb substitutions or V/Bi substitutions in non-Pb-containing samples the amount of 2223 phase in the samples is low and the intergrain connections poor. The BSCCO system has proved to be suitable for investigations of the relationship between interlayer coupling and superconductivity since it is possible to extend the distance between the two Cu–O blocks by either I 2 or HgX 2 (X\halogen) intercalation.Mercury diiodide intercalation does not a§ect the T# while it drastically increases the c-axis parameter:21 this picture is di§erent from the one of iodine intercalation where a large T# depression and smaller lattice expansion occur. Xenikos et al.22 have also studied redox intercalation of iodine in Bi 2 Sr 2 Can~1 CunOx (n\2 or 3). The c axis was elongated by about 3.6Åper intercalation stage and T# was reduced by 15Kin the 2212 and by 10K in the 2223 compound. Moreover only oxygen-deficient compositions reacted with the halogen to yield intercalation compounds whereas oxygenenriched hosts underwent decomposition. The origin of the modulated structure in the BSCCO family is still attracting significant interest since it has still not yet been completely clarified.The e§ect of 3d metal substitutions for Cu on the incommensurate modulation was examined in Bi 2 Sr 1.8 La 0.2 Cu 1~xMxOy (M\Fe Co Ni or Zn).23 Solubility limits of the transition metals on the Cu site are x\0.5 for Fe x\1 for Co and x\0.1 for Ni and Zn. Independent of the nature of the metal all the substitutions decreased the modulation periodicity. These results cannot be explained by the commonly accepted ‘extra oxygen’ model but they do support the argument that the origin of the superstructural modulation in Bi–cuprates is due to the structural mismatch between the Bi 2 O 2 layers and the perovskite blocks. Awana et al.24 have reported that 50% Ce can be substituted on the Ca site in Bi 2 Sr 2 CaCu 2 O 8`d causing a monotonic decrease of the c lattice parameter.The amount of Ce required to suppress superconductivity is about 25% nearly half that required for other rare earth elements. Magnetic susceptibility measurements revealed the non-magnetic nature of Ce in Bi 2 Sr 2 Ca 1~xCexCu 2 O 8`d implying a valence state of ]4. Isovalent substitutions on the Ba site in particular Sr for Ba have still been the leading theme of the last year’s research on Tl-containing superconducting cuprates. The role of Bi substituting on the Tl site has been studied by various groups. Wahlbeck et al.25 analysed the chemical reaction yielding (Tl/Bi) cuprates and found that (Tl,Bi)-1212 forms preferentially to (Tl,Bi)-1223 when only Sr is present. The plate-like morphology is useful for crystallite alignment during long length wire production.In addition the particle size of Tl 1.1~xBixSr 1.6 Ba 0.4 Ca 2 Cu 3 Oy increases remarkably for high Bi substitutions; however as the limit of substitution of Bi (x\0.25) was exceeded many impurity phases formed and J# was degraded.26 The structure of the as-synthesised Bi substituted Tl-1201 cuprate has been studied.27 Intergrowth of the 1201 structure and a new 1201 type superstructure characterized by substitution of Bi for Sr in the rock salt layer was found in a composition of formula (Tl 2@3 Bi 1@3 ) 0.97 Sr 1.86 Bi 0.14 CuO 5~d. Epitaxial superconducting Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 Oy thin films bearing a J# close to 3]106Acm~2 at 492 M.G. Francesconi and P.R. Slater 77Kand zero field have been prepared by laser ablation:28 a detailed description of the parameters used in the deposition was reported.The compound (Tl 1~yPby)Sr 2 (Ca 1~xYx)Cu 2 O 7 provides a suitable system for the investigation of structural and electronic phenomena leading to superconductivity since a composition-induced metal–superconductor–insulator transition is observed by the variation of both x and y. The hole distribution in (Tl 0.5 Pb 0.5 )Sr 2 (Ca 1~xYx)Cu 2 O 7 has been studied by X-ray absorption spectroscopy. 29 The intensity of the pre-edge peak increases with increasing Ca in the 0OxO0.5 range indicating that the Ca/Y chemical substitution induces hole states in the Cu–O planes near the Fermi level. The refinement of the crystal structure of (Tl 0.5 Pb 0.5 )(Sr 2~xBax)Ca 2 Cu 3 O 9 (0OxO0.6) revealed a significant o§-centering of the Tl site.30 The distance from the split site to the ideal position on the four-fold axis was found to be 0.26Å for the Ba containing samples but only 0.07Å for the Ba free sample.In order to overcome the lack of sensitivity of X-ray- and neutron-di§raction techniques towards more than two cations on the same site resonant synchrotron X-ray di§raction was used to investigate the cation distribution in Tl 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O 9 .31 The crystal structure of Tl 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O 9 has been studied at 300K and around T# (118 K) and the results suggested that subtle changes may occur in the crystal structure around T#. Local structural changes in a Tl 2 Ba 2 Ca 2 Cu 3 O 10 single crystal between 90 and 290K have been investigated.32 The oxygen atoms in the Tl layers were found to be disordered at two split sites above T# (120 K) becoming ordered at a fixed site around T#.The pressure dependences of Tl 2 Ba 2 Ca 2 Cu 3 O 10 up to 21GPa and of Tl 2 Ba 2 Ca 3 Cu 4 O 10 up to 14GPa were measured.33 For both phases a clear deviation from the parabolic pressure dependence was observed in the T# vs. P curve and this was interpreted as an indication of the presence of crystallographically inequivalent Cu–O layers; namely the inner and the outer Cu–O layers. The compound TlSr 2 YbCu 2 O 7 shows semiconducting behaviour indicating an underdoped state in agreement with the formal Cu valence of 2.0 by assuming Tl3`. The isovalent substitution of Ca on the Sr site induced superconductivity in the TlSr 2~xCaxYbCu 2 O 7 system for values of xP0.2.34 The authors suggested that superconductivity may be attributed to a mechanism of internal self-doping due to overlapping between the Tl 6s and Cu 3d(x2[y2) electronic bands.It is commonly accepted that substitutions on the Cu sites in YBa 2 Cu 3 O 7~x (YBCO-123) reduces the T# although this decrease seems to be more related to the specific Cu site being substituted as opposed to the oxidation state of the substituent ion. The cations Zn2` and Ni2` were reported to substitute on the Cu(2) site and suppress T# even by low doping levels while trivalent ions like Fe Co and Al were reported to substitute predominantly on the Cu(1) site resulting in a more moderate decrease in T#. Contrary to YBCO-123 compounds Raman scattering studies on YBa 2 (Cu 1~xNix) 4 O 8 (YBCO-124) indicate that Ni substitutes for Cu in the chain sites.35 The authors claim that this evidence is also supported by the smaller T# suppression caused by Ni doping compared to Zn doping.The reduction of T# for RBa 2 Cu 3~xZnxO 7~y (R\Yb Er Y Dy Gd Eu Sm or Nd) was found to depend strongly on the ionic radius of the rare earth element R.36 This e§ect resembles the ion e§ect on T# in R 1~xPrxBa 2 Cu 3 O 7~y where Pr substitutes on the R site and in RBa 2 Cu 3~xGaxO 7~y where Ga substitutes on the Cu(1) site in the 493 Conducting solids covering ionic and electronic conductors chains. In contrast Zn is more likely to substitute on the Cu(2) site in the Cu–O planes in the RBa 2 Cu 3~xZnxO 7~y system. The T# J# and structural properties of Co-doped YBa 2 Cu 3 O 7~d thin films fabricated by pulsed laser ablation have been measured and compared with the properties of the undoped YBa 2 Cu 3 O 7~d thin films.37 A much higher sensitivity to deposition conditions was found for the Co-doped YBa 2 Cu 3 O 7~d films in contrast to the undoped YBCO-123 films in which case there is a relatively large range of conditions for which the properties of the films remain unaltered.Considerable changes of structural and superconducting properties have also been shown by YBa 2 Cu 3~xAlxO 7~d single crystals annealed in di§erent atmospheres.38 The Al3` ions act as defects on the Cu(1) site and their influence on the physical properties is strongly related to their spatial distribution. Under reducing conditions at high temperature (TP950 K) the Al3` ions are likely to form clusters within the Cu(1) layer and this clustered distribution minimizes the amount of oxygen in the Cu(1) layer.The Al clusters are destroyed when the material is annealed in oxygen at high temperature. The declustering procedure is driven by a gain in entropy hence a statistical distribution of Al is gained. Annealing Ba-free YSr 2 Cu 3~xFexOy samples in an oxygen atmosphere at high pressure has been found to restore the superconducting properties in a remarkable way.39 The microscopic origin of this behaviour was studied and found to be connected to an Fe co-ordination number change from 4 (quasi-tetrahedral) to 5 (trigonal bipyramidal) in the chains. Specifically samples for x\0.3 prepared under standard conditions showed a T# \30 K a shielding fraction of 4% and three di§erent distributions for the Fe cations two in the chains and one in the planes.After annealing at 915 °C at 27MPa oxygen pressure the T# increased to 60 K the shielding fraction became almost 100% and Fe cations showed just one Fe site located in the chains. From studies of Ca substitution on the Y site in YBCO-123 compounds it was discovered that unlike the case of fully oxygenated systems the hole concentration in the Y 1~xCaxBa 2 Cu 3 O 7~y (yB0.3) system increases with the Ca/Y substitution and promotes a T# increase.40 Nevertheless for Ca concentrations higher than 0.15 the increased number of holes is compensated by a decrease in the O content. The additional carriers introduced by Ca substitution on the Y site in Y 1~xCaxBa 2 Cu 3 O 7~y were found to be located in the Cu–O layers.41 The identification of the phases formed during di§erent reactions between YBCO- 123 and CuO under high oxygen pressure to form YBa 2 Cu 4 O 8 led to the conclusion that even in Ba deficient compositions the preferred location of Ca in YBa 2 Cu 4 O 8 is still the Y site.42 Substitution of 20 atom%Ca for Nd in NdBaSrCu 3 O 7~d resulted in an increase of the T# from 64 to 77 K indicating that the as-prepared sample is underdoped.Calcium appears to influence the T# in a complicated way which has been analysed for di§erent Ca/Sr substitutional ranges and explained in terms of an underdoped state charge-transfer phenomena and Ca site occupancy.43 Investigations of the ion-size e§ect on transport properties in R 0.9 Ca 0.1 Ba 2 Cu 3 O 7~d systems (R\Tm Ho Gd or Nd) has revealed a strong decrease of the n H (Hall number) value as the ionic radius of the R cation increased.44 The authors attributed this behaviour to a strengthening of localisation e§ects due to disorder and lattice compression induced by Ca doping.Many models have been proposed in the past to explain the suppression of the T# in Pr-doped YBCO compounds the connecting thread between them being the oxida- 494 M.G. Francesconi and P.R. Slater tion state shown by Pr in these materials. Assuming an oxidation state of]4 for Pr Ca substitution was performed on the Nd site in Nd 1~2xCaxPrxBa 2 Cu 3 O 7~d in order to neutralize the e§ect of Pr doping on the same site.45 The linear T# decrease with the double Ca/Pr doping supported the initial idea of a charge-neutral doping and hence a ]4 Pr oxidation state.Further evidence was found in the variation of the Cu–O in-plane distance determined by neutron di§raction with the average ionic radius on the rare earth site. Thermal analyses and high temperature X-ray di§raction highlighted opposing behaviours of YBa 2 Cu 3 O 7~x and PrBa 2 Cu 3 O 7~x polycrystals at 300 °C.46 Superconducting YBa 2 Cu 3 O 7~x showed an endothermic anomaly and a weight loss while semiconducting PrBa 2 Cu 3 O 7~x showed an exothermic anomaly and a weight increase in the DTA and TGA patterns respectively. The lattice parameters of YBa 2 Cu 3 O 7~x increased remarkably at 300 °C. These results suggest that an unknown phase transition might be among the causes of the di§erent behaviours of the two compounds. The thermoelectric power and superconducting transition in high T# superconductors Dy 1~xPrxBa 2 Cu 3 O 7 have been systematically measured for 0OxO0.6.47 The carrier concentration was found to decrease with increasing x in a linear relation for x[0.15 this hole reduction is consistent with the hole depletion model and therefore the suppression of T# with Pr substitution in 123 compounds seems to come mainly from a hole-depletion e§ect supporting a Pr oxidation state of 4].While the structural- and superconducting-properties of oxidized REBa 2 Cu 3 O 7~d are retained for most of the RE atoms except for Tb Pr and Ce the properties of RESr 2 Cu 3 O 7~d have been found to be altered in a di§erent way. The influence of di§erent annealing conditions on the oxygen stoichiometry and superconducting properties of GdBa 2~xSrxCu 3 O 7~d (0OxO1.6) samples has been studied.48 Nearly single phase materials could be prepared for xO1.4 with T# and orthorhombicity decreasing monotonically with increase in Sr content.The relationship between oxygen content and the structural/superconducting properties still raises questions of paramount importance in the research on the n-type superconductor family. The oxygen dependence of the transport properties has been studied in overdoped Nd 2~xCexCuO 4Bd.49 This phase is only normally superconductobserved for the first time superconductivity in this system with x[0.18 in a Nd 1.78 Ce 0.22 CuO 4Bd film cooled from 800 °C to room temperature under vacuum. This suggests that the range of Ce substitution in which superconductivity is observed may be extended by a careful control of the oxygen content.The authors found that an excess of oxygen introduces spin impurities in the sample which may be responsible for the lack of superconductivity normally; it is well known that annealing under reducing conditions is necessary to induce superconductivity in the Nd 2~xCexCuO 4Bd materials and also that these n-type superconductors are characterized by the lack of apical oxygen. Single-crystal neutron di§raction analysis of structures of both reduced and oxygenated Nd 2~xCexCuO 4Bd revealed that the apical oxygen site may be partially occupied in oxygenatedNd 1.85 Ce 0.15 CuO 4Bd (950 °C inO 2 for 24 h) and that the occupancy decreased in the reduction step (900 °C in N 2 for 24 h) to produce the superconducting material.50 Owing to the lack of apical oxygen in the n-type superconductors the coupling between the Cu–Olayers might be expected to be weaker and thus these materials display an enhanced anisotropy.The pressure dependence of T# is remarkably di§erent from the hole-type superconductors; dT#(P)/dP is positive for 495 Conducting solids covering ionic and electronic conductors La 2~xSrxCuO 4 and Nd 2~x~ySrxCeyCuO 4 (co-ordination numbers for Cu are six and five respectively) while for Ln 2~xCexCuO 4 (Ln\lanthanide) it was found to be very small or even negative. Recent measurements of the electrical resistivity dependence on the pressure showed that dT#(P)/dPBO independent of the lanthanide host.51 The authors’ hypothesis is that this phenomenon may be related to the absence of charge transfer into the Cu–Oplanes as a result of the absence of apical oxygen.Lattice e§ects in the Ln 2~xCexCuO 4 system could be separated from charge-transfer e§ects by electrical resistivity and Seebeck coe¶cient measurements. It was demonstrated that the lattice contraction alone lowers T# and therefore the intrinsic contribution to dT#(P)/dP is negative and in most cases dominates the charge transfer. Structural distortion as a function of Tb substitution has been studied in the (Nd 1~xTbx) 1.85 Ce 0.15 CuO 4 series.52 For low Tb contents the samples showed superconductivity although the T# decreased rapidly with increasing x. For high Tb contents X-ray-neutron- and electron di§raction-studies indicated that a long lattice deformation occurs that is characterized by the rotation of the CuO 4 squares around the c axis (the orthorhombic phase observed in Gd 2 CuO 4 ).These data suggest that the rotation of the CuO 4 squares reduces T# resulting in a strong e§ect for larger rotation angles. In the La 2~xSrxCuO 4 system dopant and temperature-induced structural phase transitions from orthorhombic (Cmca) to tetragonal (I4/mmm) take place and are mainly characterized by the tilting of the angle of the CuO 6 octahedra decreasing as the dopant concentration or the temperature increase. This produces non-tilted octahedra and consequently flat Cu–O planes in the superconducting phase. The La K-edge polarized XAFS of La 2~xSrxCuO 4 indicated that the Sr dopant and the temperature induced orthorhombic]tetragonal phase transitions are of di§erent natures.53 The Sr-induced transition exhibits a partial displacive character at least up to x\0.15 which is manifested through a decrease in the tilt angle of the CuO 6 octahedra.The temperature-induced phase transition in contrast does not show any displacive character and is only due to disorder; the tilt angle remains the same over the whole temperature range studied. The series La 1.85 Sr 0.15 Cu 1~xZnxO 4 also shows a tetragonal to orthorhombic phase transition as the Zn content increases beyond 15 atom %; this phase transition resembles the one occurring in La 2~xSrxCuO 4 as the Sr content decreases below 0.085.54 The contraction along the c axis and the tilt of the CuO 6 octahedra increase with the Zn content in the orthorhombic phase. Owing to the presence of extra oxygen in interstitial sites La 2 CuO 4`d shows p-type superconductivity as does the related compound La 2~xSrxCuO 4 .However the investigation of the oxygen rich material is hindered by the occurrence of a phase separation into two distinct orthorhombic phases with di§erent oxygen contents. The suppression of this phase separation may be achieved through the partial substitution of trivalent cations such as Bi3` or Nd3` for La3` which introduce a random potential into the LaO layer. The compounds La 1.95 Bi 0.05 CuO 4`d with various values of d were obtained by an aqueous KMnO 4 solution oxidation method leading to d values twice as large as the ones obtained under high oxygen pressure.55 The maximum value of T# is 35 K about 10K higher than the values obtained when La 1.95 Bi 0.05 CuO 4`d is oxidized under high oxygen pressure. In addition superconductivity appears in tetragonal and orthorhombic phases suggesting that orthorhombicity is not strongly re- 496 M.G.Francesconi and P.R. Slater lated to superconductivity in these compounds. Room-temperature chemical oxidation has also been exploited in the preparation of La 2`xBaxCuO 4`y (0OxO0.15) compounds.56 The excess of oxygen is located at the interstitial sites (1 4 14 z; zB14 ) and the structural changes in the oxidized La 2~xBaxCuO 4`y (0OxO0.1) are similar to the structural changes in the oxygenated La 2~xSrxCuO 4`y (0OxO0.15); the most important structural changes due to oxidation were found to be the shortening of the Cu–O in-plane bond length and the release of tension within the MO layer. In contrast for x[0.1 the oxidized La 2~xBaxCuO 4`y samples showed an increase in the Cu–O in-plane distance which might be due to a competition between the size/charge Ba2` e§ect and the interstitial oxygen e§ect.The T# improved significantly under oxidation for samples with x\0.15 therefore the extra oxygen seems to dope the Cu–O planes e§ectively. New superconductors and related phases The commonly held view that only cuprate materials containing stoichiometric longrange CuO 2 layers can exhibit superconductivity is gradually being eroded. Dagotto and Rice57 predicted that doping holes into spin ladder cuprates (materials which can be thought of as a crossover from chains to planes resulting from the assembling of chains one next to each other to form ladders) could potentially induce superconductivity and this was subsequently proven experimentally by Uehara et al.58 The latter group showed that superconductivity could be observed in Sr 0.4 Ca 13.6 Cu 24 O 41.84 by applying pressure with a T# onset of 12K at 3GPa pressure.58 Superconductivity at 50K was discovered in a new tetragonal alkaline-earth cuprate Sr 0.8 Ba 1.2 CuO 3`d synthesised at ambient pressure.59 Stoichiometric CuO 2 layers are absent in Sr 0.8 Ba 1.2 CuO 3`d as in the related high pressure synthesised system Sr 2 CuO 3`d.Electron di§raction patterns suggest that the oxygen vacancies located in the Cu–O planes form a superlattice which is of the same nature as the superlattice in Sr 2 CuO 3`d (Fig. 1). One of the most important areas of research in new superconducting compounds has centred on the synthetic structural and electronic aspects of partially replacing oxygen by halides.Superconductivity up to 80K has been reported in the double Cu layer compound of chemical formula Sr 2.3 Ca 0.7 Cu 2 O 4`zCl 1.3 characterised by a tetragonal unit cell (space group I4/mmm).60 As in the single Cu layer compounds chlorine was found to occupy the apical site co-ordinating to Cu. As there are no aliovalent cations on the Sr/Ca site and no interstitial chlorine hole doping is thought to be induced by partial replacement of the monovalent halogen with divalent oxygen. The first superconducting oxide fluoride,61 Sr 2 CuO 2 F 2`d has been classified as the n\1 member of the new homologous series Sr 2 Can~1 CunO 2n`dF 2By whose higher members have been synthesized by a high pressure route.62 The n\2 and n\3 members showed superconducting transitions at 99 and 111K respectively.The new T* type oxide chlorides CaRCuO 3 Cl (R\Y Ho or Er) prepared under high pressure show many structural similarities with p-type superconductors bearing a T* structure. Nevertheless attempts to introduce holes and induce superconductivity in these compounds by increasing the oxygen content failed due to the formation of the RCu 2 O 4 spinel phase which is thermodynamically very stable under high oxygen pressure.63 Tatsuki et al.64 have reported the first synthesis of Ba 2 CuO 2 Cl 2 under a pressure of 6GPa at 1000 °C. The compound Ba 2 CuO 2 Cl 2 has a K 2 NiF 4 type 497 Conducting solids covering ionic and electronic conductors Fig. 1 Possible oxygen ordering within the Cu–O layers in Sr 0.8 Ba 1.2 CuO 3`d. (a) Regions of CuO 4 chains are separated by chains of CuO 6 octahedra.(b) Chains of doubleCuO 5 square pyramids. (c) Stacking of the layers giving rise to a monoclinic cell tetragonal structure with lattice parameters a\4.1026(3) and c\16.442(1)Å similar to A 2 CuO 2 Cl 2 (A\Ca or Sr) reported previously. The first evidence of ‘probable p-type’ superconductivity in an actinide-containing cuprate has been reported in the (Bi 2~yPby)Sr 2 (Eu 2~xThx)Cu 2 Oz series.65 The sample with x\0.2 and y\0.8 bears the highest T# in the system (about 15 K). A series of new Pb-based 1222 cuprates have been reported.66 The system (Pb,Mg)(Sr,R) 2 (R@,Ce) 2 Cu 2 Oy (R,R@\rare-earth element) is isostructural with (Pb,Cu)(Sr,R) 2 (R@,Ce) 2 Cu 2 Oy (space group I4/mmm) with the (Pb,Mg)O layer replacing the (Pb,Cu)O layer. Resistivity measurements showed the compounds to be 498 M.G.Francesconi and P.R. Slater semiconducting. On the other hand superconductivity was discovered in (Pb 0.5 Cd 0.5 )(Sr 0.9 R 0.1 ) 2 (R@0.7 ,Ce 0.3 ) 2 Cu 2 Oy although this was dependent on the nature of R and on the O content.67 The (Pb 0.5 Cd 0.5 )(Sr 0.9 R 0.1 ) 2 (R@0.7 ,Ce 0.3 ) 2 Cu 2 Oy compounds are also isostructural with the (Pb,Cu)(Sr,R) 2 (R@,Ce) 2 Cu 2 Oy compounds. 3 Other electronic conductors and superconductors Boride carbide intermetallic compounds One of the most interesting features of the RNi 2 B 2 C (R\rare earth ion) materials is the evidence of coexistance of magnetism and superconductivity. Coupling of antiferromagnetic ordering and flux lines has been observed at a microscopic level in ErNi 2 B 2 C; the vortex lines rotate away from the direction of the applied field below the Neel temperature.68 Moreover the vortex structure was found to be square instead of hexagonal as typical of type II superconductors.Measurements of anisotropic magnetisation M(H,T) on ErNi 2 B 2 C single crystals showed further evidence of coexistence of superconductivity and a weak ferromagnetism for T\2.3K and H\12 kG.69 Composition-driven transitions have been found in HoNi 2 B 2 C samples with di§erent Ho/Ni and Ni/B ratios.70 Magnetoresistance measurements revealed the occurrence of three distinct states the re-entrant superconductivity typical of these compounds coexistence of magnetism and superconductivity and a non-superconducting magnetic state. Lynn et al.71 performed neutron di§raction on HoNi 2 B 2 C and HoNi 1.985 Co 0.015 B 2 C in order to investigate the magnetic order in superconducting and non-superconducting compositions.Both systems exhibit long-range magnetic order below B8K showing three types of magnetic order. In the particular regime where the three magnetic structures are observed simultaneously the compositional dependence of the intensities of the di§raction peaks suggest that they occur in spatially separated areas. A low-temperature commensurate antiferromagnetic structure and a higher temperature incommensurately sinusoidal modulation of the spin amplitude has also been found in HoNi 2 B 2 C by muon rotation measurements.72 The magnetic structure of TmNi 2 B 2 C has been investigated by neutron di§raction. 73 The compound TmNi 2 B 2 C is the only member of the RNi 2 B 2 C family to show modulation along the (110) direction and magnetic moments aligned along the c axis.The Tm moments align antiferromagnetically along the c axis in the (110) plane below 1.5K with an incommensurate magnetic structure. The magnitude of the moments is sinusoidally modulated along the diagonal of the ab plane. The authors suggested the possibility that this modulation allows the coexistence of the magnetic order with the superconducting state. Superconductivity in intermetallic compounds of general formula (RC)m(TB)n where R\rare earth and T\transition metal had previously been reported only for m\1 n\2 (i.e. YNi 2 B 2 C). This has now been extended with a report of superconductivity in the [LuNi 1~x(Lu/V)xBC] system which is the first superconductor characterised as m\2 and n\2.74 In addition superconductivity was also found in other related systems YNi 1~xCuxBC and LuNi 1~xCuxBC with T# around 8.9 K the second series showed enhancement of T# upon Cu substitution.75 499 Conducting solids covering ionic and electronic conductors Other systems After the discovery of the boride–carbide superconductors there has been renewed interest in other metal carbide systems especially carbide halides.Resistivity susceptibility and specific heat measurements have been performed by Henn et al.76 on the new layered superconductors Y 2 I 2 C 2 and Y 2 Br 2 C 2 (T# \9.97 and 5.04K respectively) to characterize the superconducting state. An analysis of the specific heat anomalies at T# revealed strong electron–phonon coupling which is not observed in the related threedimensional superconductor YC 2 .The e§ect of intercalation of Na and substitution of Th for Y has been studied in Y 2 Br 2 C 2 .77 The increase of T# on intercalation of Na was attributed to an increase in the carrier concentration. The degree of increase depended on the degree of ordering with a greater increase after annealing (Y 2 Br 2 C 2 T# \5.05 Na 0.23 Y 2 Br 2 C 2 T# \5.6 and 6.2K before and after annealing respectively). In contrast substitution of Th lowered T# (Th 0.4 Y 1.6 Br 2 C 2 T# \3.6 K) appearing to contradict the intercalation results. The authors therefore suggested that in this case the T# increase due to increased carrier concentration is outweighed by the T# decrease due to the introduced disorder. Mattausch et al.78 have reported a new carbide–halide superconductor La 9 Br 5 (CBC) 3 with a T# of 6K.The electronic band structure closely resembles that of the superconductor Y 2 Br 2 C 2 despite the di§erent structures of the two compounds. From such band structure calculations the size of the C–B–C angle was found to be significant suggesting that the bending vibration of the CBC ions is important for the superconducting properties. Superconductivity (T# \2.6–2.9 K) has been reported in the Ni based ternary carbide LaNiC 2 which is the first Ni based ternary carbide superconductor.79 From the observed specific heat data the authors suggested that this phase is a nonconventional BCS superconductor. A new layer structured nitride superconductor has been prepared by Yamanak et al.80 by Li intercalation into the van der Waals gap of b-ZrNCl using Bu/Li.80 The final composition was Li 0.16 ZrNCl and the material showed a decrease in resistance at 12.5K with zero resistance at 11.5 K.Johrendt et al.81 have reported a new phase Y 2 AlGe 3 with a novel structure consisting of a three-dimensional framework of Al and Ge with Y in the cavities; Al surrounded tetrahedrally by Ge atoms which are arranged in the form of planar Ge2~ infinite chains and isolated from each other respectively.81 This phase exhibited superconductivity at 4.5 K. New ternary Nb tellurides AxNb 6 Te 8 (A\Ca Sr Ba La or Nd; x\0.22–0.32) have been prepared by molten salt ion exchange reactions with TlxNb 6 Te 8 .82 Most of these phases showed metallic conductivity followed by superconductivity at low temperatures 4–6K significantly higher than for Nb 3 Te 4 (1.8 K).Somewhat surprisingly the starting phase TlxNb 6 Te 8 is non-superconducting. Resistivity anomalies were observed for La 0.32 Nb 6 Te 8 and Nd 0.24 Nb 6 Te 8 at 46 and 43K respectively similar to that observed at 40K for Nb 3 Te 4 single crystals and these may be related to the onset of a charge density wave type instability. Nagata et al.83 have reported a new thiospinel CuIr 2 S 4 which exhibits a metal– insulator transition at 226 K accompanied by a structural change from tetragonal symmetry in the insulating phase to cubic in the high temperature metallic phase. A 500 M.G. Francesconi and P.R. Slater systematic study of the solid solution with CuRh 2 S 4 which is a superconductor at 4.7 K was also reported. Organic superconductors Komatsu and Siato84 have reported the first example in an organic compound of the realisation of superconductivity by carrier doping a Mott insulator.84 Although Kand K@-(BEDT-TTF) 2 Cu 2 (CN) 3 have the same band structure the former is a Mott insulator with an exactly half filled band while the latter is a metal.The EPR spectrum of the K@ salt shows a signal due to Cu2` thus indicating that the metallic state and superconductivity of this phase is due to carrier doping by substituting Cu` by Cu2`. Schlueter et al.85 have reported the first study of the e§ect of isotopic substitution on T# in the K(ET) 2 M(CF 3 ) 4 (X 2 HCCH 2 X) (M\Cu Ag or Au) family of organic superconductors where M(CF 3 )4~ is a large discrete (non-polymeric) anion.85 Substitution by D for the 8H atoms of the ET electron donor molecule causes T# to increase from 2.9 to 3.1 K thus demonstrating that the inverse isotope e§ect is present in ET-based systems containing a discrete anion.The same group has also published a useful review on organic superconductors.86 Clevenger et al.87 have reported the successful combination of an organic superconductor and inorganic superconductor into a single hybrid structure. Thin films of YBa 2 Cu 3 O 7 on single crystal MgO were prepared by laser ablation. Organic superconductor/ inorganic superconductor structures were fabricated through the vapour phase deposition of (BEDT-TTF) 2 I 3 onto the surface of the YBa 2 Cu 3 O 7 film. Results from powder X-ray di§raction and atomic force microscopy indicated that the organic superconductor was deposited in a poorly crystalline predominantly disordered fashion.Crystalline films could however be achieved by prior adsorption of a monolayer of a long chain alkylamine onto the YBa 2 Cu 3 O 7 film due to the resultant change in the interfacial properties of the film. 4 Ionic conductors and intercalation compounds Lithium ion conductors There appears to be no decrease in research into Li batteries but rather just the opposite with electrode materials remaining the most researched area. In this widely studied field it is important to keep abreast of both the recent discoveries as well as past results. In this respect a number of useful review articles have been published including one on cathode materials and another on carbon materials for the anode.88,89 Increasingly low cost and environmental safety (low toxicity of the constituent materials) are being sought and as a result the intercalation properties of manganate phases continue to attract considerable attention for their potential use in secondary Li batteries.The spinel LiMn 2 O 4 has been one of the most widely studied cathode materials but the intercalation/deintercalation characteristics are still not fully understood. To this end Kanamura et al.90 have reported a detailed structural study of LixMn 2 O 4 in the range 0.5[x[0.13. It was concluded from XRD that an irreversible structural change takes place during the deintercalation process in the region x\0.5. This was supported by EPR spectroscopy which showed the presence of some Mn2`. Therefore 501 Conducting solids covering ionic and electronic conductors Fig.2 Structure of LiMnO 2 ; Li (spheres) located between MnO 2 layers instead of a two phase region in the range 0.5[x[0.13 as previously assumed the results could only be explained by the presence of a third phase possibly amorphous and containing Mn2`. A problem with the use of LiMn 2 O 4 as a cathode material is the Jahn–Teller distortion observed on intercalation of Li as Mn is reduced to Mn3` resulting in a change of structure from cubic to tetragonal. This is a problem since the elongation of the octahedra at the structural transition results in an increase of 16% in the c/a ratio introducing mechanical stress. Therefore the cathode loses integrity on cycling. Doping studies have therefore been of interest to try to remove this problem.91 The spinel Li 1`xMn 1.5 Ni 0.5 O 4 has been prepared by a low-temperature sol–gel route and studied by Amine et al.91 This material can intercalate a second Li to give Li 2 Mn 1.5 Ni 0.5 O 4 with the same cubic spinel structure.In contrast to Li 2 Mn 2 O 4 which exhibits two plateaus at 4 and 3V vs. Li/Li` because of the structural transition caused by the Jahn–Teller e§ect this Ni-doped material showed only a single 3V plateau with a large discharge capacity of 160mAh g~1 and fairly good cyclability. In this case as there is no transition to a Jahn–Teller distorted state there is little mechanical stress expected. Some new cation deficient mixed oxide (Mn–Co Mn–Fe Co–Fe) spinels have been synthesised using a solution technique and preliminary studies of their Li intercalation properties reported.92 The phases were cubic except for those with high Mn contents which were tetragonal due to Jahn–Teller distortion.The most promising phase was 502 M.G. Francesconi and P.R. Slater Mn 2.15 Co 0.37 O 4 for which a specific capacity of 62Ah kg~1 was recovered after five cycles. Armstrong and Bruce93 have reported the synthesis and electrochemical performance of layered LiMnO 2 with a structure analogous to LiCoO 2 (Fig. 2). This is of considerable interest since LiCoO 2 is one of the best cathode materials to date but by replacing Co by Mn both cost and toxicity can be reduced. The synthesis was performed by low-temperature ion exchange from NaMnO 2 . The charge capacity of B270mAh g~1 compares well with that of LiCoO 2 and LiMn 2 O 4 and preliminary results indicate good stability over repeated charge–discharge cycles.Layered LiMnO 2 was also prepared independently using a similar route by Capitaine et al.94 who reported that a small amount of Mn was present within the Li layer such that the composition can be written Li 0.97 Mn 0.03 (Li 0.03 Mn 0.97 )O 2 . Chen et al.95,96 have prepared a hexagonal form of manganese dioxide AxMnO 2 ·yH 2 O (A\Na x\0.35 y\0.7; A\K x\0.25 y\0.6) by hydrothermal decomposition of aqueous AMnO 4 . The materials are similar in properties and structure to Ax(H 2 O)TiS 2 with MnO 2 layers between which the A` ions and H 2 O reside. Up to 0.6 Li may be reversibly intercalated in the voltage range 3.6–2V. Lavela et al.97 have examined Li insertion before and after Cu extraction in spinel sulfides Cu 2 MSn 3 O 8 (M\Mn Fe Co or Ni).97 The Ni and Co containing samples exhibited the best performances and a significant increase in the cell voltage and reversibility was observed after extraction of Cu in the Co system to give Cu 1.1 CoSn 3 O 8 .Tin-119 Mo� ssbauer spectroscopy showed that the Sn atoms in the Co and Ni systems had a higher tendency to be reduced by Li insertion. In addition to Mn containing systems V and Co based materials continue to be the most studied. Davies et al.98 have examined mixed TiO 2 –V 2 O 5 cathodes and showed that such ternary mixtures exhibited good reversibilities giving energy densities slightly greater than that observed for V 6 O 13 . The improvement in the properties of the mixed systems has been attributed to the preferential reduction of TiIV over VV near the low voltage limit which prevents reorganisation of the microstructure of the material.An open circuit voltage of 3.5V was observed compared to 2.8V for V 6 O 13 and the average voltage on discharge remained steady at B2.3V for each successive discharge. The improved average cell voltage accounts for the theoretical energy density achievable using the mixed electrode exceeding that of V 6 O 13 . This result indicates that materials which were previously thought unsuitable because of irreversibility at low voltages might still find favour as cathodes if a second material can be found with suitable electrochemical behaviour. The synthesis of pure samples of NbVO 5 has been reported for the first time using a sol–gel route.99 Thishase as well as the related TaVO 5 material has empty channels and so both compounds are promising materials as electrodes in Li batteries.Both NbVO 5 and TaVO 5 could insert Li up to limits of 1.95 and 1.57 respectively. After the reduction step of the first cycle the structures of the pristine materials changed and the new compounds showed good reversibility for Li intercalation. Hydrothermal synthesis has been used to prepare a new vanadate phase Li 0.6 V 2~dO 4~d·H 2 O.100,101 The synthesis involved reaction between vanadium pentoxide and lithium hydroxide in the presence of an organic templating agent the tetramethylammonium ion. The hydrated lithium vanadium oxide possesses the simplest layered structure of any vanadium oxide containing only VO 5 square pyra- 503 Conducting solids covering ionic and electronic conductors Fig. 3 Arrangement of the vanadium oxide planes in Li 0.6 V 2~dO 4~d·H 2 O (vanadium light spheres; oxygen dark spheres) mids whose apices alternate up and down in a layer (Fig.3). After dehydrationB1 Li per V 2~dO 4~d formula unit could be reversibly cycled indicating a potential use as a cathode in Li batteries. Nazar et al.102 have also prepared a new layered vanadate by hydrothermal synthesis using 1,4-diazabicyclo[2.2.2]octane as a templating agent.102 The new phase (C 6 H 14 N 2 )V 6 O 14 ·H 2 O has a structure composed of a new arrangement of edgeshared VO 5 square pyramids that are corner shared with VO 4 tetrahedra to form puckered layers between which the template cations are located. Electrochemical Li insertion studies have shown that after removal of the template by calcination the vanadate can reversibly intercalate up to 8Li per mol of V 6 O 14 .The electrochemical Li intercalation properties of the oxide ion conductor Bi 4 V 2 O 11 have been examined.103 Three compositions were studied Bi 4 V 2 O 11 Bi 3.6 Pb 0.4 V 2 O 11~y and Bi 4 V 1.8 Cu 0.2 O 11~y and the performances of all three were equivalent. They were shown to insert a surprisingly large number of Li atoms 8 per V atom during the first discharge at an average potential of 1.7 V implying a theoretical specific energy of 655Wh kg~1. The reason why such high Li intercalation is possible requires further study. The large amounts of Li intercalation suggest extremely low oxidation states for V and Bi and it is proposed that at least part of the Li is reduced when inserted forming metallic Li between the Bi–O and V–O layers.The potential of these materials for battery applications is however slightly reduced by the irreversibil- 504 M.G. Francesconi and P.R. Slater ity which was found after the first cycle such that in subsequent cycles only 5.75 Li per V could be reversibly cycled. A Li-rich vanadium bronze Li 6 V 5 O 15 has been prepared from reaction of Li 2 CO 3 with V 2 O 5 at 680 °C by Hua et al.104 This phase exhibits a high specific capacity ([340Ah kg~1) at 0.2mAcm~2 and a 1Vcuto§ voltage along with good reversibility and good structural stability. The authors suggest that this material has a potential use in a low voltage secondary Li battery. Nanocomposites comprising of conductive poly(aniline) chains interleaved between the layers of sol–gel derived V 2 O 5 have been examined as potential electrode materials.105 The reversible capacity (after mild oxygen treatment) is higher than that of the simple V 2 O 5 xerogel at intermediate charge/discharge rates at constant current. In addition the Li insertion is completely reversible in the nanocomposite even after insertion of 3Li per formula unit while the pure xerogel displays some irreversibility. The Li di§usion is also more rapid in the nanocomposite. The improved properties therefore seem to be related to the faster kinetics for Li transport in the composite. Kloster et al.106 have examined mixed ionic–electronic conducting V 2 O 5 -polymer electrolyte xerogel nanocomposites with electronic conduction in the V 2 O 5 phase and ionic conduction in the polymer electrolyte [(CH 2 O) 0.1 (CH 2 CH 2 O) 0.9 ]nLiCF 3 SO 3 .The nanocomposites showed high electronic conductivity parallel to the V 2 O 5 layers with neglible perpendicular to the layers. The oxide LiCoO 2 remains a widely studied electrode material for Li batteries due to its excellent electrochemical characteristics.107 However even though it has been studied for the last 15 years the existence of the deintercalated Li free end-member phase CoO 2 has remained an elusive unanswered question. To this end Amatucci et al.107 have examined the deintercalation of LiCoO 2 using in situ X-ray di§raction and have isolated the end member phase CoO 2 for the first time. Surprisingly an increase in crystallinity was observed as x approached 0 instead of the expected destruction of the core structure of LiCoO 2 by a drastic increase in structural disorder; CoO 2 is hexagonal believed to be isostructural with CdI 2 with cell parameters of a\2.822 and c\4.29Å.Electrochemical studies showed that 95% of the Li could be reinserted after complete delithiation. The nitride Li 3~xCoxN (0OxO0.5) with the hexagonal Li 3 N structure has been synthesised from reaction of Li 3 N with Co under N 2 .108 The phase as prepared has some Li vacancies such that the composition should be written Li 3~x~zCoxN but these can be filled by intercalated Li. Lithium can be readily deintercalated and reintercalated in a Li/Li 2.6 Co 0.4 Ncell forming Li 2.6~yCo 0.4 N (0OyO1) over the cell voltage range 0 to 1.1V giving a specific capacity of B480mAh g~1 which is larger than that of C materials and does not change during proceeding cycles. The high specific capacity and low voltage therefore suggest that this phase has potential as an anode in a Li secondary battery.Shodai et al.109 have also investigated the same system as well as analogues with Ni or Cu in place of Co. They examined a larger voltage range 0–1.4 V and achieved a larger degree of Li deintercalation for Li 2.6~yCo 0.4 N (0OyO1.6) resulting in a higher capacity 760mAh g~1. In both studies the system became more amorphous on deintercalation of Li and it is this amorphous state that exhibits the good electrochemical properties. Cochez et al.110 have reported the electrochemical insertion of Li into the new tin-containing spinel sulfides Cu 2 CoSn 3 S 8 and CuCoSn 3 S 8 . Rocking chair cells with 505 Conducting solids covering ionic and electronic conductors LiCoO 2 as the other electrode showed good cyclability in the 1–2V range although the cell capacity was low (13mAh g~1) indicating that they would only be of use if the weight of the cell was not a limiting factor.The oxide LiFeO 2 with a corrugated layer structure isostructural to orthorhombic LiMnO 2 has been synthesised by Kanno et al.111 by an ion exchange reaction between c-FeOOH and LiOH·H 2 O under hydrothermal conditions. Lithium cells consisting of Li anodes and this new phase as the cathode showed good charge/discharge reversibility in the voltage range 1.5 to 3.5V with a capacity up to 100mAh g~1. During charging the phase became amorphous and it was the amorphous phase that participated in the reversible charge–discharge mechanism. This is the first example of a Li–Fe oxide system which shows good electrode characteristics for Li secondary cells.Sputter-deposited iron oxide thin films have been investigated as possible replacements for Li metal in secondary batteries.112 The conductivities of the amorphous thin films were much higher than those of crystalline forms. Reversible Li intercalation was demonstrated with specific capacities close to 330Ah kg~1 at a C/2 charge/discharge rate with 100% depth of discharge. Panero et al.113 have reported a new type of rocking chair battery based on a graphite anode and a polymer [e.g. poly(pyrrole)] cathode. Good cyclability and appreciable energy density were demonstrated although further work is required to optimize the properties. The intercalation process is a dual one with Li entering the graphite and the counter ion (e.g ClO 4 ~ if LiClO 4 is used in the electrolyte) inserting into the poly(pyrrole) structure.As a result of this dual nature these batteries are therefore termed dual-ion or ‘dion’ batteries. The advantages of these cells over conventional Li ion cells are that they are inexpensive and heavy-metal free. However disadvantages are the lower voltage and the fact that an excess of electrolyte is required. Zheng et al.114 have examined the Li intercalation properties of carbon materials made by heating organic precursors between 550 and 1000 °C in an inert gas atmosphere. High capacities which exhibited large hysteresis were found in all samples heated below 800 °C and all these contained substantial hydrogen with the capacity dependent on the hydrogen content.The authors therefore suggest that the Li atoms can bind quasi-reversibly on hydrogen-terminated edges of graphene fragments in carbonaceous materials. The nasicon-type structure is known for its high ionic conductivity. The open nature of the three-dimensional framework (Fig. 4) also suggests the potential for good Li insertion properties. To this end the systems M 2 (SO 4 ) 3 (M\Ti/Fe V/Fe or Fe) and LixM 2 (XO 4 ) 3 (M\Ti V/Fe or Fe; X\P or As) have been characterised for their Li insertion characteristics.115,116 Iron(III) sulfate gave open circuit voltage of 3.6V vs. Li and a reversible capacity of B1.8 Li per formula unit. Changing the oxyanion from SO 4 2~ to PO 4 3~ in these framework compounds decreased the redox potentials significantly e.g. from 3.6 to 2.8V for the Fe3`/Fe2` couple.The authors discuss the comparative advantages (e.g. low cost low toxicity compared to LiCoO 2 ) and disadvantages (e.g. more open framework reduces the volumetric density) of framework cathodes for Li battery applications. Intercalation of Li into ramsdellite Li 2 Ti 3 O 7 has been investigated.117 Maximum intercalation limits of 0.6 and 1Li per Li 2 Ti 3 O 7 were observed by chemical and 506 M.G. Francesconi and P.R. Slater Fig. 4 Framework structure of nasicon-related Fe 2 (SO 4 ) 3 showing the corner linking of SO 4 tetrahedra and FeO 6 octahedra electrochemical intercalation respectively and the ramsdellite network was preserved during intercalation. Electrochemical spectroscopy and powder X-ray di§raction studies indicated that there are two di§erent Li intercalation sites.Manthiram and Tsang118 have synthesised an amorphousMoO 2`d (dB0.3) phase at ambient temperature by a simple chemical route (reduction of aqueous K 2 MoO 4 with KBH 4 ). The electrochemical behaviour was distinctly di§erent from that of crystallineMoO 2 exhibiting an excellent cyclability with a capacity ofB220mAh g~1 in the range 3 to 1V in Li cells. The good capacity at su¶ciently low voltages suggests that this material may be a candidate for an anode material in secondary Li batteries provided a high voltage cathode is used in conjunction. The electrical conductivity and Li di§usion coe¶cient in nanocomposites prepared by intercalation of poly(ethylene oxide) (PEO) into MoS 2 has been studied.119 The nanocomposites were semiconducting with relatively high electrical conductivity.Intercalation of PEO resulted in higher Li di§usion coe¶cients than in pure molybdenum disulfide and the improved properties suggest the potential for use of the intercalate as an electrode material in Li batteries. On the electrolyte front new polmer electrolytes continue to be sought and in addition studies have been performed to try to rationalise the magnitude of the ionic conductivity in these systems. Concerning the latter the highest conductivities are observed in the amorphous region and so plasticizers are commonly added to the electrolyte to reduce the crystallinity. The precise function of the plasticizer e.g. ethylene carbonate (EC) in Li polymer electrolytes has not however been fully studied. Wang et al.120 have presented IR results of LiClO 4 –EC mixtures that show that there is a strong interaction between Li and EC which mainly occurs on the C––O group of the molecules.The structure of the ClO 4 ~ anion is also a§ected forming solventshared ion pairs Li`–EC–ClO 4 ~. Xu et al.121 have shown that the CN stretch region of the SCN~ anion for NaSCN 507 Conducting solids covering ionic and electronic conductors and LiSCN dissolved in PEO (average molar mass 400) shows dramatic di§erences from the salts dissolved in polyethylene oxide glycol (PEG) (average molar mass 400). The di§erences were attributed to anion solvation by the OH end of the PEG polymers. The authors argued that the di§erent molecular status of the above electrolytes in the two solvents should be considered by electrochemists studying batteries since this is a fundamental factor determining the e¶ciency of a battery.Lyons et al.122 have prepared polysilane comb polymers incorporating ethoxyethoxybutane in the side chain of the polymer. An ambient temperature conductivity of 1.2]10~7 S cm~1 was observed for M[CH 3 CH 2 OCH 2 CH 2 O(CH 2 ) 4 ] Si(CH 3 )Nn containing lithium triflate at a concentration of four oxygens per Li cation. The Arrhenius plots showed curvature typical of a Vogel–Tamman–Fulcher (VTF) relationship for conduction. Continuing their previous studies Sumathipala et al.123 have prepared a further mixed conducting LISICON type c solid solution Li 4~2xCoxGeO 4 (0.15OxO0.8). The material with x\0.25 had the highest net conductivity 8.4]10~6S cm~1 at 27 °C and the highest ionic transference number 0.19. A study of the ionic conductivity of the perovskite La23 ~xLi 3xTiO 3 system has shown that the temperature dependence of the conductivity can be modeled by a VTF type relationship and as a result the authors suggest a mechanism of conduction involving the tilting and/or rotating of the TiO 6 octahedra.124 Lithium intercalation was shown to be possible although the capacity was not very high (maximum of 0.15 Li) and the process was not fully reversible.This material therefore has limited potential as an electrode material. In addition intercalation is a problem when considering this material as an electrolyte since any intercalation electrode with a potential more cathodic than 2.8V will react under short circuit conditions leading to some reduction of Ti4` to Ti3` thus introducing electronic conduction.Neudecker and Weppner125 have prepared the single phase compound Li 9 SiAlO 8 (a 50 50 solid solution of Li 4 SiO 4 and Li 5 AlO 4 ) which has an ionic conductivity of 2.3]10~7 S cm~1 at 25 °C with an ionic transference number close to 1 even under extremely oxidising and reducing conditions. Upon extreme polarisation (E vs. Li/Li`O6.5 V) below 100 °C stable tarnishing layers were observed at the Li 9 SiAlO 8 - electrode interfaces which resisted further decomposition. The authors therefore suggested that this material may be a suitable electrolyte for investigating cathodes which exhibit very high positive voltages vs. lithium. Improved ionic conductivity (5]10~7S cm~1 at room temperature) two orders of magnitude higher than LiNbO 3 films has been observed in glassy LiNb oxynitride thin films.126 The film structure was highly crosslinked and the film transmittance was [85% in both visible and solar ranges suggesting suitability for use in electrochromic devices.Kelder et al.127 have synthesised BPO 4 via a soft chemical route and shown that a significant amount of Li can be dissolved in the BPO 4 lattice to give Li ion conducting solid solutions 12 xLi 2 O–BPO 4 . The maximum room-temperature conductivity was 2]10~6S cm~1 for x\0.07 with an activation energy of 0.3 eV. Sodium ion conductors A new thiophosphate NaTi 2 (PS 4 ) 3 chemically analogous to nasicon but with a di§erent structural arrangement of the linked octahedra and tetrahedra has been 508 M.G. Francesconi and P.R. Slater Fig. 5 Framework structure of NaTi 2 P 3 S 12 showing edge-linking of TiS 6 octahedra and PS 4 tetrahedra (Na ions are not shown) synthesised by Cieren et al.128 The structure is built up from TiS 6 octahedra and PS 4 tetrahedra linked to each other only by edges such that each TiS 6 octahedra is bonded to 3 PS 4 tetrahedra with each tetrahedra likewise bonded to two octahedra (Fig.5). The resulting framework structure has wide tunnels present along the c axis in which the Na ions are located. These tunnels suggest the possibility of insertion properties as well as ionic conduction. Suda et al.129 have prepared sodium ion conductors in the system Na 2 O–Y 2 O 3 –P 2 O 5 –SiO 2 by sol–gel synthesis with the gelation induced by passing moist air or N 2 into the reaction mixture. Sintered samples of NaxY 0.8 P 0.4 Si 3.6 O 12 (x\5.33–5.60) were single phase with high ionic conductivity (x\5.33 p\0.07 S cm~1 at 300 °C).The ionic conductivity of NaLnTiO 4 (Ln\La Nd Sm or Gd) has been studied and found to be lower by one order of magnitude than that of Na 2 Ln 2 Ti 3 O 10 .130 This was attributed to a corrugation of the LnO layer and accompanying contraction of the NaO layer due to poor charge balance between the NaO and LnO layers in NaLnTiO 4 . On the commercial applications side sodium b-alumina has been used as an electrolyte to add Na electrochemically to molten aluminium alloys on both laboratory and industrial scales in order to modify the microstructure and properties.131 Traditionally this has been achieved by physically adding a sodium rich alloy to the melt. The authors argue that the electrochemical route is potentially a much cleaner and e¶cient route with the dross and fumes associated with the previous route being eliminated.It also lends itself to improved control via a solid-state sodium sensor. 509 Conducting solids covering ionic and electronic conductors Rubidium ion conductors Isasi et al.132 have reported the synthesis and ionic conductivity of a new defect pyrochlore Rb 3 CrTe 3 O 12 . The formula can be written as Rb32 (Cr12 Te32 )O 6 showing that both the Rb andOsites are partially occupied with Rb` ion conductivity through well defined three-dimensional tunnels in the structure. The conductivity at 600K is approximately 1.6]10~4Scm~1 with an activation energy of 0.8 eV. The structure of the polymer electrolyte poly(ethylene oxide) 4 ·RbSCN has been determined by powder X-ray di§raction.133 This represents the largest cation system to be studied to date and it is isostructural with the K-containing analogue.The authors compare crystallographic studies for di§erent cation sizes from Li` to Rb`. In all cases the PEO chains adopt a helical conformation with the cations located in the helix. Thus even for ions as diverse in size as Li` and Rb`,PEO exhibits a remarkable ability to wrap round the cations with a change in chain conformation on going from Na` to K` to accommodate the larger size cations. The crystallographic studies indicate strong interactions between each chain and its dedicated ions but only weak interactions between the chains. The authors therefore suggest that the well established decrease in conductivity with increasing salt concentration in amorphous polymer electrolytes may be due to intrachain cross-linking rather than the widely accepted view which has attributed it to interchain crosslinking.Silver ion conductors Silver(I) ion conductors show some of the highest ionic conductivities for any ion commonly exhibiting higher conductivity than Na containing analogues despite the larger size. This is demonstrated by the compound Ag 2 La 2 Ti 3 O 10 which has been synthesised by ion exchange from M 2 La 2 Ti 3 O 10 (M\Na or K) with molten AgNO 3 .134 As in the case of MLaTiO 4 (M\Na or Ag) the ionic conductivity was much higher for the Ag containing system than for the Na analogue. This higher conductivity is probably related to the higher polarizability of the Ag` ions. Below 200 °C the conductivity was purely ionic while above 200 °C some electronic conductivity was observed.Wada et al.135 have prepared the sulfide Ag 8 TiS 6 . This compound has a conductivity around 10~3 Scm~1 at ambient temperature with a transference number for Ag` close to unity. Doping of Cd K and Na into the ionic conducting mixed system Ag 2 HgI 4 –Cu 2 HgI 4 has been investigated.136 In Cd doped Ag 2 HgI 4 (67 mol %)–Cu 2 HgI 4 (33 mol%) the phase transition from the low temperature b- to the high temperature a-form occurred at lower temperature and the system showed higher conductivity. On the other hand K doping only improved the conductivity prior to the phase transition while for Na doped samples the conductivity was higher than the parent system above the phase transition. Singh et al.137 have shown that addition of AgI to the glass system Ag 2 O·P 2 O 5 ·MoO 3 results in a conductivity enhancement by almost three orders of magnitude.Studies with electrochemical cells showed the electrolyte to be stable and to exhibit only small polarisation suggesting potential technological applications use. Agrawal et al.138 have investigated the ionic conductivity of silver borate glassy systems. The host salt was a ‘quenched [0.75 AgI 0.25 AgCl] mixed system’ instead of the commonly used host AgI. The highest ambient temperature conductivity 510 M.G. Francesconi and P.R. Slater (2.23]10~2S cm~1) was observed for the composition 0.7[0.75AgI 0.25AgCl] 0.3[0.833Ag 2 O 0.167B 2 O 3 ]. The conductivity was dependent on sample preparation and was highest for samples prepared by quenching.Proton conductors Oxygen deficient perovskite materials have been shown to exhibit high proton mobility which results from water absorption into the anion vacancies. Concentration cell measurements on the proton conducting perovskite Ba 3 Ca 1.18 Nb 1.82 O 9~d indicated that only 13 of the oxygen vacancies participated in the conduction process with the remaining oxygen vacancies virtually immobilised.139 A proton conductivity [10~2S cm~1 was observed at 600 °C and the conductivity was independent of P(O 2 ) over a wide range indicating that the electronic conductivity was virtually nil. Results from the use of this electrolyte in a hydrogen fuel cell were also reported. Mixed zirconium and cerium containing perovskites have been examined to make use of the good proton conductivity of the cerates and good mechanical stability of the zirconates.Solid solutions with formula SrYb 0.05 (Ce 1~xZrx) 0.95 O 3~y (x\0 0.25 0.5 0.75 or 1) were synthesised. The x\0.25 sample exhibited the best characteristics with good proton conductivity and high thermal cycling resistance.140 As a result of these promising results the authors plan further work to characterise this system more substantially. Shimura et al.141 have studied possible proton conduction in the pyrochlore systems Ln 2 Zr 2~xYxO 7~y (Ln\La Nd Gd or Sm) and Y 2 Ti 2~xMxO 7~y (M\In or Mg). Proton conduction at high temperature was demonstrated for the former under a hydrogen-containing atmosphere while there were no clear signs of proton conduction in the latter system with the conduction being electronic rather than ionic.Other cation conductors Ionic conductivity due to trivalent cations is not common but has been observed in rare earth–bA-alumina at high temperature. Warner et al.142 have prepared and characterised the phases LaAl 11 O 18 and LaAl 12 O 18 N reporting ionic conductivity due to La3` at high temperatures. Impedance data revealed an intra-granular ionic conductivity with high activation energies of 2.5 and 1.6 eV respectively. At B1540K the conductivity was comparable to that of Sr–Li–b-alumina with pB3]10~5 S cm~1. Oxide ion conductors It is rapidly being established that solid oxide fuel cells (SOFCs) are potentially one of the most commercially viable of the di§erent types of fuel cells. As a result of this interest in SOFCs continues to grow ensuring a consistent body of research on oxide ion conductors both purely ionic (for the electrolyte) and mixed conductors (for the electrodes).A review by Steele143 on materials for high temperature fuel cells has discussed materials selection and the relationship between design fabrication microstructure and performance. In the industrial area of SOFCs Westinghouse with their tubular design continue to dominate. Since 1986 significant progress has been made in the evolution of cells with 511 Conducting solids covering ionic and electronic conductors higher power lower cost and improved thermal cycling capability and details are given in the article by Bratton and Singh.144 In addition a 5MW integrated SOFC/combustion turbine power plant design for distributed power applications has been developed which can achieve e¶ciencies approaching 70%.145 One of the main objectives of SOFC research is to develop cells that can operate usingCH 4 and ultimately natural gas as a fuel.In the normal design this is achieved by steam reforming the CH 4 prior to supply to the fuel cell. In order to improve energy utilisation an alternative route is to reform internally using the partial oxidation of CH 4 since the operating temperature for a solid oxide fuel cell is su¶ciently high. Such a process yields simultaneous generation of thermal energy electrical power and gaseous mixtures of CO and H 2 which is available directly for the synthesis of CH 3 OH. Hiei et al.146 have shown that the electrical power generated as well as the activity for the partial oxidation of methane is strongly dependent on the oxide ion conductivity of the electrolyte used.The compound La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3~x gave the highest electrical power with Ni and La 0.6 Sr 0.4 CoO 3 as anode and cathode respectively. For this set-up and a feed gas of CH 4 –O 2 \4 to the anode the electrical power was 336mWcm~2 and yield of synthesis gas (CO and H 2 at molar ratio of 2) was 70%. Antonucci et al.147 have also studied oxidative reforming of CH 4 reporting results for a 150W tubular SOFC stack prototype with an yttria–scandia stabilised zirconia (YSSZ) electrolyte Pt–PrO 2 cathode and Pt–Ni–CeO 2 YSSZ anode. Although the feasibility was shown a cost e§ective catalyst (without noble metal) is needed for the anode to make the cell commercially viable. Ishihara et al.148 have reported the electrical power generation characteristics of a SOFC with La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3~x as the electrolyte.This material has a higher conductivity than yttria stabilised zirconia (YSZ) and was found to be e§ective in enabling a decrease in the operating temperature. The power density of the cells was strongly a§ected by the electrodes both anode and cathode. The maximum power density was observed for aNi anode and Sm 0.6 Sr 0.4 CoO 3 cathode attaining values as high as 0.44Wcm~2 at 1073K and 0.14Wcm~2 at 973 K. The attained power density is three times larger than that obtained using YSZ as the electrolyte at 1273K and an order of magnitude higher at 973 K. Similar studies have been made by Feng et al.149 demonstrating high power density at 800 °C. They utilised doped lanthanum cobaltite as the cathode and both ceria and the gallate mixed with NiO as composite anodes.It was found that the energy wasted due to the electrolyte resistance was not as signifi- cant as the anode overpotential. Better anode materials therefore need to be found to utilise the high ionic conductivity of doped lanthanum gallate to lower the operation temperature of solid oxide fuel cells. Bismuth oxide stabilised in the high temperature oxygen deficient fluorite structure of d-Bi 2 O 3 has the highest known oxide ion conductivity. Problems have however been observed with ageing at temperatures as low as 600 °C due to a transformation to a rhombohedral phase which has much lower conductivity. Small additions of CeO 2 (less than 5 mol %) have been shown to suppress the aging in stabilized Bi 2 O 3 –Y 2 O 3 oxides.150 The conductivity of the Bi 2 O 3 –Y 2 O 3 –CeO 2 electrolyte was 0.1 S cm~1 at 600 °C and no degradation was observed after annealing at 650 °C for over 300 h thus indicating the achievement of kinetic stability without loss of oxide ion conductivity.As in the case of doped LaGaO 3 there is a need for a good anode material at low temperature to be able to utilise this electrolyte. 512 M.G. Francesconi and P.R. Slater The perovskite phase Y 0.9 Ca 0.1 FeO 3 has been proposed as a cathode material for solid oxide fuel cells by Kim and Yoo.151 The majority ionic carriers were identified as oxide ions and the ionic conductivity was in the range 10~4–10~2S cm~1 at fuel cell operating temperatures with the activation energy dependent on the oxygen partial pressure.The defect structure indicated anti-Frenkel disorder. A comprehensive study of YSZ confirmed the change in activation energy for conduction at ca. 650 °C and identified this to be structural in origin.152 The structural evidence was from neutron di§raction which showed broad di§use scattering peaks below 600 °C. The di§use scattering observed at low temperature is believed to be due to a short range ordering of oxygen vacancy-dopant cation clusters to form microdomains. Above 650 °C the intensity of the di§use peaks decreased significantly indicating that short range ordering only occurs at low temperatures. The discovery of high oxide ion conductivity in doped LaGaO 3 has targetted perovskite systems for study with a view to obtaining new oxide ion conductors.Thangadurai et al.153 have reported new anion deficient perovskite oxides AM 1~xAlxO 3~x (A\Na or K;M\Nb or Ta; 0\xO0.5) exhibiting good oxide ion conduction. The conductivity increased with increasing x up to x\0.5 attaining a value of B2]10~2S cm~1 at 900 °C for KNb 0.5 Al 0.5 O 2.5 which is comparable to Ba 2 In 2 O 5 .153 Aluminium-27 NMR spectra showed that both tetrahedral and octahedral Al co-ordination were present when A\K suggesting only a short-range ordering of oxygen vacancies in this phase. In contrast for A\Na the Al was exclusively tetrahedrally co-ordinated in a long-range ordered brownmillerite type structure. For the latter phase there was an abrupt increase in conductivity above 800 °C which may be due to an order–disorder transition similar to Ba 2 In 2 O 5 .Ishihara et al.154 have presented results on the oxide ion conductivity in doped Ga-based perovskite type oxides LnGaO 3 (Ln\rare earth) as well as its related oxide A 3 Ga 2 O 6 (A\Sr or Ba). Although the oxide ion transference number in the latter phases were almost 1 the ionic conductivity was much lower than for doped LnGaO 3 due to the low solubility of aliovalent cation dopants. The ionic conductivity of the Sr 2`xLa 1~xTaO 6~x (0OxO1) (Sr 2 LaTaO 6 – Sr 3 TaO 5.5 ) solid solution with the (NH 4 ) 3 FeF 6 type structure has been examined.155 Oxide ion conductivity with a maximum value of 6.2]10~3S cm~1 at 800 °C was observed for the composition with x\0.75. This conductivity is nearly identical to that for CaO-stabilised ZrO 2 . The compound Bi 4 V 2 O 11 is proving to be an extremely versatile host for doping and is able to accept significant amounts of trivalent ions independent of their size.Lee et al.156 performed doping studies at the V site with ions as diverse in size as B Al Cr Yand La. The highest conductivity (1.4]10~4S cm~1 at 300 °C) was observed for the La doped system Bi 4 V 1.8 La 0.2 O 10.8 although this is lower by one order of magnitude than the best BIMEVOX system. One advantage however of the La doped system is the reproducibility and reversibility on heat/cool cycles of its conductivity data. The oxide fluoride Nd 2 Eu 2 O 3 F 6 shows high electrical conductivity 5]10~2S cm~1 at 923K under P(O 2 )\0.4 Pa with the charge carrier being mainly theO2~ ion.157 XPS studies have been performed to obtain further information on the conduction mechanism.These studies suggested that the valency of Nd was greater than 3]while that of Eu was less than 3]. This variance of the valency is thought to result in a weakening of the metal–oxygen bonds and strengthening of the 513 Conducting solids covering ionic and electronic conductors metal–fluorine bonds with the overall result of an increase in the mobility of the oxide ions. References 1 R. Usami S. Adachi M. Itoh T. 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Takahashi J. Mater. Chem. 1996 6 795. 517 Conducting solids covering ionic and electronic conductors

ISSN:0260-1818    

DOI:10.1039/ic093489

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

26 Radiochemistry By D. S. URCH Chemistry Department Queen Mary and Westfield College University of London Mile End Road London E1 4NS UK 1 Introduction This section of Annual Reports will cover recent progress in those aspects of radiochemistry where radioactivity or the e§ects of radioactivity (but not radiation chemistry) are of primary importance i.e. the production and purification of radioactive isotopes the preparation of labelled compounds and the impact of radioactivity and radiochemicals on the environment. Some short historical reviews have appeared during the past year one recalling the early work of Becquerel and the Curies,1 the other2 the development of radiochemical techniques throughout this century. Radiochemistry figures in the Encyclopedia of Applied Physics3 and a detailed review of the radiochemistry of germanium4 has appeared.The proceedings of two recent conferences on ‘isotopes’5 and on labelled compounds6 have been published whilst the impact of the use of labelled compounds in medicine has been reviewed.7 Other recent publications include two new books devoted to radiopharmaceuticals8,9 and a third on radiopharmaceutical development10 using biological models. The role of technetium11 in nuclear medicine has been reviewed. 2 Isotope production At an international conference held in 1996 some papers12–14 gave critical consideration to the future role of cyclotrons versus accelerators for the production of shortlived neutron-deficient isotopes whilst others presented more parochial reports of the production of specific isotopes (e.g. 67Ga 81Rb 123I 201Tl,15,16 22Na 55Fe 133Ba15 and 82Sr17).Useful reviews concerning the design of radioisotope facilities18 and the automation of radioisotope production19 have appeared as well as an invaluable compilation of nuclear data20 to assist in the production of ‘new’ isotopes of interest in nuclear medicine (75Br 86Y 94.Tc 124I). Further tables of cross-section data have been published to cover the production of 99Tc 123I 201Tl,21 of 111In,22 and of 65Zn 66Ga 67Ga.23 The roles of direct neutron capture and inelastic neutron scattering have been compared24 and the latter found to be the more important in the production of many radioisotopes of tin and platinum. Royal Society of Chemistry–Annual Reports–Book A 519 Light elements The reported25 production of a few atoms of element Z\[2 can only be of academic interest since the prospect of this element having any chemistry let alone radiochemistry seems remote.Rather more practical are the use of high energy-protons26 to produce 7Be from metals such as iron and copper of high-energy neutrons27 to initate the [13C(n,p)13B] reaction and of high-energy gamma rays28 to make 15O 11C and 13N from 16O (water). The isotope 18F is of great use in nuclear medicine; it can be made by [20Ne(d,a)18F],29 [18O(p,n)18F]30,31 and now by a new reaction [16O(3- He,p)18F].32 It has been shown that protons react32,33 with 18F itself [18F(p,a)15O] which can lead to short term contamination of the radiofluorine. The production of 22Na by the proton irradiation of neon (22Ne) has been proposed16,34 and it has been reported35 that 21Na can be prepared from 20Ne.The detection of an exceptionally light isotope of silicon (22Si q1@2 29 ms) has been reported36 but of more practical use is the production37 of high purity 26Al from [24Mg(a,pn)26Al]. Nuclear reactions that lead to the production of 36Cl from argon [40Ar(p,an or 2p,3n)36Cl] have been studied extensively38 to establish probable routes for the cosmogenesis of this isotope. Transition elements (first row) The bombardment of iron with 40MeV protons has been shown39 to give rise to a range of manganese (52,56Mn) cobalt (55,56,58Co) and nickel (56,57Ni) isotopes whilst neutron irradiation of natural iron produces40 both 54Mn and 60Co. The lighter isotope of cobalt (57Co) can also be made41 from iron by deutron bombardment; very pure 57Co can be made42 from 58Ni.An extraordinarily heavy isotope of nickel (78Ni) has been observed43 from projectile fission of uranium whilst an isotope of more reasonable mass (63Ni) can be made44 from copper[63Cu(n,p)63Ni]. Elements of modest mass Proton-induced reactions with various zinc isotopes (66Zn 67Zn 68Zn) give rise23 to isotopes of zinc (65Zn) and gallium (66,67Ga); 67Ga can also be produced45,46 by the deutron bombardment of natural zinc. The heavier isotope (68Ga) can most conveniently be obtained from the decomposition of 68Ge. A new generator has been described47 in which the radioactive germanium is adsorbed onto a ceria column. Proton irradiation of arsenic (as Cu 3 As) and krypton gives rise48 to radioisotopes of selenium and rubidium respectively [75As(p,3n)73Se] and [82Kr(p,n)82.Rb].When arsenic is bombarded with alpha particles49 radioisotopes of bromine are formed (76,77Br). A lighter isotope results20 from the nuclear reaction of krypton with protons [78Kr(p,a)75Br] or selenium with deutrons [74Se(d,n)75Br]. The isotope 81.Kr is widely used in nuclear medicine and results from the decay of 81Rb. New techniques for the production of this isotope from 82Kr have been described50 which yield products of high radiochemical purity. Yttrium isotopes also are used in nuclear medicine and an improved generator for 90Y (from the decay of 90Sr) has been reported51 whilst the proton irradiation of 86Sr yields 86Y.20 Transition metals (second row) and some heavier elements Molybdenum-99 can be extracted from the products of 238U fission52 but the better route is from the neutron irradiation of 98Mo.New generators for the production of 99.Tc from the decay of adsorbed 99Mocontinue to be published53,54 and details have 520 D. S. Urch been given55 of methods to recover 98Mo from spent generators. There is continued interest in ways to make lighter isotopes of technetium e.g. [94Mo(p,n)94.Tc],20 [93Nb(a,2n)95.Tc]56 and [96Mo(3He,pnc)97Tc],57 and technetium itself can be the subject of proton irradiation for the production of 97Ru (q1@2 2.9 d),58 [99Tc(p,3n)97Ru]. Cyclotron irradiation has been used59 to prepare radioisotopes of palladium (103Pd) and rhodium (100Rh and 101.Rh). When silver is bombarded with alpha particles a range of di§erent isotopes is produced60 depending on the energy of the beam e.g. [107Ag(a,n)110.In] [107Ag(a,2n)109In] [107Ag(a,pn)109Cd] and [109Ag(a,2n)111In].22 The isotope 111In also results22 from the proton bombardment of cadmium [111Cd(p,n)111In] and [112Cd(p,2n)111In].A very light neutron deficient isotope of cadmium has been reported61 to result from the reaction of iron and chromium nuclei [50Cr(56Fe,2p2n)102Cd]. Light nuclei of iodine are also of interest because of their favourable properties for nuclear medicine. Iodine-123 can be produced by the proton irradiation of either 123Te62 or 124Xe63 whilst 124I can be made from 124Te by either proton20,48 or deutron64 bombardment. Rare-earth and even heavier elements A new technique has been described65 for the production of complexed 153Sm which is used in nuclear medicine for bone imaging. Complex formation can also be used66 in the separation and purification of 153Gd other procedures67 have been based on chromatography.It has been found68 that most isotopes of terbium with masses between 153 and 161 are formed when natural gadolinium is bombarded with alpha particles but a heavier isotope 166Tb (q1@2 21 s) has been identifed69 in the proton induced fission of 238U. A new generator for 172Lu (daughter of 172Hf) has been described.70 The irradiation of tungsten with protons using a cyclotron has been shown71 to be a useful route to 183Ta [186W(p,a)183Ta]. Two isotopes of rhenium of masses 186 and 188 are being intensively studied for their radiopharmaceutical potential. The former can be produced either by the [185Re(n,c)186Re]72 reaction or by the proton irradiation of tungsten [186W(p,n)186Re]72,73 and purified chromatographically.74 The heavier isotope is usually obtained from the decay of 188W and techniques for the production75,76 of this isotope in high-flux reactors as well as numerous generators75 –78 for dispensing the resulting 188Re have been described.Some new very light and very short lived isotopes of platinum (166Pt q1@2 0.3 ms 167Pt q1@2 0.1 ms 168Pt q1@2 2ms and 170Pt q1@2 14.8 ms) have been produced79 by the bombardment of molybdenum with krypton nuclei. Rather more useful is the report59 that 188Pt 189Pt and 191Pt can be produced from iridium and osmium targets. Gold-198 results from the alpha bombardment of natural gold80 and new very short-lived isotopes of lead (180Pb)81 and polonium (190Po)82 have been found. Astatine-211 being a short-lived (q1@2 7.2 h) alpha emitting isotope has great potential in tumor radiotherapy and techniques to optimise the reaction [209Bi(a,2n)211At] have been described.83 The production of 210Fr would appear to be a little more di¶cult and making just a thousand or so atoms84 is cause for celebration (well publication).It has been found85 that beams of chlorine or argon nuclei can react with rare-earth elements to produce light and short-lived isotopes of radium (masses 202–205). Actinides and beyond Agenerator for the preparation of 225Ac from the decay of 229Th has been described,86 521 Radiochemistry and special techniques reported for the purification of 234Th,87 232U,88 and 237Np.89 Very light isotopes of protactinium result90 from the bombardment of erbium with vanadium nuclei (213,214Pa) but high-energy protons are su¶cient to bring about the [238Pu(p,4n)235Am] reaction91 leading to a light isotope of americium (q1@2 15 m).Other light isotopes of transuranic elements can be made92 using cyclotron beams of protons or 3He nuclei [236U(p,2n)235Np] [237Np(3He,p 2n)237Pu] and [237Np(3He,p3n)236Pu]. Methods for making (it is hardly production or preparation) a few atoms of the super heavy elements (Z[104) have been reviewed93 and it is amazing how just a few common experimental facts can bolster theory no end. Why now it is claimed the nuclear properties of these elements at or near the new region of stability (108 whatever happened to 110?) all agree closely with the predictions of theory (just don’t look back at the theoretical predictions of a decade ago!). Even so it is of interest to be able to report that the bombardment of 232Th with 40Ar can give either 272(108)94 or 265(106),95 depending on the argon beam energy and that 16O reacts with a curium target96 to make some atoms of 261(104) (q1@2 78 s).The latter experiment even allowed some rather rapid chemistry to be attempted. This has not yet proved possible with elements 110–112 which are made by reactions of beams of nickel97 and zinc98 nuclei with 208Pb or 209Bi targets 269(110) 271(110) 273(110) 272(111) and 277(112) have so far been claimed. 3 Preparation of labelled compounds The greatest call for compounds labelled with radioisotopes comes from nuclear medicine. The role of such compounds in diagnosis99,100 and therapy101 has been extensively reviewed as have the techniques for the preparation of compounds labelled with short-lived positron emitting isotopes102–105 that can be used for tomography (PET).More specific reviews have covered the role of organotin compounds106 in radiopharmaceutical synthesis procedures107 for the labelling of DNA and automation in a radiopharmaceutical laboratory.108 Tritium The techniques for the preparation of tritium labelled compounds are now well established but there is increasing emphasis on routes to produce labelled compounds with very high specific activities. Wholly tritiated methyl iodide has for example been used109 in the preparation of labelled L-methionine and [3H]methyl derivatives of cyclodepsipeptides.110,111 Multiple labelling can also be achieved either by catalytic exposure to tritium gas as in the labelling of colchicine112 or glucose113 or by the reduction of unsaturated compounds with tritium gas.Some recent examples of the latter are preparation of labelled 3-aminoquinuclidene,114 thymidine monophosphate 115 an opioid receptor116 (CH––CH 2 reduced to CH3H–CH 2 3H) a pyrimidine derivative117 and protoporphyrinogen oxidase.118 Reduction that leads to the incorporation of a large amount of tritium label can also be carried out using pertritiated sodium tetrahydroborate as in the preparation of labelled geranyl derivatives119 and prostoglandins.120 High specific acitivty labelling can also be achieved by dehalogenation (usually bromine is replaced by tritium) which can often be e§ected simply by exposure to tritium gas. This method has been used to prepare labelled ocfentanil and 522 D. S. Urch brifentanil derivatives,121 multiply labelled diphenylacetic acid,122 2-[3H]adenosine- 5@-triphosphate123 and 1,2-dihydronaphthoic acid.124 In most cases the labelled compound was but an intermediate in the synthesis of a larger molecule.Techniques for the tritium labelling of amines have been considered125 and a method outlined126 for the preparation of tritium labelled alkenes. Carbon (mostly 11C) The short half-life of 11C exercises the mind when planning syntheses so that whilst [11C]methyl iodide is widely used in the labelling of small (methyl ketones127) and large molecules such as agonists128 and antagonists,129 improved yields can be obtained130 by trapping and adsorbing the reactants. Adsorption of potentially reactive species has also been used131 to facilitate the preparation of [11C]thymidine.Other procedures born either of cunning or desperation have involved the use of supercritical ammonia132 to make [11C]guanidines or the adsorbing of 11CO onto hot ruthenium and then allowing the carbon atoms to react with pentene133 so as to eventually make [11C]hexane even pyrolysis to 700 °C has been tried,134 pyrroles with [11C]side chains then form labelled pyridine derivatives. Hot atom reactions too have been used,135 since the gamma irradiation of C 60 which induces the [12C(c,n)11C] reaction gives a good yield of [11C]fullerene; 14C labelled fullerenes are made136 when a plasma arc is struck between graphite rods impregnated with [14C] progesterone. The quite specific needs of 11C-labelling continues to spawn the preparation of special intermediates such as [11C]cyanogen bromide,137 [11C]cyanamide138 and N-[1-11C]acetylpyridinium chloride.139 Fluorine (18F) General procedures for the production of 18F radiopharmaceuticals have been extensively reviewed;140–142 a more specific article143 has considered the more particular use of 18F·F as a way of incorporating 18F into molecules.One of the simplest labelling methods is to use nucleophilic displacement by the [18F]~ anion of a suitable group such as mesityl as in the synthesis of (fluoro)thienylcyclohexylpiperidines,144 or tosyl in the preparation of fluoromisonidazole.145 The e¶ciency of the labelling reaction can often be enhanced by the use of crown ethers (or similar compounds)146 to overcome solubility problems in other cases microwave irradiation147 has been found to have a beneficial e§ect.Optimum procedures for the [18F]fluoridation of proteins and peptides,148 of dopamines149 and of ethyl groups150 have been discussed. Phosphorus sulfur potassium and calcium Most of the radiochemistry that utilises radiophosphorus is concerned with the labelling of proteins,151 and recombinant antibody fragments152 via nucleoside triphosphate groups. These can be labelled with 32P at the a or c positions153 or it is a pleasure to report with 33P (c-[33P]ATP).154 Labelled nucleotide phosphorothioates can be purified155 by thin layer chromatography. Techniques for the preparation of a range of sulfur compounds labelled with [35S] including [35S]sulfonamide have been reviewed.156 The radiochemistry of the potassium isotopes and their potential for use in nuclear medicine has been evaluated.157 Hydroxyapatite has been labelled158 with [45Ca].523 Radiochemistry Copper gallium selenium bromine and yttrium New bifunctional complexing agents have been synthesised to permit the incorporation of radioactive copper (either as 64Cu or 67Cu) into monoclonal antibodies159,160 (in the jargon of the trade one speaks of ‘conjugating’ the chelating agent with the antibody) or octreotide161 (a somatostatin receptor ligand). Similar stategies have been used for the labelling of complex biomolecules162 with 67Ga as in the synthesis of new bifunctional ligands163 based on thio derivatives of ethylenediamine. The role of gallium radiopharmaceuticals in nuclear medicine has been reviewed.164 Details of the preparation of [75Se]-labelled methionine165 and [77Br]-5,7-dibromo-4-oxo-1,4- dihydroquinoline-2-carboxylic acid166 have both been reported.Bifunctional ligands which can ‘conjugate’ to proteins and chelate specific radioactive ions are also being developed to bind 90Y into monoclonal antibodies167 and the like. Diethylenetriaminepentaacetic acid and its derivatives168 are popular ligands for this purpose. Technetium The radioisotope 99.Tc is one of the most widely used in the whole of nuclear medicine. Its role is constantly under review169,170 as is the use of chelates171 which are parts of larger bifuntional ligands the chelate binds the technetium the other functional part of the ligand joins to the selected biomolecule. The technetium that is used in this way is mostly derived from generators (decay of immobilised 99Mo) and there are problems of continuing concern about the isotopic purity of the 99.Tc that results,172 the chemical purity and composition of the eluate (Tc complexes with bu§ers can be found and sometimes complexes with column substrate material)173 and even when is the best time to use the eluate.174 The bifunctional ligand approach is now widely used in the labelling of large protein molecules.The chelate may contain sulfanyl sulfanylacetyl or sulfanylacetamido groups bound to short-chain fatty acid aromatic acid or amino acid molecules.175 The acid group provides the point of attachment to the protein as in the labelling of the cyclic GP IIb/IIIa receptor antagonist.176,177 Other recently developed chelates have been based on hydrazinopyridine178 or hydrazinonicotine179,180 derivatives; a series of molecules based on 2-aminocyclopentene-1-dithiocarboxylic acid have also been shown181 to form very stable complexes with technetium.A more direct way of labelling proteins is to recognize that they will contain somewhere in their structure the amino acid cysteine and that this molecule has an S–S bond. This can be cleaved by reduction e.g. with 2-sulfanylethanol and the resulting thiol groups will bind readily to technetium (the so-called Schwartz method). This approach has been used to incorporate 99.Tc into the octapeptides vapreotide182 and sandostatin183 and into monoclonal antibodies such as 3H11184 and anti-CEA.185 A simpler method of S–S bond clevage has been proposed by UV irradiation. Labelling then proceeds much as before ; this technique has been applied186 to the preparation of the labelled antitumor antibody PR1A3.Whilst all these clever methods most certainly do lead to technetium becoming chemically bound to large biomolecules it is as well to ask two questions to what extent has labelling altered the whole system and also to what extent has labelling altered the target molecule? New versions of liquid chromatography have been developed187 in an attempt to resolve the former problem and so measure and separate the labelled polymers labelled oligomers etc. all of which are formed during the labelling of proteins. The second 524 D. S. Urch question is more serious since it is axiomatic that there are no naturally occuring proteins that incorporate technetium. How then does the presence of this atom modify the physiological activity of the protein,188 is it benign or not and how would one find out? Always with an eye to possible medical applications many other molecules have been shown to form complexes with technetium and of course their radiopharmaceutical potential has been assessed; tris(hydroxymethyl)phosphine,189 a porphryin 190 diethylenetriaminepentaacetic acid191 and hydroxyethylthiotamoxifen.192 Rather more bizarre are the reports of the preparation of 99.Tc-labelled sulfur colloids193 and tin aerosols!194 Ruthenium and indium The radiopharmaceutical potential of 106Ru complexes195 and the radiopharmaceutical uses of 111In complexes164 have both been the subject of recent reviews.As with other isotopes used to ‘label’ proteins the bifunctional ligand approach is now all the rage but with a strange lack of inspiration in the choice of chelating group for indium it is some derivative of diethylenetriaminepentaacetic acid or nothing.With variations in the bridging groups to the protein moiety this chelator has been used to label albumin,196 lipoproteins,197 monoclonal antibodies167 and tamoxifen derivatives.198 Iodine Radioiodine exchange with covalently bound iodine is one of the simplest ways of producing radioiodine-labelled compounds as in the recent reports of the labelling of iodobenzvesamicol199 and iodotyrosyl peptides.200 Chloramine-T is often used to assist this reaction (adenosine201 and tyrosine202 derivatives thyronine and thyroxine203). Chloramine-T is also used to help in ‘demetallation’ reactions where organostannyl or similar groups are replaced by radioiodine.This has been used in the preparation of m- p-[123I]iodomethylphenidates,204 [123I]labelled piperidine205 and [125I]pyridine206 derivatives m-[125I]iodohippuric acid207 and proposed208 as a general method for the labelling of iododeoxyuridine with any isotope. Another facilitator for iodine exchange is Iodogen (1,3,4,6-tetrachloro-3a,6a-diphenylglycouril) which has been used in the labelling of 2-iodolisuride209 as well as many iododerivatives of peptides and proteins.210 Iodine exchange can also be catalysed by copper(I) and this procedure has proved e§ective in the preparation of 2-[123I] iodophenylmetyrapone211 and a whole range of iodo-derivatives of nitroimidazole.212 The lability of radioiodine compounds is notorious (or ought to be) and a detailed study has been made213 of storage at various temperatures of m-[131I]iodobenzylguanidine.So rapid are the reactions of decomposition that it is recommended that this compound (and by implication other 131I compounds too) not be used after 3.5 h at room temperature. But help is at hand in the form of a silver membrane which it is reported214 can remove just about all the free iodide ions from solution. Heavier elements It would appear that solutions of dysprosium or holmium nitrate when treated with alkaline sodium tetrahydroborate and then subject to neutron irradiation yield215 suspensions labelled with either 165Dy or 166Ho. The ease with which holmium undergoes the n,c reaction and the stability of fullerene sytems to irradiation has 525 Radiochemistry encouraged216,217 the preparation of fullerene encapsulated materials which may have radiopharmaceutical uses.Rhenium too by analogy with technetium is being actively studied for applications in nuclear medicine. There is a distressing tendency for very simple ‘one-step’ procedures to be described218–220 for the production of proteins monoclonal antibodies and the like all labelled with either 186Re221 or 188Re; radioactive the products of such procedures may be but what exactly has been labelled is a question unaddressed and so necessarily unanswered. Rather more detailed and careful work has gone into the labelling of some antibodies with 186Re using quite specific chelating groups222 based on sulfanylacetyltriglycine. Special chelates have also been reported223 for the labelling of monclonal antibodies by 203Pb.The labelling of similar materials with 211At224 and 213Bi225 has also been reviewed. 4 Environment The impact of radioactive substances upon the environment is an ever increasing area of research with concern about this impact increasing at an even faster rate. At every stage radiochemistry is involved implicated and required. A bibliography226 has appeared and conferences have been held (e.g. on Environmental Impact of Radioactive Releases227). The main concern of this section of the Report will be to review recent work on problems associated with waste disposal both from the nuclear power industry and from the use (mostly medical) of radiochemicals. Long-term strategies Perhaps stategies is too optimistic a word when what has been achieved over the past forty years is considered.228 Some publications give a detailed and even critical account229 of the problems to be solved but that is not quite the same as a strategy.Current thinking230–232 would seem to be in favour of near-surface sites for the ‘disposal’ of low-level waste; but the word ‘disposal’ is of course just a euphemism. Radioactive waste is not disposed of it is placed somewhere and then we all hope and pray it won’t get out. That it might (thinking the unthinkable) has been considered in a ‘1000 year prediction’233 and also in a contemplation on the e§ects of both climatic and geological change234 upon deep underground storage facilities235 for high-level waste sobering if not comforting reading. Whilst most attention is focused on the problems associated with artificial radioactivity some thought has been given to ‘natural’ problems such those associated with mining; it is proposed that waste ore containing radium be subject to vitrification236 (see below for how long that might last!).Transmutation etc. Apart from shooting long-lived radioactive waste out into space,237 which really is disposal (but hoping that the cannisters do not become malevolevent asteroids to plague some future generation) and burying the stu§ under the bottom of the sea,238 the brightest idea so far is that of transmutation.239 The really long-term problem would then be turned into a shorter-term one. In some cases this could be achieved by c-irradiation240 (favoured for 137Cs) or by particle bombardment using acceler- 526 D. S. Urch ators;241 neutron irradiation has been investigated as a way of removing 99Tc242 (which is a pity when one thinks back to how much time and e§ort was expended more than sixty years ago hunting for element 43).A tokamak transmutation reactor has been proposed243,244 in which the high-level wastes (actinides and fission products) would form the ‘blanket’ round the reactor; e¶cient detoxification(!) is promised. Two quite specific problems attract the proponants of transmutation the first is the nature of the spent fuel from light water reactors245,246 and the other is the plutonium problem247 (i.e. what on earth to do with it). Optimists245 even foresee the construction of special reactors which would not only transmute long-lived radioisotopes into short-lived ones but which would produce energy as well and so the whole process would be economically profitable.Storage geology Most counties with nuclear power plants even those that just use radiochemicals are now contemplating the construction of permanent storage facilities for all types of waste. Since this inevitably involves underground storage the geology of the proposed site248–250 becomes the subject of much scrutiny. The Germans favour251 the use of old potash and salt mines as well as the salt dome252 near Gorleben. More general but very detailed consideration253,254 of the problem has been made by Canadian researchers. The concept of ‘indefinite containment’ is bandied about255 but this holy grail seems if one is going to be really honest unrealisable. It should be conceded that some leakage is inevitable and so it becomes a question of how much in what chemical form and at what rate will which isotopes escape.This in turn is a function of the geological environment and the extent to which it may have been disturbed256,257 by the construction of the storage facility itself . Deep bore holes are now being considered258 –260 as ‘final storage’ locations for spent fuel elements (usually aluminium clad) for depleted fuel261 and for surplus weapon-grade plutonium.262A particular problem which has attacted attention recently is the mobilisation of uranium263–265 in such locations when in contact with saline ground waters. There are those who have proposed266 that such sites would be useful sources of enhanced geothermal energy. Storage containers and containment Other problems to be considered are the containers in which waste would be held; advanced microwave ceramic technology is now said267 to be the solution promising ‘virtual permanence for extended periods of geological time’ whilst those advocating the use of concrete268 are worrying about groundwater infiltration concrete degradation etc.as well they should. The word ‘immobilization’ is popular with those who advocate combining actinides and such like into cement269,270 or glass-forming cements271 or ceramics272 or felspathoids273 or alloys.274 But for how long will the atoms said to be immobile stay that way? All a radioactive atom has to do is decay and it has enough recoil energy to scotch any notion of ‘immobility’! So much for eons of ‘containment’. Yet there are some silicate structures that would appear to be more radiation stable than others and zircon275,276 in particular has been advocated as a host for plutonium.Alternatives such a zeolites277 and glasses278,279 (i.e. vitrification) have also been suggested. Any such host will of necessity get hot as radioactive atoms decay and studies280 on glasses have shown that transition-metal ions tend to migrate to the 527 Radiochemistry colder regions whilst zirconium and calcium would seem to like it hot. Kaolinite proved an e§ective moderator to ion transport. Both borosilicate281 and ‘vitreous cement’282,283 type glasses have been advocated for the retention of radioactive waste and their long-term (sic.) stability for a seven year281 period has been tested. Whilst waste nuclear fuel might be contained in copper284,285 or titanium286 canisters (whose corrosion characteristics have been investigated) the overall construction material of a waste disposal site will be mainly of concrete.The use of concrete in such structures has been the subject of a recent book287 whilst the rates at which ground water might percolate through cement and concrete and leach out radioactive species ‘contained’ within has been the subject of some speculation288–291 (the PC word is ‘modelling’). Other problems associated with the long-term storage of nuclear waste are the e§ect which radiolysis will have on water292 (the increase in peroxide concentration will enhance its oxidising power) and the probability of hydrogen formation.293 Radioisotopes in solution Improved radiochemical procedures have been reported for the determination of 54Mn,294 90Sr,295 129I296 as well as americium and curium297 in aqueous solutions.Activated carbon has been shown298 to be very e§ective at removing technetium from contaminated ground water. It is also e§ective in removing many other radioisotopes from drinking water according to a recent Finnish report.299 Analysis of sewage sludge300 and river sediments301 can also yield evidence of environmental contamination (e.g. 137Cs post-Chernobyl). On a more global scale similar analyses have been made of marine sediments from the Adriatic,302 the Arctic303 and from the Kara Sea304 (that is where nuclear submarines go to die). Adsorption of radioisotopes This is a vast topic and one of increasing importance since most radioisotopes can to a greater or lesser extent be adsorbed onto something. Adsorption onto mineral surfaces has been reviewed305 and the current literature surveyed306 for relevant data.More specific studies have concentrated on clay minerals,307 on bentonite,308 on the adsorption of 110.Ag onto various oxides309 and on the interaction of uranyl anions310 with goethite [a-FeO(OH)] at varying partial pressures of carbon dioxide. The interaction of goethite and granite with radioactive selenium and tin isotopes has also been investigated311 but most studies have concentrated on the adsorptive properties of clays and similar layered minerals. Apart from specific investigations such as the adsorption of thorium on montmorillonite,312 there has been an even greater concentration on the three elements barium (133Ba),313,314 strontium (90Sr)313–318 and caesium (137Cs).317,318 The adsorption of strontium and caesium radionuclides on glauconite,319 silica gel320 and even Red Mud321 (bauxite residue) has also been studied.But of all these isotopes it is the behaviour of 137Cs in soil322 which has received the most attention; to skim just a few of the topics 137Cs in forest soils,323 in peat,324 in lake sediments,325 in rich soils,326 in tropical soils,327 in the soils of the Scottish uplands328 and in fungi.329 Decontamination Adsorption behaviour can often determine the techniques used in decontamination 528 D. S. Urch and these techniques can also be closely related to those used to extract and to concentrate specific isotopes. Thus strontium-90 can be extracted330 from fuel reprocessing solutions by cation exchange and actinides can be removed331 by extraction chromatography procedures which give rise to the concept of recycling332 of radioisotopes.More generally active carbon has been found333 to be an excellent general adsorbant of radioactive ions. Other decontamination procedures can involve chelation334 –336 of transition-metal ions (e.g. removal335 of 60Co) or fluorinated surfactants. 337 Electrolysis has been proposed338 for the decontamination of stainless-steel surfaces. Microbes Some strains of bacteria have been found to be positively fond of plutonium339 and can be used for water decontamination (99%). A more general review340 concludes that many microbes have the power (or is it the will?) to remove metal ions including radioactive ones from solution probably by binding to their cell walls. The ability of microbes to live almost anywhere has to be taken into consideration when planning long-term waste disposal293,341–343 even in deep locations in crystalline rock.It is not inconceivable that bacteria will evolve that actually thrive on nuclear waste and use the abundant energy for themselves caring not a jot for the radiation. Food The contamination of food by radionuclides following reactor accidents344 or even just from normal reactor discharge (3H 14C 35S)345 is a cause for concern. Analyses following accidents of things as varied as catfish,346 sheep,347 wild boar348 and tea349 have shown that radiocaesium is a common problem with 90Sr also being found in tea 131I can be a short-term problem350 in milk (and milk products). A radiochemical study of cooking345 has shown that up to 60% 35S and 3H is lost by boiling or roasting.The radiocaesium problem can be met by the use of ammonium hexacyanoferrate( III)344 or Prussian Blue,351 or livestock reared in contaminated areas can be moved away shortly before slaughter352 (shortly because caesium is quickly flushed from the system). Contamination The nature of the Chernobyl reactor accident has been the subject of re-evaluation353,354 and reports of the consequent contamination continue to appear (Saxony355 and Poland356). Analyses have also been reported357 of plutonium levels in soils near the Semipalatinsk (former USSR) nuclear test site. The role of natural fires in transporting some long-lived isotopes (36Cl 137Cs 129I) long distances has been investigated.358 5 Miscellaneous Decay Most radioactive isotopes decay by the emission of 4He nuclei or electrons (positive or negative) and gamma rays but recent work has described359–361 rare events in which 529 Radiochemistry light elements appear to undergo fission or decay by the emission of clusters of nucleons.Another rare situation is a nucleus in which the population inversion of an excited state is possible this could lead to the development of a c-ray laser. Hafnium-178 has been identified362 as such a nucleus and a procedure for its isolation has been proposed. c-Decay can sometimes be subjected to angular perturbations which are chemical in origin . This e§ect can be used for isotopes such as 111In and 181Ta to yield structural information363 concerning the chemical environment of the radioisotope. Work continues on ways to transform the energy of radioactive decay directly into electric power and recent publications have described batteries powered by 238Pu 90Sr364 and 147Pm.365 The half-life of 126Sn has been redetermined366 as 260 years.Hot-atom chemistry A detailed account has been given367 of the reactions of recoil tritium with ortho- and para-hydrogen and also dideuterium at low temperatures. The observation of a large isotope e§ect suggests that some tunnelling reactions take place. Tritium adsorbed on metal surfaces can react368 with 1-haloalkanes to produce tritium labelled 2-haloalkanes. Whilst it is clear that this is not a reaction induced by nuclear recoil it could well be that the energy of tritium decay plays a part in initiating this (ion–molecule?) reaction. More conventional hot atom reactions are involved in the reactions of recoil 13N with aqueous ethanol369 to produce [13N]ammonia.The importance of recoil energy in understanding the chemical reactions of the heavy elements as they proceed through their decay chains has been emphasised.370,371 Recoil energy is also the basis for the isolation372 of the heaviest elements. Thin-layer sources Thin uniform sources of radioisotopes are useful since losses of activity can be avoided. This is particularly important for a-sources373 and even c-sources if they are low energy 241Am.374 Electrodeposition can be used (65Zn)375 or nuclear beam implantation (7Be).376 Ion-beams have been suggested377 as a way of producing glass beads which would have 32P embedded in their surface. These beads could then be used for radiotherapy.Geochemistry cosmochemistry Uranium–lead and thorium–lead ratios can be used for dating378 moanzite inclusions in granite whilst deviations from 234Th 210Pb ratios can be used379 to measure current rates of sedimentary deposition. The 231Pa 230Th ratio in the Atlantic and related isotopic ratios from deep ocean sediments have been used380 to establish the lifetimes of particular patterns of water circulation . It is proposed381 to use accelerator mass spectrometry to detect 236U at levels 10~13 that of 238U. If this can be done it would provide a neutron flux indicator for the past hundred million years. Indeed there is considerable interest382 in attempting to estimate the rate at which nuclei such as 3H 7Be 10Be 14C 21Ne 26Al and 36Cl38 would be produced by nuclear reactions induced by galactic cosmic rays and also by solar protons.It is of course necessary to take account of the energies of the bombarding particles and of the position on the earth’s surface and also depth under the surface. In particular 36Cl can be produced by spallation reactions383 involving 40Ca. Calcite and calcium feldspars will therefore 530 D. S. Urch often have detectable amounts of 36Cl from which the original mineral can be dated. Other reactions that can lead to 36Cl are muon capture by 40Ca and neutron capture by 35Cl. This latter reaction is more likely near uranium deposits384 and measurment of 36Cl in such locations can be used to determine ion mobility. Using proton beams with energies up to 400MeV cross-section data have been measured385 for the production of nuclei such as 10Be 26Al and 36Cl.These data are of value in estimating not only terrestial nuclear reactions (above) but also extraterrestial reactions386 such as would occur in asteroids. In this way it is possible to date386 meteorites and also to rationalise387 the higher 11B 10B and 41Ca:40Ca ratios found in them. References 1 J.P. Adlo§ and H. J. MacCordick Radiochim. Acta 1995 70/71 13. 2 S.T. Contis and K. Rengan J. Radioanal. Nucl. Chem. 1996 203 273. 3 G.L. Trigg (Editor) Radiochemistry and Seismology Encyclopedia of Applied Physics VCH Weinheim Germany 1996 vol. 16. 4 S. Mirzadeh and R. M. Lambrecht J. Radioanal. Nucl. Chem. 1996 202 7. 5 Proceedings of International Con§erence on Isotopes Beijing 1995 J. Radioanal. Nucl. Chem. 1996 206. 6 J. Allen and R. Voges (Editors) Synthesis and Applications of Isotopically Labelled Compounds Wiley Chichester 1995.7 G. 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ISSN:0260-1818    

DOI:10.1039/ic093519

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

27 Inorganic mechanisms By N. WINTERTON ICI Chemicals & Polymers Ltd Runcorn Technical Centre The Heath Runcorn Cheshire WA7 4QD UK 1 Introduction Understanding the factors which control the mechanisms of inorganic and organometallic redox and substitution reactions in solution is important environmentally biologically industrially as well as intrinsically. The literature on these topics for 1996 subject to the exclusions noted in the 1995 report is organised as before. Espenson1 and Atwood2 have updated their texts on kinetics and mechanisms. Texts3–6 and a volume of Chem. Rev.7 contain surveys of mechanistic aspects of the biological roles of metals. The increasing contribution made by computational methods to the study of mechanisms has been reviewed.8,9 A program which corrects for concentration gradients in stopped-flow kinetic observations on second-order processes with rate constants up to 108 mol l~1)~1 s~1 has been tested10 experimentally.The design of a stopped-flow instrument for the simultaneous monitoring of absorbance and fluorescence up to 200MPa has been described.11 Short-lived intermediates such as five-co-ordinate base-o§ methylcobalamin have been characterised by time-resolved X-ray absorption spectroscopy.12 2 Redox reactions Marcus13 has reviewed electron-transfer reactions in chemistry in a volume on protein electron transfer.6 Current challenges in electron-transfer mechanisms have also been addressed.14 Long range electron transfer Biological electron-transfer has been reviewed.15–23McLendonand co-workers24 have shown that Marcus theory can be applied to in vivo reactions of cytochrome (cyt) c.Intramolecular electron transfer between the Cys3–Cys23 radical ion and CuII in wild-type and single-site mutated azurins25 occurs via a through-bond mechanism. The pHdependence of these processes has also been reported.26 The e§ect of the intervening medium on electron transfer in Ru-modified azurin cyt c and myoglobin has been studied.27 Photoinduced electron-transfer rates for a series of diimine–RuII complexes with FeIII(cyt c) with *Gq\[1.12V decrease with increasing exergonicity28 and fit a bimolecular model for electron transfer over an equilibrium distribution of reactant Royal Society of Chemistry–Annual Reports–Book A 541 separationseachwithadi§erentformationprobability.Thedriving-forcedependenceof the reduction of FeIII(cyt c) modified at His-33 by a series of ruthenium moieties reveals29 that the most exergonic reductions have rates which are much higher than expectedfor processesleadingdirectly toground-stateproducts.It issuggested29thatan electronically-excitedferrohaem is the initial product. Rates of oxidation ofFeII(cyt c)by [FeIII(CN) 5 (Rpy)]2~ depend on the e§ect of R on the stability of the precursor complex to electron transfer.30Therates of electron transferbetweenmetHbIIIand cytb 5 II deviate from linear Arrhenius dependence31 because of specific interactions between the two proteins. Other long-range electron-transfer processes within protein–protein complexes have been described.32–35 The very slow rates of electron self-exchange between wild-type Pyrococcus furiosus 4-Fe ferredoxin compared with two Asp-14-Cys and Asp-14-Ser mutants are ascribed36 to Asp ligation to the 4-Fe cluster.Positively charged residues near to the electron-transfer site account37 for the rate of electron self-exchangefor the blue copperprotein,umecyanin beingamongthe lowestknownfor this class of protein. Electron self-exchange has also been studied38 for Clostridium pasteurianum rubredoxins. Luminescence quenching of [Ru*(bipy) 3 ]2`by a copper site in sub-unit II of Thermus thermophilus cytochrome ba 3 is observed39 at low protein concentrations. At higher concentrations quenching is saturated as a result of groundstate complexation. Reduction of Pseudomonas aeruginosa cyt cd 1 nitrite reductase by [Fe(edta)]2~ leads first to reduction of haem c followed by a very slow first-order intramolecular reduction of haem d 1 one of the slowest natural electron transfers recorded in biological systems ascribed40 to unfavourable orientation e§ects and/or large separations.Slow intramolecular electron transfer is also seen41 for the reaction between a bacterial dihaem protein and [Co(bipy) 3 ]3`@2`. Increased *G8 for electron transfer in sperm-whale myoglobin modified at the distal histidine residue (His-64) is a result of hydrogen-bonding e§ects in the distal pocket.42 Rates of intramolecular electrontransfer in cytb 5 covalentlymodifiedwithRuII–polypyridinemoieties atCys-73 or Cys-65 with*RuIIandFeIII separatedby a well defined 12 covalent-bondlink show43 either zero or slightly negative *V8. Cobalt(II) toRuIII intramolecular electron-transfer rates have been measured44 for a cobaltocytochromec modifiedat His-33 byRu(NH 3 ) 5 .Reversibleouter-sphereelectron-transferrates for[Ru(NH 3 ) 5 L]3`@2`(L\substituted py) and cyt c vary45 with the ability of L to penetrate the haem groove on cytochrome c. Sykesand co-workers46–49 have studied the kinetics of the reduction of theR2protein of E. coli ribonucleotide reductase with [Co(sep)]2` and [Co([9]aneN 3 ) 2 ]2`46 and with hydrazine,47 as well as of the reaction of the oxidised form of rubredoxin from Clostridium pasteurianum with [Co(sep)]2` and its reduced form with [Co(terpy) 2 ]3` and[Ru(NH 3 ) 6 ]3`.48 Electrostatic e§ects in the reactions of [2Fe-2S]ferredoxinsand a series of cobalt complexes have been reported.50 Circular dichroism has been used51 to investigate the specific sites of reaction of plastocyanin with optically-active CoII complexes.The pH dependence of the oxidation of ferric microperoxidase-8 by photogenerated [Ru(bipy) 3 ]3` suggests52 that the ferryl cation-radical porphyrin is formed by intramolecular electron transfer within a ferric cation-radical porphyrin. Reaction with cysteine has also been studied.53 Electron transfer in de novo-designed metalloproteins has been described.54 A new theoretical method55 for describing tunnelling processes in long-range multiple-pathway electron transfer in proteins and an approach designed to permit analysis of such processes in terms of protein secondary and tertiary structure56 have 542 N.Winterton been developed. Marcus and co-workers57 report procedures designed to reduce the computational complexity associated with an extended Hu� ckel theory analysis of electron tunnelling in proteins by selecting for the calculation only those amino acids that are important to electron transfer.The validity of a multiple-scattering approach to electron coupling has been debated.58,59 Aspects of DNA-mediated electron transfer15,60 –65 and oxidative DNA and mRNAcleavage66–75 have been studied mechanistically. In a series of mixed-valence diferrocenylpolyenes [Fc(CH––CH)nFc]` (n\1–6) metal–metal coupling falls o§ exponentially76 with separation with an exponent of 0.087Å~1 one of the smallest attenuations reported. Photoinduced electron- and energy-transfer have been studied in [(tterpy)RuII(k-L1)RhIII(tterpy)]5` 77 [(bipy) 2 Ru(k-L2)Os(bipy) 2 ]5`,78 in [(tterpy)RuII(k-L)OsII(tterpy)]2`,79 linked by rigid spacers L\L3 (with metal–metal separations of 11 15.5 and 20Å for n\0 1 and 2 respectively) or L4,80 in [(bipy) 2 RuII(k-L5)CoIII(bipy) 2 ]5`81 in [(bipy) 2 MII(k- L6)M@II(bipy) 2 ]4` (M,M@\Ru or Os),82 and [(bipy) 2 RuII(k-L7)RuIII(bipy) 2 ]5`,83 and [(bipy) 2 RuII(k-L8)RuIII(decbipy) 2 ]5`.84 Related systems85 involving other covalent assemblies including diads and triads containing metalloporphyrin units have been studied further particularly as photosynthetic models.86–94 Rates of long-range electron transfer through an aryl ether dendrimer framework have also been reported.95 Intramolecular and intervalence electron transfer Further investigations have been undertaken of transients formed in the electronic excitation of various fac-[ReI(CO) 3 L(a-diimine)]` 1 fac-[ReIX(CO) 3 (a-diimine)] and relatedMn and Ru complexes.96–109 Mey and co-workers103 have shown that (for 1 L\afa2~) the rates for afa3·~ to ReII and (for 1 L\oqd) oqd·~ to ReII electrontransfer (k ET ) are remarkably slow with k ET ~1 being 5.6 and 15 ks respectively.These long lifetimes are thought to arise from weak electronic coupling through the aryloxy bridges. Solvent e§ects have been reported105 on the intramolecular folding that precedes quenching by a nitrobenzene moiety incorporated into a polyether macrocycle of Re-to-pyridine charge transfer in 1 L\L9. Formation and decay of alkyl radicals formed by Re–R homolysis have been studied by FT-EPR for [ReR(CO) 3 (dmbipy)] and [RuR(CO) 2 (dmbipy)] (R\PhCH 2 2-Pr or Et).98 Metal–metal bond fission has also been studied for [ReR(CO) 3 (a-diimine)] [R\Ph 3 Sn or Mn(CO) 5 ].108 Other related photochemical and photophysical studies are noted for a-diimine complexes of Ru,110–122 Rh,114,123 Ir,114 Os117 and Cu.124,125 Intramolecular electron-transfer rates in valence tautomeric complexes [Co(3,5- dtbsq) 2 (L)] (L\bipy phen dmbipy or dpbipy),126 have been reported.Related processes in analogous ReI and MnI complexes,127 in homologous [Co(L)(NH 3 ) 5 ]2` (L\trans-stilbenecarboxylate) complexes128 and in homopolymetallic complexes such as [(bipy)(terpy)Ru(CN)Ru(NH 3 ) 5 ]3`,129 [(bipy) 2 Ru(bbbpy)Ru(bipy) 2 ]5`,130 [(bipy) 2 Ru(k-bpt)MRu(k-2,3-dpyp)Ru(bipy) 2N2 ]7` [containing both electron-rich (bpt~) and electron-poor (2,3-dpyp) bridging ligands],131 [(NC)Ru(bipy) 2 (CN)Ru(bipy) 2 (NC)Ru(bipy) 2 (CN)Ru(NH 3 ) 5 ]5`,132andCN~-,129,133 1-(4-cyanophenyl)imidazole-,134 and other ligand135-bridged Ru dimers and heteropolymetallic complexes such as [(NC) 5 FeIII(pyCN)RuII(NH 3 ) 5 ],136 [(NC) 5 Fe(CN)Os(NH 3 ) 5 ]~,137 [(NC) 5 Fe(CN)MPt(NH 3 ) 4N(NC)Fe(CN) 5 ]4~,138 [L(CN)Fe(CN) 5 ]3~ (L\cobalamin)139 and [(bipy) 2 ClOs(4,4@-bipy)Ru(NH 3 ) 5 ]3`@4` 140,141 have been described.543 Inorganic mechanisms N N N N N N n N N N N N N N N n L1 L2 L3 L4 N N N N N N N N N N L5 L6 N N N N N N 544 N.Winterton N N N N N N N N N N NHC(O) C(O)NH N N N N N N H H N O O O O O NO2 O O O O O O O L7 L8 L9 Outer-sphere electron-transfer and self-exchange reactions Swaddle has reviewed142 the e§ectiveness with which theory and experiment coincide in the estimation and measurement of volumes of activation *V8 for outer-sphere electron transfer and for aqueous systems the use made of the departure of *V8 from predicted values in assessing the role of counter ions contributions from inner-sphere processes and other phenomena.Takagi and Swaddle143 have examined solvent e§ects on *V8 for self-exchange for the couple [RuMCF 3 C(O)CHC(O)CF 3N3 ]`@0. The *V8 value for the self-exchange reaction of cytochrome c is in reasonable agreement144 with expectations from other observations. Values of *V for heterogeneous electron transfer for three couples are is about half those for homogeneous bimolecular processes involving the same couples,145 as expected from Marcus theory. Volume changes associated with the [ML 6 ]3`@2` couples (M\Fe Cr Ru or Co; L\H 2 Oor NH 3 ) have also been reported.146 Relevant theoretical studies147,148 are noted.Outer-sphere processes have been studied involving [RuII(NH 3 ) 5 L]2` (L\3- or 4-NH 2 py) with [CoIII(edta)]~,149 [Fe(phen) 3 ]2` with periodate (autocatalysis disappears in excess of IO 3 ~),150 [Nb(CN) 8 ]5~ and alkaline S 2 O 8 2~ BrO 3 ~ and IO 4 ~ (which reacts by two parallel pathways involving monomeric IO 4 ~ and the hydrated dimer I 2 O 10 H 2 4~),151 [Fe(CN) 6 ]4~ and [CoIII(HxPyOz)(NH 3 ) 5 ](m~3)~ (HxPyOz m~ represents 12 di§erent oxophosphorus anions),152 [VO(H 2 O) 5 ]2` and [MnIII(cdta)(H 2 O)]~,153 Ti(aq)3` and [Co(CN) 5 X]3~ (X\Cl Br or I),154 and [Co(NH 3 ) 5 (Him)]3` and [Fe(CN) 6 ]4~.155 Both outer-sphere and inner-sphere processes are involved in reactions of [CoII(sep)]2` with [CoIII(Hdmg) 2 (H 2 O) 2 ]` and [CoIII(dmgBF 2 ) 2 (H 2 O) 2 ]`.156 In reactions of S 2 O 3 2~157 and N 3 ~158 with [NiIV (L10)]2`or [NiIV(L11) 2 ]2`,157 NiIV to NiIII reductions are outer-sphere and NiIII to NiII inner-sphere.The anions [Fe(CN) 6 ]4~ and [FeII(CN) 5 (H 2 O)]3~ react with trans 545 Inorganic mechanisms N NOH N N N H H HON N NOH NH2 N S S N HO OH L10 L11 L12 [CoIII(pyca)(NO 2 )(en) 2 ]` by outer-sphere and inner-sphere processes respectively.159 Kinetics of a series of outer- and inner-sphere reductions of [Co(L12)]3` have been reported.160 Electron-transfer self-exchange rates have been measured (using NMR spectroscopy) for three ruthenium ammine complexes161 (solvent e§ects are associated with solvent–solute hydrogen bonding) for the PMe 3 complex of cytochrome c162 (rates extrapolated to infinite ionic strength are in the order myoglobin–PMe 3 complex> cytochrome b 5\cytochrome c–PMe 3\cytochrome c) [Fe(Me 3 - NCH 2 C 5 H 4 )Cp]2`@` (encapsulated in cyclodextrin163 or sulfonated calixarene164).Self-exchange rates have also been estimated for [Co(Hdmg) 2 (H 2 O) 2 ]`@0 and [Co(dmgBF 2 ) 2 (H 2 O) 2 ]`@0,165 [ReX 2 (dppee) 2 ]`@0 (X\Cl or Br)165 Min CH 2 Cl 2 from studies of oxidation by [CoIII(nox) 3 (BBu) 2 ]` (see also ref. 166)N [OsO 2 ([14] ane(NMe) 4 )]2`@`167 and for FeIIFeIII–cyclidene complexes.168 Iron(II)–iron(III) selfexchange has been studied in a room-temperature melt comprising bipy–Fe complexes functionalised with oligo(ethylene glycol) groups.169 Chelate-ring conformational e§ects on precursor assembly are revealed170 from studies of stereoselectivities in the oxidation of [CoII(pn) 3 ]2` by *-[CoIII(ox) 2 (Gly)]2~ and in the association of the latter with (lel) 3 - and (ob) 3 -[Co(pn) 3 ]3`.The *H8 and *S8 di§erences between *–* and *–" pairs involved in the stereoselective oxidation of [Co(en) 3 ]3` and a series of chiral anionic CoIII complexes suggest171 that electron transfer proceeds by two di§erent mechanisms depending on hydrogen bonding between the reactants. Oxidation (inner-sphere) by *-[Co(ox) 3 ]3~ of [CoIIM5(R,S)- MetrienN(H 2 O) 2 ]2` produces172 a small enantiomeric excess of a *-product [CoIII(ox)M5(R,S)-MetrienN]` when the polyamine is in a cis-b(SS,RR) configuration and of the"-product from a cis-b(SR,RS) configuration. Enantioselective quenching of rac-[TbIII(pydca) 3 ]3~ by ferricytochrome c173 and by *- and "-[RuII(phen) 3 ]2`174 have been described.In the latter study *V8 values for identical diastereomeric processes in water and methanol are of opposite sign believed to be associated with di§erences in solvation of the encounter complexes. Inner-sphere electron and atom transfer Several inner-sphere reactions associated with outer-sphere processes have already been discussed.156–160,172 One-electron oxidation of [Fe(H 2 O) 6 ]2` by 546 N.Winterton [Cr(O 2 H)(H 2 O) 5 ]2` Mgiving [CrO(H 2 O) 5 ]2` and [Fe(OH)(H 2 O) 5 ]2`N is similar175 in rate to its oxidation by H 2 O 2 . Acid-catalysed oxygen-atom transfer (two-electron oxidation) of I~ by [Cr(O 2 H)(H 2 O) 5 ]2` and two trans-[Co(O 2 H)L(H 2 O)]2` complexes MO 2 H2`]3I~]3H`HI 3 ~]M3`]2H 2 O are more rapid than similar processes involving H 2 O 2 .This arises from pK! di§erences between co-ordinated M(H 2 O 2 )n` and H 3 O 2 `. The anion [Co(CN) 5 (O 2 H)]3~ reacts with L-methionine to give [Co(CN) 5 (L)]2~ (L\L-methionine-S-oxide).176 Other oxygen-atom-transfer processes have been studied mechanistically including transfers from V to S M[VVO(O 2 )L]n L\(quin) 2 n\3[; (H 2 O) 2 (pyca) 2 n\0; (H 2 O) 4 n\1] and [Co(en) 2 (SCH 2 CH 2 NH 2 )]2`N,177 from Ti to Ti M[TiIV(tpp)O] and [TiIII(oep)Cl] via two parallel pathways one associative and one involving Cl~ dissociationN,178 from S to Mo and N to Mo MMe 2 SO and py-N-O to [MoIVO(S 2 PR 2 -S,S@)L]N,179 Mo to P M[MoVIO 2 (S 2 PR 2 -S,S@)L] and PPh 3N,179 from Mo and W to N,180 and N and S to P (di§erent mechanisms for transfer of O from R 2 SO to PR 3 and from NH 2 OAc to PR 3 ),181 from Re to S M[ReVII(Me)O(O 2 ) 2 ] to thiophene via nucleophilic attack of thiophene S on a co-ordinated peroxy groupN,182 from Re to P M[ReVII(Me)O 3 ] and H 2 P(O)OHN,183 and from Cl N S and V to Re MClO 4 ~ py-N-O Me 2 SO and VO(aq)2` and [ReV(Me)O 2 ]N.183 Photoreduction of [ReVII(Me)O 3 ] by [Fe(CN) 6 ]4~ is believed184 to proceed via [Me(O) 3 Re(NC)Fe(CN) 5 ]4~.An unusualNtoO oxygenatom- transfer results in O 3 formation from reaction of [Fe(oep)(NO 2 )] with O 2 .185 Nitrogen monoxide gives NO 2 with [Fe(oep)(NO 2 )].186 The relationship between oxygen-transfer and 95Mo NMR chemical shifts for MoVIO 2 complexes has been investigated.187 Nitrogen-atom-transfer between [MoN(OR) 3 ] and [Mo(NRR@) 3 ] has been established using labelling techniques.188 Stereospecific NH transfer has been reported for the reaction of [Ta(Me)Cp 2 ] with aziridines.189 Molybdenum to P sulfur-transfer kinetics have also been described.190 Rates for the halide-transfer self-exchange processes [M(X)Cp 2 ]`/[MCp 2 ] (M\Ru or Os; X\Cl or Br) in MeCN span191 a range of 106 with I[Br[Cl; Ru[Os.Oxidation of indium(I) [from indium amalgam and Ag(CF 3 SO 3 ) in acetonitrile followed by dilution with water] by [CoIIIX(NH 3 ) 5 ]2` (X\Cl Br or I) occurs stepwise.192,193 The first slow step giving InII is inner-sphere. Halide-atomtransfer between the organometallic free radical [Re(CO) 5 ] and [CuX(L13)]` and [MX(L14)]` (M\Cu or Ni; X\Cl Br or I) are all di§usion-controlled processes.194 Miscellaneous redox reactions The cation [Mn 2 O 2 (phen) 4 ]3` forms an outer-sphere adduct with NO 2 ~ which is reduced by one-electron steps to MnII.195 However the related adduct [Mn 2 O 2 (phen) 3 (H 2 O) 2 ]3` is unreactive.Reductive cleavage by catechol of [MFe(dmgBPh 2 ) 2NO] in the presence of ligands L proceeds via monoligated intermediates. 196 Metal catalysis of the reduction of [NiIV(L15) 2 ] by NH 2 OH197 or thiols198 involves FeII 197 (via acid-dependent and -independent pathways) or CuI 198 reduction of NiIV to NiIII. Nitrogen monoxide reduction of [Cu(Me 2 phen) 2 ]2` in methanol involves199 inner-sphere adduct formation reaction with solvent to give an N-co-ordinated nitrite complex and dissociation to [Cu(Me 2 phen) 2 ]` MeONO and H`. Reductive nitrosylation of ferrihaemoproteins200,201 and related studies of NO formation from CuI–NO 2 complexes202,203 are reported.Reduction of CuII by excess thiourea obeys204 the rate law:[d[CuII]/dt\k@[CuII]2[tu]7 involving 547 Inorganic mechanisms a rate-determining bimolecular decomposition of two complexed CuII species. Copper( II)-catalysed reduction of [IrCl 6 ]2~ by tu involves rate-determining oxidation of [Cu(tu) 5 ]2` by [IrCl 6 ]2~. Ammine deprotonation is believed to be rate-determining205 in the disproportionation of the N7-co-ordinated guanine nucleoside complexes trans-[RuIII(L)(py)(NH 3 ) 4 ]3`. Pulse radiolysis studies of the formate reduction of Hehba-bu§ered HCrO 4 ~ involving the reduction of CrVI to CrIII via unstable CrV and CrIV show206 that CrV is not generated directly from CrVI arising instead207 from oxidation of CrIV by CrVI via hydride shifts from HCO 2 ~ to Cr––O.Other reductions of CrV 208,209 and CrVI 210–212 have been described. Reduction of trans-[Pt(CN) 4 X 2 ]2~ (X\Cl or Br) by thiols is first order in complex and [RSH]505 via parallel processes involving halide-bridged electron transfer for which RS~ is[105 times more reactive than RSH. The basicity of RS~ is the predominant determinant of reactivity.213 Electronic and steric e§ects of phosphine ligands have been quantified for the redox stability of trans-[Ru(NO 2 )(PR 3 )(PR@3 )(terpy)]`.214 Mechanisms have been described for the oxidation of SCN~ by CeIV 215 or alkaline diperiodatocuprate(III),216 phosphite by diperiodatonickelate(IV),217 iodide by diperiodatoargentate(III),218 of the cornershared double-cube [Mo 6 MS 8 (H 2 O) 18 ]8` (M\Pb or Bi) by [Fe(H 2 O) 6 ]3`,219,220 OsVIII catalysis of SbIII oxidation by [Fe(CN) 6 ]3~,221 chromate reduction by [Fe(H 2 O) 6 ]2`,222 TlI 223,224 and hypophosphite225 oxidation by permanganate and VV,226,227 oxidation of fluoro complexes of UIV by XeF 2 ,228 HSO 3 ~ oxidation by [Fe(bipy) 3 ]3`,229 oxidation of [Ru(NH 3 ) 5 (L)]2` (L\butyl sulfoxide) by cis- [Ru(NH 3 ) 4Mpy-4-C(O)NH 2N2 ]3`,230 of S 2 O 3 2~ 231 and other substrates232,233 by [FeVIO 4 ]2~ formation of [RhI(bipy) 2 ]` from [RhIII(ox)(bipy) 2 ]`,234 and oxidation of CrIII by N-bromosuccinimide.235,236 The cation [Pt([14]aneN 4 )]2` reacts with OH· to give intermediates believed to contain PtIII 237 whereas [Pd([14]aneN 4 )]2` is reported to su§er ligand attack.238 Reaction of [CoIII(NH 3 ) 5 (X-pyO)]3` with Me 2 C·OH involves239 reduction of the co-ordinated pyridine-N-oxide followed by intramolecular electron transfer to CoIII.Reactions of oxygen-containing oxidants and reductants Reaction of the superoxo complexes [CrIII(16O 2 )(H 2 O) 5 ]2` and [CrIII(18O 2 )- (H 2 O) 5 ]2` at pH1 produces240 no detectable 16O18O ruling out the process 2[Cr(O 2 )(H 2 O) 5 ]2`H2[CrO(H 2 O) 5 ]2`]O 2 . Homolytic processes are favoured for the initial stages of decomposition in acid whereas at higher pH disproportionation to [CrIII(O 2 H)(H 2 O) 5 ]2` [Cr(H 2 O) 6 ]3` and O 2 occurs via initial formation of O 2 ·~. The cation [Cr(O 2 )([14]aneN 4 )(H 2 O)]2` reacts with [Fe(H 2 O) 6 ]2` in a three-stage 548 N.Winterton process Cr(O 2 )2`]Fe(aq)2`]H`HCr(O 2 H)2`]Fe(aq)3` followed by the formation and decay of a CrV species suggested241 to be [CrO([14]aneN 4 )]3`.Oxygenbinding kinetics242 for five-co-ordinate CoII complexes of lacunar-cyclidene ligands L16 are uncomplicated by solvent dissociation (since ligand or solvent binding in the sixth position is prevented). Variation of oxygen-binding rate with bridge length (optimum at C 6 ) is associated with entropic e§ects arising from constraints on O 2 entrance into the cavity. Dioxygen dissociation was solvent independent but sensitive to the axial base. Reaction with O 2 of a series of alkoxo-bridged diiron complexes di§ering in accessibility to the diiron(II) centre display kinetics which are first order in the two reacting species when access is unimpeded whereas impeded access leads to a reduced reaction order in [O 2 ].243 In the latter case a two-step mechanism was proposed.Photoexcited [PtII(dpdt)(4,4@-Bu 2 bipy)] is deactivated with 3O 2 yielding 1O 2 which then dehydrogenates the thiolate ligand.244 Dioxygen also results245 from the reaction of [MnIII 2 (2-OHsalpn) 2 ] with excess Bu5OOH. Reaction of dioxygen with [Cu(L17)]` at [78 °C yields a trans-1,2-diperoxocopper(II) complex [(L17Cu) 2 - O 2 ]2`. This decays in MeCN with first-order kinetics by converting a pyCH 2 ligand moiety to pyC(O). Isotope e§ects and activation parameters point246 to an isomerisation to k-g2 g2-bonding of the peroxo group prior to four-electron oxidation of the ligand. Kinetics of the interconversion247 of trans-1,2-diperoxo and (the more stable) k-g2 g2-peroxo-bonding modes in [Cu 2 (O 2 )(L18)]2` have been studied in detail.Interconversion does not occur directly but involves either partial or complete O 2 dissociation. Using related ligands Karlin and co-workers248 conclude that oxygenation of the pyCH 2 -moiety in L19 does not proceed via oxygen activation. Studies of the reaction with O 2 of a dicopper(I) complex with the oxidised form of L19 yields a gem-diolate which suggests that both oxygen atoms are used in ligand oxidation via oxodicopper(II) rather than via peroxodicopper(II) complexes. Interconversions betweenM 2 (k-g2 g2-O 2 ) andM 2 (k-O) 2 moieties are thought249 to be generally relevant to both metal-complexed catalysed reductive cleavage of O 2 and water oxidation to O 2 . Reaction of O 2 with [VO(ma) 2 (H 2 O)] involves initial attack250 of O 2 on VIV leading ultimately to cis-[VV(O) 2 (ma) 2 ]~.A correlation is found (o\[1.1) between the rate constant for O–O cleavage in [Fe 2 (k1,2 -O 2 )(N-Et-hptb)(O 2 CC 6 H 4 X)]2` and the Hammett p value for X showing the e§ect of electron-donating substituents.251 Peroxodiphosphate formation by CuII-catalysed oxidation of the phosphinate ion by oxygen has a maximum rate at pH3.5.252 Kinetics of the reaction between H 2 O 2 and AuIII in aqueous HCl display253 an inverse dependence on [H`] and [Cl~]. Thiocyanate inhibition of vanadium bromoperoxidase-catalysed bromide oxidation by H 2 O 2 is the result of competitive SCN~ oxidation.254 Kinetics and mechanisms have been studied for the oxidations by H 2 O 2 of [Fe(CN) 6 ]4~,255,256 [CrIII(NCS) 2 (tn) 2 ]`,257 [WV(CN) 8 ]3~,258,259 [Ru(Me 2 phen) 2 (H 2 O) 2 ]2`260 copper(II)–Schi§ base complexes261 of Br~ catalysed by [VO(O 2 )(Hheida)]~ and [VO(O 2 )(bpg)]262 and of transient formation263 (see also ref.264) from H 2 O 2 and Mn-reconstituted horseradish peroxidase. The question of OH· formation from H 2 O 2 and edta hedta and tcma complexes of FeII has been reviewed.265 The mechanisms of a series of H 2 O 2 oxidations catalysed by [Re(Me)O 3 ] have been described.182,183,266 The complex [Re(Me)O 3 ] catalyses267 the heterolytic decomposition of cumyl hydroperoxide. The radical OH· is formed when photoexcited UO 2 2` 268 or [Fe(ox) 3 ]3~ 269 is quenched by H 2 O 2 . tert-Butyl hydroperoxide reacts with [FeIII(L20)]2` to give270 549 Inorganic mechanisms N N N N N N (CH2) n N N N N N N N N N N (CH2)4 L16 L17 L18 N N N N N N N N N NH2 N O N NH Br H H L19 L20 [FeIII(L20)(OOBu5)]` which su§ers O–O homolysis to generate [FeIII(L20)O·]`.Similar Fe–g1-hydroperoxide FeV––O and related species are implicated in a range of organic oxygenations.253,271–274 Oxidation of [Fe(CN) 6 ]4~ by RO 2 · is an outersphere process275 in contrast to he inner-sphere processes involving RO 2 · and [Fe(H 2 O) 6 ]2`. The cation [FeIII(O 2 R)(H 2 O) 5 ]2` then decomposes by H`- and Fe2`- catalysed processes. Catalysis of superoxide dismutation 2 O 2 ·~]2 H` HO 2 ]H 2 O 2 has been reported for MnII-macrocyclic,276,277 dimanganese278 and CuII 279 complexes. The absence of pH dependence for copper–zinc superoxide dismutase suggests280 that Zn and the histidyl imidazolate residue that bridges Cu and Zn facilitate peroxide dissociation.Water-soluble FeIII–porphyrin complexes catalyse isomerisation of peroxynitrite to nitrate at the expense of processes leading to nitrite probably281 via FeIV––Ointermediates formed by reversible homolysis of co-ordinated ONOO~. Oxidation of CoII to CoIII by HSO 5 ~ in the presence of [MoO 4 ]2~ occurs282 via one-electron oxidation of cobalt after molybdate loss from a heteropolyanion [H 6 CoIIMo 6 O 24 ]4~ finally yielding [H 4 CoIII 2 Mo 10 O 38 ]6~. TheHSO 5 ~ oxidation of MnIII–porphyrin complexes proceeds via oxygen-atom transfer.283 Kinetic studies of the oxidation of [(NC) 5 FeII(im)RuIII(NH 3 ) 5 ]~ and [(NC) 5 FeII(im)RuII(NH 3 ) 5 ]2~ by S 2 O 8 2~ reveal284 the role of RuII in assisting the oxidation of the FeII centre. The di§erence in reactivity with S 2 O 8 2~ (after correction for di§erences in reduction 550 N.Winterton potentials and charge) between [FeII(CN) 5 L]3~ and the corresponding [RuII(NH 3 ) 5 L]2` are ascribed285 to hydrogen bonding between the peroxydisulfate and the ammines in the latter case and non-adiabaticity in the former.Oxidation by S 2 O 8 2~ of CuII macrocyclic complexes286 and decomposition of H 2 O 2 and H 2 O and O 2 catalysed by a dinuclear FeIII complex287 and MnII complexes288–291 have also been described. The redox chemistry of sulfur is discussed in the next section. Non-metal redox reactions Kinetic studies of the acid-catalysed reduction of nitrite by Me 3 N·BH 3 (giving H 2 and N 2 O) are consistent292 with rate-determining attack of H~ on H 2 NO 2 ` (or NO`) to giveHNOas a reactive intermediate. Iodination of R 3 N·BH 3 has also been studied.293 Carbon dioxide is reduced to CO and formate with [Fe0(tpp)] (in the presence of Lewis294 or weak Brønsted295 acids) and other metal catalysts.296,297 The radical H· gives either H 2 and ·N 2 H 3 with N 2 H 4 in aqueous acid298 (via hydrogen-atom abstraction) or ·NH 2 and NH 3 from N 2 H 5 ` (via addition-fragmentation).The constancy of the rate of decomposition of HOONO in solutions of varying viscosity argues against a free-radical process for this unimolecular reaction.299 Peroxynitrite oxidations of AsIII and SIV proceed via O-bridged precursor complexes for which reactions via H 2 AsO 3 ~ or HSO 3 ~ with HOONO dominate.300 The reactivity series with peroxynitrite SnII[SbIII[AsIII[SIV?PI[PIII reflects the relative accessibilities of the electron-rich sites of the reductants.No evidence has been found301 for the direct nitrosation of H 2 O 2 by ·NO a process which only takes place in the presence of O 2 . The rate law supports the intermediacy of ONOONO (or isomeric forms) the precursor to ·NO 2 andN 2 O 3 . The reaction of ·N 3 and ·NOin the presence of H 2 O 2 has also been studied.302 The kinetics of the oxidation of NH 2 OH by aqueous bromine,303 of the reaction between OBr~ and NH 3 or C(O)(NH 2 ) 2 ,304 and reactions of N-haloamines 305,306 -amino acids307 and -amides308 have been reported. The biological role of NO309 and the atmospheric role of nitrous acid310 have both been reviewed. The 105-fold acceleration by freezing311 of the reaction of nitrous acid with oxygen has been investigated in detail.312 The radical ·NO is thought313 to be an intermediate in theUVirradiation of alkaline 15NO 2 ~ in the presence of the aci-form of nitromethane CH 2 ––14NO 2 ~ since the adduct CH(14NO 2 )(15NO)2·~ is formed.Three studies314–316 have been reported of the mechanism of ozone decomposition in water. Photodissociation of aqueous O 3 ~ and its recombination have been studied317 on the picosecond time-scale. The kinetics of the HCO 2 H–H 2 O 2 318 and ox2~–H 2 O 2 319 reactions and the rates of the uncatalysed and 1O 2 -forming ketonecatalysed decomposition of peroxymonosulfuric acid320 have been reported. Formations of SO 4 2~ ad S 2 O 6 2~ from the photodecomposition of HSO 3 ~ arises321 from self-reaction of SO 3 ·~. In oxygen-saturated solution a short-chain process occurs leading to SO 4 2~ and S 2 O 8 2~ the latter arising from recombination of SO 5 ·~.Two other groups322,323 have studied these processes. The rate constant of the reaction of SO 5 ·~ and HO 2 · (possibly forming a tetroxide transient) has led to the suggestion324 that this process is a chain-terminating step in the chain-oxidation of SIV to SVI in cloud water.325,326 Kinetic data for the oxidation of SIV by H 2 O 2 in aqueous base have been interpreted in terms of two processes,327 SO 3 2~]H 2 O 2 and SO 3 2~]HO 2 ~. Additional industrially- and environmentally-relevant studies have been reported including the FeIII–FeII-complex catalysed H 2 S oxidation by air,328,329 bisulfite photooxida- 551 Inorganic mechanisms tion,330 metal-catalysed oxidation of SIV by oxygen331,332 and the involvement of HONH(SO 3 )~ and HON(SO 3 ) 2 2~ in such processes.332–335 Photochemical decomposition of ClO 2 in water involves Cl· loss from thermallyequilibrated ClOO.336 The reaction of ClO 2 ~ (generated in situ from ClO 2 and I~) with I~ is both substrate-inhibited and auto-catalytic involving subsequent ClO 2 ~–I 2 and HOI and HIO 2 disproportionation reactions.337 The reaction of BrO 3 ~ and I 2 of stoichiometry 2 BrO 3 ~]I 2H2 IO 3 ~and Br 2 involves an acid-dependent induction time associated with the sudden depletion of I 2 and formation of IBr as a transient.338 A 17-step mechanism is proposed.Bromine hydrolysis has been re-investigated,339 with the forward rate constant k& for Br 2 (aq)]H 2 OHHOBr]Br~]H` being 97 s~1 (25.0 °C k\0.50M) and for the reverse k" 1.6^1010M~2 s~1.Oxidations by ClO 2 ~340 and BrO 3 ~341,342 of sulfur-containing organic substrates have been studied in detail by Simoyi and co-workers. Rates of oxide-radical and hydratedelectron reactions with IO 3 ~ have been reported.343 Oscillating reactions and chemical chaos A special issue of Faraday Trans.344 on chemical instabilities was devoted to the work of Peter Gray. New means of developing chemical patterns from coupling di§usion and complex kinetics have been reviewed.345 Studies relevant to the chemistry of oxyhalogen–sulfur systems346 were discussed in the previous section.340–343 Further reports have appeared on various aspects of the Belousov–Zhabotinsky (B-Z) reaction.347–388 Studies have shown that oxygen shortens the duration of B-Z oscillations.347 The e§ect of light356–362 has been reported including light-induced quenching of oscillations358 and spiral waves.361 Temperature,365 stirring,366,367 dc electric368,369 and electric current370 e§ects perturbations by Ag`,350 the dependence of system dynamics on initial reagent concentrations,353,354 the development of di§erent modes of oscillatory behaviour from mixed substrates compared with those seen from single substrates,355 the identity of malonic-acid derived363 and other364 intermediates and stochastic resonance371,372 have all been examined.Magnetic resonance images have been obtained375 for the CeIII- and RuII-catalysed B-Z reactions but not for the FeII-catalysed reaction. Belousov–Zhabotinsky reactions have been carried out gel-376,377 or membrane-immobilised,378–381 cation-exchange resin bead-loaded382 and in water-in-octane reverse microemulsions.383 In the microemulsion the rate is ca.10-fold larger than in homogeneous aqueous solution. Rate depends on [NaBrO 3 ]2 in the microemulsion and on [NaBrO 3 ] in the homogeneous aqueous solution. The origins of the induction period for the bromate–ferroin clock reaction389,390 and Leisegang-precipitation phenomena from CoII and NH 4 OH in gelatin have been studied.391 Chinake and Simoyi392 have drawn attention to the environmental relevance of non-linear kinetic behaviour in sulfur-compound oxidation. Photoinduction and photoinhibition of oscillations are observed393 in the BrO 3 ~–HSO 3 ~–[Fe(CN) 6 ]4~ (BSF) system in a continuous-flow stirred tank reactor (CSTR). A mechanistic model for the BSF system is the subject of debate.394,395 Chemical instability in the BrO 3 ~–HSO 3 ~396 and the H 2 O 2 –HSO 3 ~397 flow systems has been studied.Swinney and co-workers398 have studied the IO 3 ~–HSO 3 ~–[Fe(CN) 6 ]4~ system in a thin-gel reactor. Chaotic temporal pH changes occur399 as a result of coupling of the CO 2 hydration equilibrium and slow CO 2 removal with theH 2 O 2 –HSO 3 ~–[Fe(CN) 6 ]4~ oscillatory system in a CSTR. pH 552 N.Winterton Oscillations are also seen in H 2 O 2 –400 or BrO 3 ~–401 HSO 3 ~–solid marbleflow systems. Intermediacy of HOS 2 O 3 ~ and its reaction with H 2 O 2 and S 2 O 3 2~ are proposed402 for the CuII-catalysed thiosulfate oxidation by H 2 O 2 which displays oscillatory behaviour in a flow system. Copper(II)-catalysed thiourea–H 2 O 2 and S 2 O 8 2~–SCN~ oscillators have also been reported.403,404 Oscillations without kinetic bistability have been observed405 in a [MnO 4 ]~–cyclic diketone system in a CSTR.Other permanganate-based oscillators have been reported.406 Travelling waves in the IO 3 ~–HSO 3 ~ system,407 models of the Briggs–Rauscher (malonic acid–IO 3 ~–H 2 O 2 )408,409 and Bray–Liebhafsky (I 2 –IO 3 ~–H 2 O 2 )410 systems have been reported. Other related studies have appeared.411–426 3 Substitution Jordan427 has proposed a new model which analyses the e§ect of leaving groups Y on *V8 for dissociative substitution in ML 5 Y. Apparently contradictory views of the origins of non-linear temperature behaviour of product ratios in processes showing selectivity can be reconciled428 by viewing the phenomenon as arising from the non-linear change with temperature of the ratio of the concentrations of two intermediates.Six-co-ordination Rates of hexameric and tetrameric CrIII–aqua–hydroxo complex formation from [Cr 3 (k-OH) 4 (OH)(H 2 O) 9 ]4` and [Cr(OH)(H 2 O) 5 ]2` and of the cleavage of the trimer and tetramer have been investigated by Drljaca and Spiccia.429,430 Interconversion and cleavage of singly- and doubly-bridged ions [(H 2 O) 5 Rh(k- OH)Cr(H 2 O) 5 ]5` and [(H 2 O) 4 Rh(k-OH) 2 Cr(H 2 O)) 4 ]4` are CrIII-centred substitution processes.431 Hydrolysis of [(tmpa)Cr(k-O)(k-L)Cr(tmpa)]2` (L\sulfate or molybdate) displays432 a fast water-induced fission of the ligand bridge (facilitated by cis-labilising k-O) followed by L loss from either [CrL(tmpa)(k-O)Cr(tmpa)(H 2 O)]2` or its conjugate base. Anation of [Cr(OH)(H 2 O) 5 ]2` by [Mo(CN) 8 ]4~ is an associative- interchange process.433 Complexation kinetics have been reported for [Cr(H 2 O) 6 ]3` and L-histidine,434,435 for [Rh(OH)(H 2 O) 5 ]2` and DL-methionine436 or pyridine-2-aldoxime,437 for [Ni(H 2 O) 6 ]2` and 8-hydroxyquinoline,438 for [Co(H 2 O) 6 ]2` with folic acid439 or [Co(His)(NH 3 ) 5 ]2`,440 for [Al(H 2 O) 6 ]3` and amino acids or H 3 nta441 or 7-substituted 8-hydroxyquinoline-5-sulfonates.442,443 Formation (from molybdate and the corresponding aquo-complex) and hydrolysis of [Co(OMoO 3 )(NH 3 ) 5 ]` involve MoVI–O not CoIII–O cleavage.444 The *V8 measurements (with other data) for SCN~ substitution into [Cr(Hedta)(H 2 O)] and [Cr(edta)] ~ support445 the suggestion that the lability of CrIII–edta complexes is associated with activation by transient chelation of the pendant arm of the pentadentate edta.Waterexchange in the ion pair p-[Co(NH 3 )(H 2 O)(tren)]3`·Cl~ occurs as readily as anation by Cl~ suggesting446 that entry of Cl~ andH 2 O from positions adjacent to the group trans to tren NH 2 (the p site) a§ects re-entry of the leaving water. Anation of (a,b)- [Co(OH)(tetren)]2` by S 2 O 3 2~ gives both S- and O-bonded [Co(S 2 O 3 )(tetren)]` via an internal conjugate-base process447 involving [Co(Htetren)(H 2 O)]2`. Substitution of H 2 O in [OsIII(NH 3 ) 5 (H 2 O)]3` by [FeII(CN) 6 ]4~ giving [OsIV(NH 2 )(NH 3 ) 4 553 Inorganic mechanisms MFeII(CN) 6 HN] involves448 oxidation of OsIII to the more labile OsIV by residual oxygen. Binuclear-complex formation between trans-[CoIII(salen)(H 2 O) 2 ]` and [Fe(CN) 6 ]3~ proceeds by an I$ mechanism.449 Formation of the analogous intermediate from [Fe(CN) 6 ]4~ precedes FeII-to-CoIII electron transfer.Nucleophilic substitution of water has been studied for [Cr(bipy)(H 2 O) 4 ]3` and azide ion,450 for [Re(CN) 4 (NO)(H 2 O)]2~ and SCN~ N 3 ~ and tu,451 for trans-[Ru(NH 3 ) 4 L(H 2 O)]2` (L\EPh 3 ; E\P As or Sb) and imidazole (by an I$ mechanism),452 for cis- [Ru(NH 3 ) 4 (Him)(H 2 O)]2` or cis-[Ru(bipy) 2 (Him)(H 2 O)]2` and imidazole,453 for [RuIII(edta)(H 2 O)]~ and cysteine,454 for [RuIII(tpps)(H 2 O) 2 ]3~ and SCN~455 (by a D mechanism) for [RuII(tpps)(CO)(H 2 O)]4~ and CN~,456 for [Cr 3 O(OAc) 6 (H 2 O) 3 ]` and urea,457 and at the tetrahedral Ni of [Mo 3 NiS 4 (H 2 O) 10 ]4` by tppts3~ Br~ I~ and SCN~ (by an I$ mechanism).458 Substitution of H 2 O by SCN~ at Mo is a much slower process.Transformation of the edge-linked double-cube [MMo 3 PdS 4 (H 2 O) 9N2 ]9` to [Mo 3 (PdL)S 4 (H 2 O) 9 ]4` by L\Cl~ Br~ or SCN~ is rapid459 and is followed by substitution of non-identical waters on each Mo. For L\SCN~ a slow isomerisation of Pd–NCS to Pd–SCN is also observed. The replacement of water by HCO 3 ~ in a series of polyvanadate ions such as [VIV 2 O 3 (OH) 3 (H 2 O) 5 ]~ has also been reported.460 Stereoselective ternary complexation of CuII with (S)-amino acid amides and (R)- or (S)-histidine and (R)- or (S)-tyrosine has been described.461 The dynamics of ternarycomplex formation between [Zn(nta)(H 2 O)]~ and bipy and Me 2 bipy,462 between [NiII(ada)(H 2 O)]~ with bipy and phen,463 between [MoIVO 2 (CN) 4 ]4~ and bipy,464 and between [FeIII(nta)(H 2 O) 2 ] and bipy465 have also been reported.From studies of the acid-catalysed aquation of [Co(O 2 H)(CN) 5 ]3~ the relative a¶nity of CoIII for a series of oxygen donors is found176 to be HO 2 ~[H 2 O[H 2 O 2 . Spontaneous acid- and base-catalysed aquation of the neutral complexes trans(O)- [Co(taud)X] (X\Cl~ Br~ or NO 2 ~) has been reported.466 Square-pyramidal and trigonal-bipyramidal intermediates produced in acid- and base-hydrolysis of CoIII complexes of the type trans-[CoCl 2 (L–L) 2 ]` have been studied by molecular-orbital and molecular-mechanics methods.467 Acid-catalysed hydrolysis of [Co(CO 3 )(Him) 4 ]` involves a rapid protonation rate-determining carbonate-chelate ring-opening and subsequent rapid loss of HCO 3 ~.468 The Hg2`-assisted removal of Cl~ from trans-[CrCl 2 (NH 2 R) 4 ]` (R\Et Pr or Bu) to give trans- [Cr(NH 2 R) 4 (H 2 O) 2 ]3` proceeds469 in two well separated interchange processes.The kinetics of dissociation of Tiron (\H 2 L) complexes of FeIII point to470 parallel processes involving aqua– and hydroxo–FeIII–L2~ and –LH~ complexes. Urea dissociation from [Cr 3 O(OAc) 6 (urea) 3 ]` is strongly labilised457 compared with [Cr(urea) 6 ]3`. Methanol may be replaced in [CoIII(L)(MeO)(MeOH)] (L\PPIX dimethyl ester) by a series of substituted pyridines in a dissociative process,471 with the electron-donatingMeO~ favouring entry of the least basic pyridine.A plot of ln k0"4 vs. pK! exhibits a minimum related to changes in pyridine n- and p-bonding in the transition state. Photolysis of [FeII(PPIX)(CO)] in dmso yields the same five-coordinate transient seen upon photolysis of [FeII(PPIX)(dmso) 2 ] namely [FeII(PPIX)(dmso)].This decays by dmso co-ordination followed by substitution of bound dmso by CO.472 Structural changes occurring on the nanosecond time-scale which result from the photolysis of the myoglobin–CO complex reveal that geminate CO rebinding is more important in the crystalline solid473 than in solution. Time- 554 N.Winterton resolved UV circular dichroism,474 molecular dynamics475 and other476 simulations and IR spectroscopy477 have been applied to similar systems. The kinetics of azide binding to ferricytochrome c both wild-type and a series of site-specific mutated variants reveal478 a two-step reversible process with saturation. Racemisation kinetics have been studied for fac-"-(])(546)- and fac-*-([)(546)- [Cr(Val) 3 ] in dmf in which "-(])(546)- and *-([)(546)-[Cr(Val) 2 (N-Val)(dmf)] are also formed.479 Isomerisation and racemisation (and other conformational and con- figurational changes) for [CrM(RO) 2 bipydoN3 ]3` in aqueous solution are all intramolecular processes.480 Related studies on [M(S 2 C 2 R1R2) 3 ] (M\Mo or W)481 and [Ga(L21) 3 ]3`482 have also been reported.Base hydrolysis Both the steric course and rate of the base hydrolysis for trans-[CoX(NH 3 ) 4 (NH 2 - Me)]2` (X\Cl Br or NO 3 ) indicative483 of a dissociative conjugate-base mechanism di§er markedly from those of [CoX(NH 3 ) 5 ]2`. Variation with temperature of *V8 for the base hydrolysis of trans-[CoCl 2 ([14]aneN 4 )]` is ascribed484 to a change from rate-determining Cl~ release to rate-determining conjugate-base formation.Base-hydrolysis of trans(O)-[Co(taud)X] (X\Cl~ Br~ or NO 2 ~)466 of cis- [CoX(en) 2 L]2` (L\NH 3 or amine; X\Cl or Br)485 of a,b-anti-[CrCl(picdien)] 2`,486 of cis-[Co(salicylato)(en) 2 (NH 2 R)]2` (R\H Me or Et)487 and of cis-[CoCl(b- Ala)(en) 2 ]2`488 have been reported. Gillard and co-workers489 argue that the higher rate of base hydrolysis for [Pt([1H 8 ]bipy) 2 ]2` compared with [Pt([2H 8 ]bipy) 2 ]2` supports OH~ attack at ligand not metal. Calculation suggests467 that the squarepyramidal and trigonal-bipyrimidal intermediates produced in base-hydrolysis of CoIII complexes (neglecting solvation e§ects) are much lower in energy than corresponding intermediates formed in acid hydrolysis. Four-co-ordination Rate constants k for complex formation between [Pd(H 2 O) 4 ]2` and a series of thioethers follow trends similar to those for other Pd complexes,490 viz.log k\c]a&p*]bh the latter representing intrinsic electronic and steric terms respectively. A trigonal-bipyramidal transition state formed491 when acetic or propionic acid and [Pd(H 2 O) 4 ]2` react to give [Pd(O 2 CR)(H 2 O) 3 ]` is stabilised by hydrogen bonding between the entering RCO 2 H and the departing H 2 O. The claim that the lability of the aqua ligand in [PtIIMC 6 H 3 R(CH 2 NMe 2 )N(pySO 3 -3)(H 2 O)] 2 is associated with the Pt–C p bond of a cyclometallated ligand has been criticised492 and defended.493 Large negative *V8 and *S8 for ligand substitution in the related compound [PtMC 6 H 2 R(CH 2 NMe 2 ) 2 -2,6N(H 2 O)]` 3 show494H 2 O substitution to be associative.Direct isomerism between trans- and cis-[Pt(CH 3 )Cl(dmso) 2 ] occurs solely by a water-catalysed process.495 All configurational isomers of [Pt(edda-N,N@)] aquate to give tridentate edda-N,N@ with a pendant glycinato residue. The related [Pt(edda-N,N)] is inert under the same conditions.496 The anion [Pt(nta)Cl]2~ has a pendant glycinato residue which is believed497 to be weakly axially associated. The kinetics of complex formation betweenM2` (M\Ni Co or Cu) and the b-diketone Hamac reveal498 two processes involving reactions of enolate and the enol tautomer only. Dissociation of [M(amac)]` has also been studied. Replacement of ClO 4 ~ by 555 Inorganic mechanisms O NH NH N R Pt N R Pt N N OH OH O + L21 2 3 – O3S OH2 OH2 nitriles in [Ir(ClO 4 )(CO)(PPh 3 ) 2 ] is an associative process.499 Initial rapid complex formation between SCN~ and trans-[AuIII(CN) 2 X 2 ]2~ (X\Cl or Br) gives [Au(CN) 2 X(SCN)]2~ followed by slower intermolecular outer-sphere reduction by SCN~ to [AuI(CN) 2 ]~.500 Substitution of Cl~ by substituted benzenethiolates RS~ in [Cl 2 FeS 2 VS 2 FeCl 2 ]3~ to give [(RS)ClFeS 2 VS 2 FeCl(SR)]3~ proceeds by noncatalysed dissociative and acid-catalysed associative processes.501 Studies of the mechanisms of reactions of Fe–S clusters have been reviewed.502 Mechanistic studies of PtII 503–509 PdII510 and other metal511 co-ordination complexes relevant to their anti-tumour activity,512–514 have been reviewed.trans Analogue507 of the active species from the prototypical anti-tumour complex cisplatin,503 trans-[PtCl(NH 3 ) 2 (H 2 O)]`515 or trans-[Pt(NH 3 ) 2 (H 2 O) 2 ]2`516 react with inosine or 1-methylinosine by displacement ofH 2 Orather thanOH~ with theN7-co-ordination site being preferred over the N1 site.The cation trans-[Pt(NH 3 ) 2 (H 2 O) 2 ]2` is 7–8-fold more reactive than trans- [Pt(OH)(NH 3 ) 2 (H 2 O)]`. Reaction of inosine 5@-INP or 5@-GMP with [PdCl 2 (R 2 NCH 2 CH 2 NR 2 )] (R\Me or Et)517 also reveals preference for N7-bonding in acid and N1-bonding in base. Both bis-chelated [Pt(Me 2 NCH 2 CH 2 PPh 2 - N,P) 2 ]2` and its ring-opened form [PtCl(Me 2 NCH 2 CH 2 PPh 2 -N,P)(Me 2 NCH 2 CH 2 PPh 2 -P)]` which exist in a [Cl~]-dependent equilibrium react with 5@-GMP to give diastereomeric forms of cis-[Pt(Me 2 NCH 2 CH 2 PPh 2 -N,P)(Me 2 NHCH 2 CH 2 PPh 2 - P)(5@-GMP-N7)]3`.518 Interestingly 5@-GMP may readily be displaced by Cl~ but not by the sulfur-donor N-acetyl-L-methionine.Nucleophilic displacements by I~ at PtII in [Pt(OAc) 2 (R 2 NCH 2 CH 2 NR 2 )] (R\4-fluorophenyl) occur predominantly via the aqua and diaqua intermediates.519 Further studies have appeared on the reactions of single-520–522 and double-stranded521,523 oligonucleotides with cis- [Pt(NH 3 ) 2 (H 2 O) 2 ]2`520,523 and -[Pt(NH 3 ) 3 (H 2 O)]2`520,522,523 or cis- [PtCl 2 (NH 3 ) 2 ] and cis-[PtCl(NH 3 ) 2 (H 2 O)]`521 as analogues of DNA platination at guanine residues in intrastrand cross-links. Chottard and co-workers520,523 show that rates of platination increase slightly with oligonucleotide chain-length for both cis- [Pt(NH 3 ) 2 (H 2 O) 2 ]2` and -[Pt(NH 3 ) 3 (H 2 O)]2` with rate constants for reaction at each guanine in a single-strand nucleotide reported.520 Chelation processes involving cis-[Pt(NH 3 ) 2 (H 2 O) 2 ]2` were also described.The double-stranded oligonucleotide d(TTGGCCAA) 2 reacts to give mono adducts with both cis-[Pt(NH 2 ) 2 (H 2 O) 2 ]2` and [Pt(NH 3 ) 3 (H 2 O)]2` more rapidly than the single-stranded d(CTGGCTCA),523 asso- 556 N.Winterton ciated with increased rate of reaction at the 5@-guanine. The cation cis-[Pt- (NH 3 ) 2 (H 2 O) 2 ]2` reacts with both more rapidly than does [Pt(NH 3 ) 3 (H 2 O)]2`. The latter not only gives the two singly-platinated oligonucleotides with d(CTGG) but also doubly-platinated species both N7-bound at adjacent guanines.522 Rate studies highlight the importance of ionic strength e§ects.Chelation of the monoadduct from cis-[Pt(NH 3 ) 2 (H 2 O) 2 ]2` is slower for the double-stranded compared with the singlestranded oligonucleotide. Sadler and co-workers521 report related studies on cisplatin with cis-[PtCl(NH 3 ) 2 (H 2 O)]` forming two monofunctional adducts with both singleand double-stranded oligonucleotides with one of the two guanines in the two substrates reacting faster than the other. Ring closure to form the complex in which both guanines in the double-stranded substrate are bonded occurs ca. 10-fold faster for one singly-bonded adduct compared with the other an observation confirmed by Chottard and co-workers.523 Dinuclear intrastrand adduct formation follows initial rapid complexation at either 5@Gor 3@Gguanines in a single-strand oligonucleotide on reaction with [Mtrans-PtCl(NH 3 ) 2NMk-NH 2 (CH 2 )nNH 2N]2` (n\2–6) with ring closure for n\4–6 being faster than for n\2–3.524 Thioethers such as L-methionine are much more reactive than thiols in reaction525 with [Pt(cbdca-O,O@)(NH 3 ) 2 ] (carboplatin) giving stable ring-opened species cis-[Pt(cbdca-O)(L-HMet-S)(NH 3 ) 2 ].The latter ring-closes to [Pt(L-Met-N,S)(NH 3 ) 2 ] very slowly. Related reactions of [PtCl(en)(MeCO-Met-S)]` with nucleotides have also been studied.526 Palladiumcomplex promoted peptide cleavage is discussed below. Relevant structural studies are noted.527–533 Activation parameters for olefin replacement in [Pd(2-pyCH–– NMe)(g2-olefin)] in CHCl 3 are consistent with an associative mechanism.534 Formation of k-amido complexes from [MPd(Ph)(PPh 3 )(k-OH)N2 ] and Bu4NH 2 shows535 second-order dependence on [Bu4NH 2 ].Two theoretical approaches to modelling the substitution process [Pd(CH 3 )(NH––CHCO 2 )(PH 3 )]]COH[Pd(CH 3 )(NH––CHCO 2 )(CO)] ]PH 3 have been compared.536 The solvent e§ects on the kinetics of cyclopalladation in [Pd(Bn 2 Medptn)(solv)]2` (rate for solv\py?dmso[dmf?MeCN) are inconsistent537 with a mechanism involving a three-co-ordinate intermediate. A fiveco- ordinate intermediate is proposed538 for the intramolecular exchange associated with the fluxionality of [PtX 2 (triphos)] (X\CN or SCN). Five- seven- and higher-co-ordination The mechanism by which CrV complexes such as five-co-ordinate [CrO(ehba) 2 ]~ cleave DNA74 has been explored.539 Reaction with pyrophosphate gives distorted trigonal-bipyramidal [CrO(ehba)(H 2 P 2 O 7 )]~ and square-pyramidal [CrO(H 2 P 2 - O 7 ) 2 ]~ in contrast to isomeric forms of six-co-ordinate [CrO(ehba) 2 (H 2 PO 4 )]2~ with phosphate.Large negative *V8 and *S8 for the substitution of X by P(OMe) 3 in trigonal-bipyramidal [PdX(pp 3 )]` are consistent540 with an associative mechanism. Very rapid initial addition of CO to [NiI([14]aneN 4 )]` to give [NiI(CO)([14]- aneN 4 )]` is followed541 by a slower first-order process believed to involve a ligand isomerisation. The unusual structure of [CuII(tren)(dzf)]2` with one Cu–N bond to dzf being much longer than the other has prompted the suggestion542 that the compound models the associative complex for ligand substitution in five-co-ordinate [Cu(tren)L]2`. Origins of the di§erence in photodissociation of NO from [MII(tpp)(NO)] (M\Fe or Co) have been explored.543 557 Inorganic mechanisms The exchange between free F~ or HF and [UO 2 Fn(H 2 O) 5~n]2~n faster for n\4 and 5 compared with other values of n is dominated by two pathways one involving exchange between two uranyl complexes probably fluoride-bridged the other involving exchange between free F~ and a uranyl complex.544 The mechanism involves ligand-promoted rate-determining water dissociation.Substitution of arsenazo III by edta or dtpa on EuIII involves a fast step and then a slow step the latter believed545 to involve acid-catalysed dechelation of arsenazo III. Other relevant dynamic546 and structural547 studies of lanthanide complexes have been reported and reviewed.548 The hydrolytic equilibrium between [Zr 8 Cl 12 (OH) 20 (H 2 O) 24 ] and [Zr 4 Cl 6 (OH) 8 (H 2 O) 16 ]6` has also been studied.549 Ligand exchange The use of variable-pressure NMR spectroscopy to study solvent exchange on transitional- metal ions has been reviewed.550 Water exchange on [Ir(H 2 O) 6 ]3` takes place via an I! mechanism and on [Ir(OH)(H 2 O) 5 ]2` via an I process with the observed551 overall rate constant taking the form k\k 1 ]k 2 /[H`].Exchange is extremely slow with a residence time corresponding to k 1 298 of ca. 300 years. Separate pathways for water exchange at sites cis and trans to the bridgingOH are reported552 from 18Oand 17O studies of [(H 2 O) 4 Rh(k-OH) 2 Rh(H 2 O) 4 ]4`. The bridging OH is inert to exchange. Theoretical approaches have been used to model transition states and intermediates for water exchange in first-row transition-metal hexaaqua complexes,553 in [Pd(H 2 O) 4 ]2` [Pt(H 2 O) 4 ]2` and trans-[PtCl 2 (H 2 O) 2 ],554 in Na(aq)`,555 in [M(H 2 O)(bipy)Cp@]2` (M\Co Rh or Ir)556 and the second co-ordination sphere of [Cr(H 2 O) 6 ]3`.557 An experimental value for the exchange rate in the latter leads to a lifetime of 128 ps for one H 2 O compared with 144 ps from a simulation.Experimental and theoretical approaches have been compared for water exchange on [Ln(H 2 O)n]3` (Ln\Nd Sm or Yb).558 An I$ mechanism via an eight-co-ordinate square-antiprism transition-state is preferred for Ln\Nd n\9. For Ln\Yb n\8 an I! process via a tricapped trigonal-prism transition-state is proposed. The need for rapid water exchange in e§ective Magnetic Resonance Imaging contrast-agents motivates a continuing focus on exchange processes involving lanthanide particularly GdIII complexes.559–562 Water exchange on the GdIII complex with [L22]6~ is shown to have a *V8 of ]0.5^0.2 cm3 mol~1 pointing to an interchange process.560 The *V8 values for water exchange,]3.1^0.2 to]7.7^0.5 cm3 mol~1 in related dendrimer complexes are consistent with I$ processes.559 Water exchange in [Mo 3 NiS 4 (H 2 O) 10 ]4` was too slow548 for direct determination by NMR spectroscopy. Solvent-exchange kinetics have been measured for trans-[Os(g2-H 2 )(en) 2 (solv)] 2` (solv\H 2 O or MeCN)563 for [Pt(CH 3 )(Me 4 en)(dmso)]`564 and cis- and trans- [ P t ( C H 3 ) C l - (dmso) 2 ].495 The kinetics of 17O loss from 17O-labelled [CrO 4 ]2~,565 and of related processes involving catalysis by tellurate,566 have been described.Oxygen- and vanadium-exchange for [V 3 O 10 ]5~ and [V 4 O 13 ]6~ is thought to proceed by cyclic intermediates.567 Other exchange processes have been studied involving Bu5OHexchange with Sn(OBu5) 4 ,568 n-propylamine with [Co(NH 2 Pr/) 6 ]2`,569 Cl~ with [PdCl 4 ]2~,570 X~ with allyltin halides,571 nitrile exchange with Ir–amidine complexes 572 CF 3 CO 2 ~ in [Tl(tpp)(O 2 CCF 3 )],573 and intramolecular arene exchange in (phosphinoalkyl)arene–RhI complexes.574 558 N.Winterton Reactions of co-ordinated ligands and linkage isomerism Redox-induced ONO to NO 2 linkage-isomerism in cis-[Ru(NO)(ONO)(bipy) 2 ]2` results in oxygen transfer between NO and ONO whereas thermal-induced isomerism does not.575 Decomposition of trans-[RuIII(NO 2 )(PR 3 )(terpy)]2` is believed to proceed via nitro–nitrito linkage isomerism.576 X-Ray studies577 on a metastable form of [Ru(NO 2 ) 4 (NO)(OH)]2~ suggest that NO is bound via oxygen.Interchange between linkage isomers of [Re(CO)L(15N14N)Cp*] [L\CO PMe 3 or P(OMe) 3 ] is nondissociative and intramolecular via a g2-15N14N intermediate.578 Nitrogen monoxide release from the decomposition of [CuMEt 2 N(N 2 O 2 )NM[9]ane(N*Pr) 3N]` in aqueous acetic acid appears not to involve prior dissociation of the Et 2 N(N 2 O 2 )~ ligand.579 Thermolysis of [Ta(CH 3 )(NN15NPh)Cp 2 ] in the presence of a 13CH 3 –14N 3 Ph analogue leads580 exclusively to [Ta(CH 3 )(15NPh)Cp 2 ]. Loss of N 2 is accelerated by electron-withdrawing substituents on the phenyl ring. The complex [MMo(NRR@) 3N2 (k-N 2 )] decomposes to [MoN(NRR@) 3 ] unimolecularly.581 The barrier to interconversion between [MCu(NH 3 ) 3N2 (k-g2 g2-O 2 )]2` and [MCu(NH 3 ) 3N2 (k- O) 2 ]2` is estimated582 from ab initio methods to be low.The cations [Os(qnt)(CO)(g2- H 2 )(PPh 3 ) 2 ]` and [OsH(CO)(PPh 3 ) 2 (Hqnt)]` are in tautomeric equilibrium at low temperatures.583 Metallotropic processes are seen in fac-[ReX(CO) 3 (bmppy)] (X\Cl Br or I)584 in related complexes of 2,4,6-tris(pyrazolyl)pyrimidine,585 2,4,6- tris(pyrazol-1-yl)-1,3,5-triazines,586and in terpy–PdII PtII 587 and –RuII 588 complexes. The view that such processes are associative has been challenged589 and defended.590 Potentially tridentate 7-substituted 8-hydroxyquinoline-5-sulfonate ligands first complex rapidly with [M(H 2 O) 6 ]3` (M\Ga591 or Al442,443) via the N and O of the 8-hydroxyquinoline.A slower stage follows in which a phenolic group attached via position 7 co-ordinates. The cation [Mo 3MPd(NCS)NS 4 (H 2 O) 9 ]4` undergoes a slow isomerisation from Pd–NCS to Pd–SCN.459 Reactions of CoIII complexes continue to be a rich source of novel reactions of co-ordinated ligands. The Hg2`-catalysed aquation of cis-[CoCl(b-Ala)(en) 2 ]2` has also been studied.488 (Related studies 469 on CrIII complexes have already been mentioned). Sulfinato-linkage isomers trans- and cis-[M5N,O(S)N- CoMOS(O)CH(CH 3 )CO 2 -O,ON(tren)]` are among the products from the photolysis or trans-[(5N,S)-CoMS(O) 2 CH(CH 3 )CO 2 -O,SN(tren)]`.592,593 Kinetics of the equilibria between CoIII–S and –C bonded forms of the ligands dathicd594 and aeaps595 point to the intermediacy of carbanions.The carbanion from aeaps reacts with water 170 times faster than its capture by CoIII; that from dathicd 270 times. Hydrogen–deuterium exchange on C2 of 1-methylimidazole is enhanced 102–103-fold by co-ordination to PtII.596 The hydrolyses of a series of mono- and di-nitrile complexes of [CoIII- (NH 3 ) 5 ]3` have been investigated mechanistically.597 For example amido-N-coordinated 2-cyanobenzamide cyclises in base to give [CoIII(NH 3 ) 5 L]2` 4 (L\1-oxo- 3-iminoisoindolino-endo-N) which is hydrolysed in acid following protonation of the exo-imine. In acid 5 (L\NHC(O)C 6 H 4 CN-2) is protonated on the amido-O followed by linkage-isomerisation to the nitrile-bound form. Hydrolysis of MeCN by NbCl 5 ,598 hydration of RCN by [RuCl(H)(CO)(PPh 3 ) 3 ],599 the e§ect of intramolecular hydrogen bonding on the rates of g1-N-bound imine formation from amines and [IrH 2 (pyCHO-N,O)(PPh 3 ) 2 ]`,600 ring closure of a CuII–b-aminoketone intermediate Mfrom Me 2 CO and [Cu(en) 2 ]2`N to give [CuII(trans-[14]dieneN 4 )]2`,601 enamine 559 Inorganic mechanisms N N HO O HO O OH O N N OH O OH O HO O O O OH OH N (NH3)5Co C O HN O 2+ 2+ 5 4 N NH (NH3)5Co H6L22 N N N N addition to NO in trans-[RuCl(NO)(py) 4 ]2`,602,603 template Schi§-base formation from amino acids and ninhydrin in the presence of Cd2`604 or Cu2`605 have all been studied.Two-centre metal-ion catalysis of acyl- and phosphoryl-transfer processes606,607 and the role of lanthanides in phosphate hydrolysis608 have been reviewed. Phosphate ester hydrolysis is accelerated by complexes of nickel(II) cadmium(II) lead(II),609,610 zinc(II),610,611–614 magnesium(II),613 copper(II),610,613,615–618 lanthanide(III),613,619–623 and other metals611,624,625 and by dinuclear complexes of zinc(II),626,627 lanthanide(III),628 and cobalt(III),629,630 peptide hydrolysis by mono-,631–634 bi-,635 and poly- nuclear636 palladium(II) complexes and ester hydrolysis by mono-637,638 and di- nuclear639 complexes of zinc(II) and by uranium(VI),640 palladium(II)641 and platinum(II)641 complexes.The rate of hydrolysis of [CoIIIL(en) 2 ]n decreases along the series L\PO 4 3~[P 2 O 7 4~[P 3 O 10 5~.642 DNA- and RNA-hydrolysis by metal complexes has been reviewed643 and mechanistic aspects studied including cleavage by lanthanide complexes,628,644–648 by CuII 649,650 in combination with H 2 O 2 (which on its own as cetyltrimethylammonium hydroperoxide promotes phosphate hydrolysis651) by other copper(II),652,653 cobalt( III),654 and other metal655 complexes.Related processes involving nucleosides and oligonucleotides have also been described.613,614,623,656–661 Second-order rate constants for the quaternisation of 1-(2-pyridyl)alkyl–PtII and –PdII complexes by organic halides are higher than the analogous 1-(4-pyridyl) alkyl compounds.662 Steric e§ects on the addition of imidazoles to [Fe(CO) 3 (1,5-g- dienyl)]` are less significant than for related reactions of pyridines.663 Incorporation of the isotope label from reaction of 18O 2 –16O 2 mixtures with [NiII(L23)] into the mono- and the bis-sulfenato complexes shows that the initially formed persulfoxide adduct gives the mono- and bis-adducts by two distinct pathways one intermolecular the other intramolecular.664 The complex cis-[Pt(S 3 O)(PPh 3 ) 2 ] is a catalytically active intermediate resulting from the reaction of cis-[Pt(SH) 2 (PPh 3 ) 2 ] with SO 2 560 N.Winterton mimicking a step in the Claus process for hydrodesulfurisation 2H 2 S]SO 2H3/8 S 8 ]H 2 O.665 Metal-ion complexation with macrocycles Intermolecular exchange of L24 in the square anti-prismatic [Na(L24)]` in MeCNand propylene carbonate occurs more slowly666 than enantiomerisation of the complex whereas for [K(L24)]` enantiomerisation proceeds mainly via intermolecular exchange.Related studies on [M(tmec12)]` (M\Li Na or K) in methanol667 and for [Lu(dota)(H 2 O)]~668 have also been described. The role of pendant groups is probed in studies which demonstrate669 that incorporation ofM2` (M\Co Ni Cu Zn Cd Hg or Pb) into the tetraaza ring of [14]aneN 4 can be facilitated by just two 2- hydroxyethyl groups attached to adjacent nitrogens.The cation [Ni(dmf) 6 ]2` and Me 2 [14]aneN 4 react670 initially to form a partially co-ordinated complex which rapidly converts to first one and then a second intermediate in which all four N atoms are bound. This then rearranges further to give trans-[Ni(Me 2 [14]aneN 4 )(dmf) 2 ]2`. Rapid initial formation in aqueous solution of a complex betweenM2` (M\Co Ni Cu or Zn) and H 3 L25 involves671 partial ligand co-ordination followed by ratedetermining product formation in processes in which [HL25]2~ is kinetically the most important ligand species. In related studies Gro� ss and Elias show672 that Ni2` rapidly equilibrates with H 2 tcta~ to form the intermediate [Ni(Htcta)].This forms [Ni(tcta)]~ in a slow first-order rearrangement. Complexation of Ni2` by polycarboxylate ions has also been studied673 using pressure- and temperature-jump relaxation methods. Lanthanide(III) complexes of cy(dtpa-en) (L26) and cy(dtpa-en-dtpa-en) (L27) exist respectively in two and four isomeric forms. In the latter case two types of exchange processes are seen,674 one very rapid which does not involve decomplexation and the other a slower process for which decomplexation is necessary. The ligand H 3 dtpa-dien (L28) rapidly forms an equilibrium mixture of two species on reaction with Eu3` with one more slowly transforming into the other [Eu(Hdtpadien)( H 2 O)]` in parallel proton-dependent and -independent processes.675 More complex processes involving incorporation of Cu2` into a series of bis-macrocycles676 and processes involving the second co-ordination sphere of metal complexes677,678 have also been investigated.The rate of ligand exchange between [Gd(do3a-b)] (do3a-b\L29) and Eu3` is directly dependent on [H`] and proceeds679 via ratedetermining rearrangement and dissociation of the monoprotonated complex. pH and [dtpa] dependence of the kinetics of displacement of ox2~ by dtpa from [Cr(ox) 3 ]3~ points680 to t parallel pathways. Related ligand-exchange reactions of [Cr(en) 3 ]3` with cddta681 and [CuII(Gly-Gly) 2 ] with [X]aneN 4 (X\14–16)682 have been reported. Dissociation of L3~ H 3 L\H 3 notmp from its complex with MgII is dominated683 by a proton-assisted pre-equilibrium followed by rate-determining dissociation of HL2~.Dissociation of MII (M\Ni Zn or Cd) from complexes with [L30]2~ and related ligands,684 of CuII from the mono- and di-nuclear complexes with L31,685 and of CuII and NiII from complexes with hexacyclen686 have all been described. The rate constant for the loss of ligand L ML\R 4 [14]aneN 4 R\H Me Et or Pr/N from [NiL]2` is 50–120-fold smaller than for [Ni(HL)]3` with both rate constants relatively insensitive to the nature of R.687 The displacement of L by cyanide ion is also described. Complexation kinetics of the cryptand 221 with Ca2`,688 of ligand interchange for the complex of 18-crown-6 with Ba2` 689 and with La3` Ca2` 561 Inorganic mechanisms SH SH N N N N N OH HO OH OH N N N O O N N N N N O O O O HN N N N N O O CO2H CO2H HO2C HO2C HO2C HO2C CO2H CO2H CO2H CO2H HO2C HO2C N HN HN NH NH N N L23 L24 H3L25 L26 L27 L28 H H HN HN N N N O O HO2C CO2H HO2C Pb2` and Ba2`690 have been reported as have studies on the rapid exchange 691 involving a series of 1 1 complexes ofK` and bis(benzo-crown ether) ligands linked by HN(CH 2 )nNH bridges (n\2–6 8 or 10).The rate of aquation of [Ni(bptan)]2` follows692 the rate law rate\k 1 [H`]2[Cl~][NiL]/(1]k 2 [Cl~]) in the presence of chloride whereas for the highly inert species [Ni(noda)(H 2 O)] the rate given by k 1 [H`][NiL]/(1]k 2 [H`]) is independent of [Cl~]. The slow rate and associated activation energy (199 kJ mol~1) for irreversible topomerisation from [ThIV(heha)]2~ of C 2 symmetry to a topomer of S 6 symmetry involves693 complete reorganisation of the chelate a process not seen in the related UIV complex.In a fascinating report a 35-nucleotide RNA has been characterised694 which catalyses the incorporation of CuII into mesoporphyrin IX. Cobalt(II) removal by CN~ from dinuclear cobalt(II) substituted derivatives of Carcinus maenas haemocyanin involves695 a rapid pre-equilibrium cyanide addition to the protein followed by slow cyanide-assisted loss of the metal ions in two steps. Complexation kinetics of [XO 4 ]n~ (X\MoVI WVI VV or AsV) with purple acid phosphatase in the FeIIFeIII form have also been reported.696 Iron release from a bacterial ferritin was measured by the 562 N.Winterton N N N N CO2H CO2H O O N N O CO2H NH HN NH HN O HO2C H H OH O HO2C HN NH L29 L30 L31 reduction of iron with S 2 O 4 2~ and the complexation of the resulting Fe2` with bipy.91% of the iron believed697 to be bulk iron reacted more swiftly than the remaining 9%. The latter is thought to be attached to the interior of the protein shell. Related studies have been reported.698,699 The kinetics of metal-ion incorporation into azaporphyrins have been reviewed.700 The porphyrin H 2 tpp in MeCN reacts rapidly with CuII to give a ‘sitting-atop’ complex [Cu(H 2 tpp)]2` with the activation process requiring701 porphyrin-ring deformation. Addition of a base results in deprotonation of porphyrin nitrogens and CuII incorporation into the N 4 ring. Kinetics and activation parameters of HgII complexation with N-p-nitrobenzyl-5,10,15,20-tetrakis (4-sulfonatophenyl) porphyrin suggests the reaction of Hg(OH) 2 with a protonated ligand.702 The kinetics of metal-ion incorporation into tpp2~ a C4-capped porphyrin a cofacial bis-porphyrin and a spheroidal bis-porphyrin in the mixed solvent system toluene–acetic acid–methanol have been described.703 Reactions of [M(H 2 O) 6 ]2` (M\Mn Fe Co Ni or Cu) with tetrakis(4-sulfonatophenyl)mesoporphyrin are reported704 to be factors of 107–1010 slower than the corresponding rate of water exchange.The rate laws705 for the solvolysis of [M(N-Metpps)] (M\Zn Cu Co or Ni) in aqueous acetic acid have a term which is first order in complex and [H`] whereas only forM\Cu or Zn is there an additional term revealing a dependence on both [H`] and [OAc~]. Catenane formation from two molecular rings706,707 proceeds by two sequential ligand exchanges between the two rings (following Pd–N cleavage) which occur at the same time as the rings twist around each other rather than by the conventional process which involves initial dissociation of one ring threading of the open-chain species through the second ring followed by reconnection.Because of the need to break or stretch several Co–Nbonds simultaneously racemisation of the dinuclear CoII complex from L32 occurs with a rate constant some 106-fold 563 Inorganic mechanisms N N N N N N L32 smaller708 than for the analogous mononuclear complex [Co(phen) 3 ]2`. Qualitatively the rates of self-assembly of pentanuclear double helicates from CuI and three oligobipyridine ligands are dependent709 on the steric bulk of the substituent in the 4,4@-positions of the bipy units.Related studies of metal-complexed catenanes,710,711 calix[4]arenes712 and knots713 have been studied and material related to self-assembly reviewed.714–718 Main-group reactions Phenyl boronic acid complexes ethane-1,2-diol via BPh(OH) 3 ~ withOH~ addition to BPh(OH) 2 being much slower than di§usion controlled.719 Values of *V8 and other parameters for the carbonic anhydrase II-catalysed hydration of CO 2 and dehydration of HCO 3 ~ suggest720 that nucleophilic attack of Zn2`-bound OH~ on CO 2 is associative and that substitution of H 2 O by HCO 3 ~ on Zn2` is dissociative in character. Carbon dioxide release and proton transfer are concerted during the dehydration process. Other studies onCO 2 have also been reported.721–723 Kinetics at 725K and 335 bar for the decomposition of aqueous urea to CO 2 and NH 3 724 and of its alkaline hydrolysis in 0.5–3.0M NaOH via OCN~725 have been reported.Hydrolysis of HC(O)Cl (from vinyl chloride and O 3 ) to HCO 2 H and HCl competes with decay to CO and HCl only at high pH.726 Computational methods727 suggest that CCl 2 reacts withH 2 Oto give CHCl 2 OH. This eliminates HCl to give HC(O)Cl reported726 to have t "\20 ks. Rapid intramolecular fluoride exchange between silicons occurs synchronously with ring inversion in fluorinated di-728 and tri- silacyclohexane729 anions. Dihydrogen loss from SiH(OEt) 3 is promoted by tartaric acid via an initial transesterification.730 Addition of water and alcohols to Ph 2 Si––CH 2 ,731 of phenols to Me 2 Si––SiMe 2 732 and the alcoholysis of silanols733 have been studied. Kinetics and isotope studies of the decomposition of aqueous NH 4 NO 2 point to734 pre-equilibrium formation of N 2 O 3 from HNO 2 followed by rate-determining attack of N 2 O 3 on NH 3 .Detailed analysis of the decomposition of NH 2 NO 2 (an ‘honorary’ organic molecule735!) shows that in dilute acid the reaction is base catalysed. An acid-catalysed process appears in more concentrated acid.735 Decomposition of RNHNO 2 to N 2 O and ROH is an acid-catalysed S N 2 process involving the aci-nitro tautomer reacting with H 2 O at low acidities and HSO 4 ~ at high.736 Nitrogen exchange between NO and aqueous HNO 3 ([1.5M) involves N 2 O 3 .737,738 Hydrolysis of nitrosoureas RNHC(O)N(R)NO in base involve rate-determining decomposition of the conjugate base.739 Related nitrosations740–742 have also been studied.The kinetics of nitrous acid-catalysed oxidation of SCN~ in nitric acid are controlled by rate-determining N 2 O 4 formation.743 The red species formed in acidic HNO 2 –SCN~ 564 N.Winterton solution is confirmed to be ONSCN. Nitric oxide reacts with amines or thiols in the presence of O 2 according744 to the stoichiometry 4NO]O 2 ]EHH2ENO] 2NO 2 ~]2H` (E\RS or RR@N). Formation of ONOONO is rate-determining and gives NO 2 and N 2 O 3 . The rate of NO autoxidation is shown to be unchanged by the presence of the amine or thiol and it was concluded that S-nitrosothiols cannot serve as carriers of NO in vivo (see ref. 745). Nitrogen monoxide formation from S-nitrosothiols (and the role of Cu2` in this process)746–748 and from thionitrate esters749 have been reported.Exchange of the tripeptide glutathione GSH with complexed ligand in [Bi(GS) 3 ] is slow at low pH and more rapid at physiological pH observations which may be relevant to the pharmacology of bismuth drugs.750 The rates of the forward and reverse reactions OH·]OH~HO·~]H 2 O have been measured and lead to values of pK(·OH) of 12.0^0.2 and pK(·OD) of 12.6^0.2.751 Sulfur reacts with RS~ in parallel pathways752 involving oxidation giving RSSR and S 3 ·~ and nucleophilic attack giving RSx ~ with a key equilibrium 2RS 4 ~HRSSR and S 3 ·~ being studied. Selenium-77 NMR spectroscopic studies suggest the following stoichiometry when selenous acid is reduced by thiosulfate H 2 SeO 3 ]4S 2 O 3 2~]4H`HSe(S 2 O 3 ) 2 2~]S 4 O 6 2~]3H 2 O with S–S bond formation occurring by direct attack at a Se-linked thiosulfate.753 The kinetics of the photochemical decomposition of XeO 4 have been reported.754,755 Solvent and other medium e§ects Relevant studies have been described in previous sections.105,169,485,487,537 Photoinduced ring closure for [Fe(CN) 5 (tn)]3~ shows a sharp decrease in quantum yield with increasing viscosity of water–glycerol mixtures.756 The role of ether phosphine ligands (‘intramolecular solvents’) on the reactivity of ‘pseudo-underco-ordinated’ complexes of ruthenium has been reviewed.757 Aquation kinetics in mixed solvents have been reported for cis- and trans-[CoCl(NO 2 )(en) 2 ]`,758 trans-[CoCl 2 (N-Meen)] 2 `,759 trans-[CoCl 2 (NH 3 ) 4 ]`,760 and [Co(NO 3 )(NH 3 ) 5 ]2`.761 Solvent e§ects on electron transfer in [NH 3 ) 4 CoIII(k-pzCO 2 )FeII(CN) 5 ]~,762 the consequences of binding of cis- [Co(en) 2 (H 2 O)]3` to a sulfonated polymer on its reactivity with [Fe(edta)]2~,763 and superoxide disproportionation764 have all been described.The di§erence in reactivity of e40-7 ~ with NH 4 ` in water and butanol is attributed to the di§erent solvation structure around the cation.765 Micellar e§ects on the replacement of H 2 O in PdII complexes,766,767 on complexation of CuII,768 on anation of (a,b)S-[Co(OH)(tetren)] 2` by SO 3 2~,769 on aquation of [Fe(Ph 2 phen) 3 ]2`770 and [Fe(Me 4 phen) 3 ]2`771 and on the reaction of [IrCl 6 ]2~ with S 2 O 3 2~ 772 have been described. Micelle formation from CoIII,773 NiII (and its role in ester cleavage774,775) CuII,776 and organometallic777 complexes have been investigated. Micellar e§ects have been studied on electron transfer between CoIII and FeII in cis-[Co(NH 3 ) 4 (H 2 O) 2 ]3` and [Fe(CN) 6 ]4~ 778 and in the ion pair [Co(NH 3 ) 5 (L)]3`·[Fe(CN) 6 ]4~ (L\N-cyanopiperidine) 779,780 on the oxidation of [Co(bipy) 3 ]2` by [Fe(bipy) 3 ]3`,781 of [Fe(phen) 3 ]3` by [CrO 4 ]2~ 782 and [Ru(NH 3 ) 5 (pz)]2` by [Co(ox) 3 ]3~.783 The reaction of [Fe(CN) 4 (bipy)]2~ with S 2 O 8 2~ in a water-in-oil microemulsion has also been reported.772 Second-order rate constants for the reaction with CO of [M(CO) 5 L] in supercritical L (L\Kr Xe or CO 2 ) follow the order Kr[XeVCO 2 .784 The e§ect of solvent density on the dissociative substitution of CO in [W(CO 2 )(CO) 5 ] was examined.Iodine recombination in supercritical argon has been probed on the femtosecond 565 Inorganic mechanisms time-scale.785 Metal-complex-catalysed hydrogenation of CO 2 to formic acid in supercriticalCO 2 786 and reaction of [Ir(CO) 2 Cp*] withH 2 and C 2 H 6 in supercritical CO 2 and supercritical C 2 H 6 787 have been studied.High-pressure photoacoustic calorimetry has been used788 to follow the formation of [Cr(CO) 2 L(g-C 6 H 6 )] from L (L\H 2 or N 2 ) and [Cr(CO) 3 (g-C 6 H 6 )]. Complexation kinetics between NiII and 5-octyloxymethyl-8-quinolinol at the hexane–water interface789 and related studies790 have been reported. Mention has already been made of the acceleration of chemical processes by freezing.312 Heterogeneous processes involving HNO 2 H 2 SO 4 HCl791,792 and ClO 2 793,794 at the surface of ice and sea-salt aerosols795 relevant to the trace chemistry of the atmosphere have been studied mechanistically.The e§ects of microwaves,796 ultrasound797 and magnetic fields798,799 have also been described. 4 Organometallics r-Bonded organotransition-metal compounds The possible evolutionary significance of the axial base to the relative importance of Co–C heterolysis and homolysis in coenzyme B 12 (adocobalamin AdoCbl) has been investigated by Finke and co-workers.800,801 Variation with the basicity of series of axial bases of the rate of Co–C bond thermolysis in adocobinamide (AdoCbi`) in anaerobic ethylene glycol and the relative importance of homolysis and heterolysis reveals that the rate of base-on homolysis is invariant for 4-pyX (X\H or NH 2 ) whereas the rate constant for base-on heterolysis increases 17-fold from X\H to NH 2 . The selection of 5,6-dimethylbenzimidazole as axial base in AdoCbl may be a consequence of its limitation of Co–C heterolysis relative to the more biologically relevant homolysis.The expected inverse dependency on solvent viscosity802 of the thermal homolysis rate constants for a- and b-(cyanomethyl)Cbi`point to the importance of di§usional e§ects. The ratio of in-cage recombination to di§usional separation k#/k$ was estimated with k#a/k#b \2.6^0.6 revealing a surprising kinetic preference of the a diasteromer. Corrin ring side chains determine equilibrium composition of a- and b-RCbi`803 in the anaerobic reaction of a- or b-alkylcobinamides RCbi` with R· via dialkylcobalt(IV)corrinoids. The *S8 values of Co–C homolysis in neopentylcobalamin analogue are influenced804 by the thermal motions of groups attached to the haptocorrin rings.He and Dowd805 suggest that free radicals are not involved in coenzyme B 12 -dependent carbon-skeletal rearrangements. A kinetic model has been developed806–808 to describe the reaction of olefins with [Co(tap)] in the presence of radicals and which involve organocobalt(III) intermediates. Axial ligands (L\py or PEt 3 ) accelerate the rate of Fe–C homolysis809 in [Fe(Me)(oep)L]. Methylation of [NiII(ctpp)] by MeI leads to C21 methylation of the macrocycle via organonickel(II) complexes.810 A reinvestigation811 of CH 4 formation from MeSCH 2 CH 2 SO 3 ~ and [NiL] (H 2 L\1,4,7,10,13-pentaazacyclohexadecane-14,16-dione) has shown the original report to be incorrect. EXAFS studies reveal812 rapid exchange of R and acac~ between [NiR(alkene)(acac)] and [Ni(acac) 2 ] and AlEt 2 (OEt) in toluene.The properties of transient p-bonded organocopper-(II) and -(III) complexes have been reviewed.813 Negative *V8 and *S8 and a large deuterium isotope e§ect suggest814 that the reaction [PtMe 4 (bipy)]]RCO 2 HH[PtMe 3 (O 2 CR)(bipy)]]CH 4 is an S E 2 pro- 566 N.Winterton cess with log k and pK! for the acid being linearly related. Rates of inversion at the a-C in [PtMCHX(SiMe 3 )NX(chiraphos)] increase in the order Cl\Br\I.815 The thermolysis of cis-[PtR 2 ] (R\g2,g1-pent-4-en-1-yl) in aromatic solvents proceeds by reversible b-H elimination.816 The RH elimination from [ZrRM(Bu5 3 Si)NHN3 ],817 silane elimination from [TaClMCH 2 SiMe 3N3MSi(SiMe 3 ) 3N],818 silanone generation from [Pt(Me)(OTf)(dmpe)] and SiPr* 2 (H)OH819 and mechanistically interesting intermediates in reactions of [Ta(CH 3 )(CH 2 )Cp 2 ],820 [Ti(CH 3 ) 2 Cp 2 ],821 [Pd(––SnR 2 )(dippe)],822 [CrVI(CH 2 CMe 3 ) 2 (–– NR) 2 ],823 [W(–– CHPh)(CO) 2 Cp]~,824 [W(–– CHR)(CO) 5 ],825 [MM–– CR@(OR)N(CO) 5 ] (M\Cr or W),826 [IrHM––CH(OR)N(PMe 3 )Cp*]` (evidence for a-H migration),827 [Os(NH 3 ) 5 (L)]2` (L\g2-vinyl ether)828 and [Fe(SiMe 2 SiMe 3 )(CO) 2 Cp*]829 have also been described.The complex cis-[Pt(CH 2 GeMe 3 ) 2 L 2 ] is more thermally labile than the Si analogue giving cis-[Pt(Me)(CH 2 GeMe 2 CH 2 GeMe 3 )L 2 ]830 by b-alkyl migration. The acid cleavage of [PtR(k-H)(k-dppm) 2 MR@] (M\Pt or Pd),831 of [Mo(CCBu5)- H 3 (dppe) 2 ],832 of [Mo(CCBu5)H 2 (depe) 2 ]833 and [Mo(g2-MeCCMe)(dppe) 2 ]834 have been described and reviewed.835 Theoretical studies of Ti–C836 and Hg–C837 cleavage have been reported.Hydrolysis of aqueous [Re(Me)O 3 ] follows the rate law [d[ReMeO 3 ] /dt\k[OH~][ReMeO 3 ] to give CH 4 and [ReO 4 ]~.838,839 At high concentrations polymerisation occurs. In the presence of H 2 O 2 MeOH is the organic product,839 with the kinetics suggesting either OH~ attack on [Re(Me)O 2 (g2-O 2 )] or HO 2 ~ on [Re(Me)O 3 ]. Decomposition via [Re(Me)O(g2-O 2 ) 2 ] represents a minor pathway. Formation and decay of alkyl radicals from Re–R homolysis in [ReR(CO) 3 (a- diimine)] has already been mentioned.98–100,108 Interestingly MLCT excitation of [MnR(CO) 3 (R@-dab)] leads to CO loss for R\Me and Mn–R homolysis for R\PhCH 2 .100 Carbonyl-insertion and alkyl-migration reactions Barriers to migratory insertions in carbonyl alkyl carbonyl acyl ethylene alkyl and ethylene acyl complexes of the type [Pd(R)L(phen)]` increase in the order R to CO\RC(O) to C 2 H 4\R to C 2 H 4 .Associated mechanistic studies840 suggest that the catalytic resting state in the industrially important copolymerisation of ethylene and CO (also studied theoretically841–844) is a carbonyl acyl complex. This is in equilibrium with an ethylene acyl species which undergoes b-acyl migratory insertion to give a metal-bound alkyl which reforms the resting state by rapid reaction with CO. The cation [PdM(CH 2 CH 2 C(O)MeN)(N,O-pyCO 2 Me-2)(PPh 3 )]` from ethylene insertion into a Pd–acyl bond has been characterised by X-ray crystallography.845 Cavell846 has reviewed recent developments in migratory-insertion reactions of M–C bonds and has reported ligand e§ects on CO insertion processes for [PdR(N–O)L].847 Reaction of [Pd(Me)(OR)(N–N)] with CO may lead either to insertion into Pd–C at [25 °C or into Pd–O at[60 °C.848 Rates of CO insertion in a series of [Pd(Me)L (th-metMe)]n` (n\0 or 1) decrease in the order L\CF 3 SO 3 ~[ Br~[MeCN[2,6-dmpy[Cl~[py849 (see also refs.850–852). Steric e§ects of a-diimine ligands N–N control t " for CO insertion in [Pd(Me)Cl(N–N)] complexes. 853 The cation [Pd(Me)(N–N–N)]` has been studied similarly.854 Rates of carbonylation of [Pt(OC 6 H 4 X-4)(triphos)]` to [PtMC(O)OC 6 H 4 X-4N(triphos)]` follow the order X\F[Me[OMe,855 and proceed via migratory insertion rather 567 Inorganic mechanisms than by nucleophilic attack of RO~ on a co-ordinated CO. Electron-withdrawing groups accelerate the rearrangement of [Pd(CH 3 )(CH 2 ––CHC 6 H 4 X-4)(phen)]` to [PdMg3-CH(CH 2 CH 2 )(C 6 H 4 X-4)N(phen)]` via b-CH 3 migratory insertion.856 The exchange [PdMg3-CH(CH 2 CH 3 )(C 6 H 5 )N(phen)]`]styreneH[PdMg3-CH(CH 3 ) (C 6 H 5 )N(phen)]`](E)-b-methyl styrene was also studied.The importance of agostic interactions in related processes has been noted.857,858 Aryl and vinyl migration are proposed for vinyl aryl ketone formation in [Ru(R@)(CO) 2 (CH––CHR)L(L@)] complexes. 859 The complex [FeR(CO) 2 Cp] reacts with PR@3 in a pre-equilibrium followed by rate-determining solvent-independent alkyl migration to give [FeMC(O)RN(CO)(PR@3 )Cp].860 Reaction with CO leads to decomposition of the Rh–Mecompound [RhIr(CH 3 )(CO) 3 (dppm) 2 ] whereas SO 2 gives the IrC(O)Me compound [RhIrMC(O)CH 3N(CO) 2 (k-SO 2 )(dppm) 2 ].861 The Rh–I-catalysed process for methanol carbonylation has been reviewed.862 Ethene inserts into Ni–C of [Ni(R)(N–O)L] via a [NiR(C 2 H 4 )(N–O)L] intermediate.863 Insertions of alkynes into Ln–N864,865 bonds or Pt–B866 bonds allylidene into Rh–O,867 CH 2 ––CHR into Rh–H,868 and pentadienes into Pd–C869 bonds have been described.Phenylacetylene inserts into the Pt–Si bond of cis-[Pt(CH 3 )(SiPh 3 )(PMe 2 Ph) 2 ] (via rate-determining displacement of PMe 2 Ph870) whereas the trans isomer is inert. Interconversion of trans-[RhCl(FcC–– – CSiMe 3 )(PPr* 3 ) 2 ] and trans-[RhClM––C–– C(Fc)SiMe 3N(PPr* 3 ) 2 ] involves871 sigmatropic 1,2-migration of SiMe 3 . Reaction of a fluoride-co-ordinated tungsten o-metallocycle with hexafluorobut-2-yne gives first a kinetic g2-vinyl product with F trans to the inserted alkyne and cis to both COs.872,873 An isomerisation to the thermodynamically-favoured product proceeds via a limiting dissociative mechanism.The process is promoted by CO or P(OMe) 3 by a g1-vinyl adduct-forming preequilibrium. Carbon dioxide inserts into Ru–H of [RuH 2 (dmpe) 2 ] to give cis- and trans-[RuH(O 2 CH)(dmpe) 2 ] and the bis(formate) complex,874 into the O–O bond of [RhCl(g2-O 2 )(PEtPh 2 ) 3 ],875 and reversibly into W–O of [W(OH)(CO) 5 ]~ to give [W(O 2 COH)(CO) 5 ]~.876 Phenyl-to-oxo migration in [Re(Ph)(OTf)(O)Tp] in dmso gives [ReO(OPh)(dmso)Tp]`.877,878 The transformation [MoMg2-C(O)CH 2 SiMe 3N (CO)(CNR@)(PMe 3 )Tp@] to [MoMg2-C(NR@)CH 2 SiMe 3N(CO) 2 (PMe 3 )Tp@] involves rate-determining deinsertion of CO to give a seven-co-ordinate Mo–alkyl intermediate.879 b-Hydrogen elimination of PhN–– CH(Ph) from trans-[IrMN(CH 2 Ph)PhN (CO)(PPh 3 ) 2 ] has also been studied.880 Ligand-displacement reactions of metal carbonyl and other low-valent compounds Geminate recombination of [M(CO) 5 ] (M\Cr Mo or W) and CO Mformed on [M(CO) 6 ] UV photodissociationN occurs within 300 fs that is after only one or two collisions with the solvent cage in alkane solution.881 Similar studies882 on [M(CO) 2 Cp*] (M\Ir or Rh) show that transients decay without CO loss. Photodissociation pathways in [Mn 2 (CO) 10 ] have been studied theoretically.883 Room-temperatureUV photochemistry of [MFe(CO) 2 CpN2 ] in alkane gives [CpFe(k-CO) 3 FeCp] within 10 ps.884 New intermediates are also seen.The solvent–cage e§ect on radical processes resulting from the photolysis of [MMo(CO) 3 CpN2 ] has been studied885 in thf–tetraglyme mixtures of varying viscosity. Thermal reaction of phosphines and phosphites with [Cp(CO) 2 FeCo(CO) 4 ] gives either neutral [Cp(CO) 2 FeCo(CO) 3 L] via a dissociative mechanism or [Fe(CO) 2 LCp][Co(CO) 4 ] by a radical-chain process. 886 Intramolecular CO exchange between metals in [MMo(CO) 3 CpN2 ] via 568 N.Winterton [Cp(CO) 2 Mo(k-CO)Mo(CO) 2 Cp],887 formation of [Re 2 (k-H)(k-alkenyl)(CO) 8 ] from [Re 2 (CO) 10 ],888 displacement of acetone in [Fe(CO) 2 (Me 2 CO)Cp]` by thioethers,889 and mechanistically-relevant structural studies on [Re(NO)(PPh 3 )(g2-O––CHR)- Cp]`890 have also been reported. Activation parameters and solvent dependence for the chelate-ring closure process [Mo(CO) 5 (N–N)]H[Mo(CO) 4 (N–N)]]CO (N–N\a series of phen ligands) support an interchange mechanism891 (see also ref.756). An associative process leading to a seven-co-ordinate intermediate is preferred for the related process [Cr(CO) 5 (dppm)] H[Cr(CO) 4 (dppm)]]CO.892 The values of *V8 for the I$ photosubstitutions [Cr(CO) 4 (phen)]]PMe 3H[Cr(CO) 3 (PMe 3 )(phen)]]CO893 decrease on increasing radiation wavelength from 366 to 546 nm. Replacement of benzylidene acetone (bda) in [Fe(CO) 3 (bda)] by bipy occurs894 by parallel dissociative and associative pathways. Reaction in heptane of the square-pyramidal [Fe 5 C(CO) 15 ] with P donors with small Tolman cone angles h leads initially to reversible formation of [Fe 5 C(CO) 15 L] having a butterfly structure.895 The latter loses CO slowly to give [Fe 5 C(CO) 14 L].Rates of adduct formation are up to 103-fold slower for [Fe 5 C(CO) 15 ] compared with the Ru analogue. Donors with h[136° react very much more slowly. The initial step in the reaction of [Ru 3 (CO) 9 (k3 -g2-Xpy)]~ (X\PhN MeN or S) with phosphines (L) is an associative attack896 on Ru with fission of the k-X bridge bond to give [Ru 3 (CO) 9 L(k-g2-Xpy)]~. The cation [RuL(g4- C 5 H 4 O)Cp]` reacts with PR 3 to give 1,1@- or 1,2-disubstituted ruthenocenes depending on P basicity and L.897 Drago and Joerg898 have proposed a scale of p-donor strengths for phosphines. Replacement of one PPh 3 by PMePh 2 or PMe 2 Ph in [RuCl(PPh 3 ) 2 (g5-C 9 H 7 )] occurs dissociatively.899 The negative *S8 for chloride displacement by PR 3 from [MnCl(CO) 2 Cp@]~ to give [Mn(CO) 2 (PR 3 )Cp@] is thought900 to involve a solvent-co-ordinated intermediate.trans Co-ordination to Cr(CO) 3 of the benzene moiety of the indenyl ligand in trans-[Cr(CO) 3 (ind)Rh(CO) 2 ] accelerates the associative substitution of CO by bidentate alkenes L\cod or nbd via a [Rh(g1- ind)(CO) 2 L] intermediate.901 An equilibrium between trans-[Cr(CO) 3 (k-g6 g3- ind)Rh(CO) 2 ] and trans-[Cr(CO) 3 (k-g4 g5-ind)Rh(CO) 2 ] is proposed. The indenyl complex is ca. 10-fold more reactive than the corresponding Cp complex. Ringslippage and hapticity-change processes have also been examined for [M(CO) 2 L 2 (ind)]`,902 [MX(CO) 2 (g7-C 7 H 6 R)]n`,903 [Cr(CO) 3 (g6-C 5 H 3 X 2 N)],904 and [RuCl(NO)(H 2 O)Cp@]`.905 Kinetics for substitution of the n donor in [Cr(CO) 3 (g6-arene)] by L to give [Cr(CO) 3 L 3 ] display a second-order rate law906 with lability decreasing along the series naphthalene[thiophene[cycloheptatriene [2,5-dimethylpyridine.Other arene-exchange processes on Cr0 and Cr907 and Rh1 908 have been described. Rates have been measured909 for the inversion at Re of the enantiomerically pure (SR)-[ReMNHCH(Me)PhN(PPh 3 )(NO)Cp] to the (RR) epimer. Replacement of PPh 3 in the (SR) isomer by PTol 3 gives 66 34RR SR products via rate-determining PPh 3 dissociation (with amide lone-pair anchimeric assistance) giving a trigonal-planar intermediate. Redox reactions Reviews have appeared by Poli,910 Espenson,911 and in a recent text by Astruc.912 The rate of solution-phase electron transfer within the ion pair [CoCp 2 ]` [V(CO) 6 ]~ 569 Inorganic mechanisms increases ca.two-fold for each additional vibrational quantum in the CO stretching mode.913 The enhancement of the rate of substitution of CO in [Cr(CO) 3 (g-C 6 H 6 )] by P(OEt) 3 from the incorporation of a ferrocenium (Fc`) group into the n-bonded arene is ascribed914 to internal electron transfer from Cr0 to FeIII. Electron-transfer catalysed replacement of CO by PPh 3 in [Ru 3 (CO) 12 ] has also been reported.915 Further reports of 17-electron species derived from metal–metal bonded complexes include studies of [M(CO) 3 (g5-C 5 H 4 R)] (M\Cr or Mo),916 [Ta(CO) 4 (dppe)],917 [Fe(CO) 2 (g5-C 5 Ph 5 )]918,919 and the isomeric diradicals from the Cr–Cr homolysis of [Cr 2 (CO) 4 L 2 (fv)].920 Dimerisation and halogen-atom abstraction processes of these species have been described.In addition third-order kinetics have been observed921 in the reaction of [Cr(CO) 3 Cp*] with thiols. A pre-equilibrium addition of RSH to the radical to give a 19-electron intermediate followed by attack of a second radical is proposed. Reaction of disulfides with the heterobimetallic radical [Cp 2 Ta(k- CH 2 ) 2 CoCp] to give the monothiolato complex [Cp 2 Ta(k-CH 2 ) 2 Co(SR)Cp] can occur either via an outer-sphere electron transfer (seen also in the reaction of [CoCp 2 ] and R 2 S 2 ) or by an initial pre-equilibrium between the reactants.922 Electron-withdrawing groups in R favour an outer-sphere mechanism. The stable radical [Fe(CO) 2Mg5-C 5 (CH 2 Ph) 5N]· has been characterised.923 The Rh–Rh bond in [RhII 2 (NCMe) 10 ]4` su§ers homolytic cleavage on photolysis in MeCN to give [RhII(NCMe) 6 ]2` which then disproportionates.924 Rhenium–rhenium bond fission in a 3,4-dirhenacyclobutene complex has also been studied.925 Initial studies have appeared926 of the reduction of the AuII–AuII bonded complex [MAuCl(dppn)N2 ]2` to [Au(dppn) 2 ]` by benzyl alcohol.The radical cation resulting from one-electron oxidation of [Mo 2 (k-Cl)(k-SMe) 3 Cp 2 ] retains the quadruply-bridged structure but the bridging chloride is labilised to substitution by MeCN and other ligands.927 An unusual isomerism involving LRe–ReLHRe(k-L) 2 Re has been observed in mixedvalence dirhenium complexes.928 Substitution of X in [FeX(CO) 2 Cp] (X\Cl Br or I) by phosphines may be catalysed929 by an electron-transfer chain process. A similar mechanism is proposed930 for the catalysis by [FeICp(g-C 6 H 6 )] of formation of zwitterionic [(CO) 3 Mo~(k-g5 g5-fv)Mo`(CO) 2 (PR 3 ) 2 ] from [Mo 2 (CO) 6 (fv)] and phosphines.Activation of alkyl C–H931,932 and ligand C–C coupling933 may be induced by metal-complex oxidation. A i2–i3 isomerisation may be induced by the oxidation of [RhI(CO)(PPh 3 )(i2-Tp@)] to [RhII(CO)(PPh 3 )(i3-Tp@)]`.934 Dimerisation of [Mn(CO) 3 (g-C 6 H 6 )]` to [MMn(CO) 3N2Mk-(g4-C 6 H 6 -g4-C 6 H 6 )N]2~ (containing a bridging tetrahydrobiphenylene ligand) on reduction proceeds by reaction with the parent cation of a [Mn(CO) 3 (g4-C 6 H 6 )]~ intermediate.935 Electrogenerated 17-electron radical cations [M(ind) 2 ]` (M\Ru or Os) react with nucleophiles (X\Cl~ or MeCN) to give the 19-electron adducts [MX(ind) 2 ]` which oxidise further to 18- electron [MX(ind) 2 ]2`.936,937 Replacement of CO by solv (solv\MeCN or thf) in [Fe(CO)(g2-dtc)Cp*]` Mformed by oxidation of [Fe(CO)(g2-dtc)Cp*] by Fc`N proceeds by an associative mechanism.938 Fifteen-electron intermediates are proposed939 to arise from the decarbonylation of [MoCl(CO)(PMe 3 ) 2 Cp]`.Related processes involving low-valent Mn and Re carbonyl complexes98–101,108,109 and halogen transfer191,194 have been discussed earlier. The complexes [Re(CO) 3 (bipy)]· and [Re(CO) 3 (bipy)]~ rather than [ReCl(CO) 3 (bipy)]·~ are the active species in the electroreduction of CO 2 catalysed by [ReCl(CO) 3 (bipy)].940 The rate of halogen transfer between [W(CO) 3 Cp]~ and [MoX(CO) 3 Cp] shows941 little dependence on X. The 570 N.Winterton kinetics of the one-electron steps PdII to PdI and PdI to Pd0 have been characterised for [Pd(mtas) 2 ]2`.942 Redox processes involving [Re(Me)O 3 ] have already been described.182–184,266 Mechanisms of oxidations of organic substrates catalysed by [Re(Me)O 3 ] have also been reported.943–950 Oxidative addition and reductive elimination Computational and experimental estimation of secondary a-deuterium isotope-e§ects suggest951 that the oxidative addition of MeI to cis-[RhI 2 (CO) 2 ]~ (see also ref.862) occurs via an S N 2 mechanism with inversion of configuration at carbon. The logs of the second-order rate constants for the reaction of [Ir(CO)(PR 3 )Cp*] with MeI to give [IrMe(CO)(PR 3 )Cp*]`I~ correlate linearly952 with measures of the basicity of the complexes. The calculated exothermicity of the addition of CH 4 to MRh(CO)Cp] to give [RhH(Me)(CO)Cp] is very sensitive to the representation of the 5s orbital.953 Tunnelling e§ects on the relative rates of reaction of the C–C and C–H bonds in C 2 H 6 and a bare Pd atom have been computed.954 Initial studies955 of the early stages (ps) of the photoreaction of [Rh(CO) 2 Tp@] in hydrocarbon solution suggest that solvated monocarbonyl intermediates form prior to RH oxidative addition to give [Rh(H)R(CO)Tp@].Quantum e¶ciency of C–H activation is lower at 458nm than at 366 nm.956 Theory supports957 an oxidative-addition process over a four-centre process for the p-bond metathesis Ir(CH 3 )]RHHMR]CH 4 . Related calculations958 for the activation of alkane C–H by PtII or PdII favour a four-centre process for the former (thereby avoiding the formation of PtIV).An oxidative addition-reductive elimination process is not favoured energetically for Pd but cannot be excluded for Pt. From related calculations Russian workers959 propose the existence of [PtCl 2 (g2- CH 4 )(PH 3 ) 2 ]. Oxidative addition of SiH 4 to [MCl(CO)(PH 3 ) 2 ] (M\Rh or Ir) has been studied theoretically.960 Slow elimination of Bu5Et from [Ir(H)(CH 2 CH 2 Bu5)Cl(PPr* 3 ) 2 ] formed from [IrCl(H) 2 (PPr* 3 )] and Bu5CH––CH 2 is thought to proceed via a [IrCl(CH 3 CH 2 Bu5)(PP* 3 ) 2 ] complex.961 The cation [Zr(CH 2 CMe 3 )L 2 ]` gives butene by reversible b-Me elimination962 for L\Cp but irreversibly for the more sterically demanding L\Cp@. Protonolysis of [Pt(R)Cl(Me 4 en)] by HCl involves963 initial formation of [PtIV(H)(R)Cl 2 (Me 4 en)] Cl~ dissociation to give [PtIV(H)(R)Cl(Me 4 en)]` C–H bond formation giving the alkane p complex [PtIICl(RH)(Me 4 en)]` followed by RH loss.The importance of five-co-ordinate intermediate formation is underlined by Hill and Puddephatt964 who have synthesised [PtH(Me) 3 (4,4@-Bubipy)] and shown it to be stable to reductive elimination of MeH. The intermediate [Pt(CH 2 CH 3 )Cl(g2-C 2 H 4 )] isolated from the reaction of [PtCl 4 ]2~ and ethene in water,965 gives [Pt(CH 2 CH 2 OH)Cl 5 ]2~ trans-[Pt(CH 2 CH 3 )(CH 2 CH 2 OH)Cl 4 ]2~ and ethanol on reaction with [PtCl 6 ]2~. Platinaoxetane formation from ethylene oxide and [PtCl 4 ]2~ has also been studied.966 Methane formation from [Ru 2 (k-CH 2 )(k- CO)(CO) 2 Cp 2 ] in the presence of SiMe 3 H proceeds via initial CO dissociation oxidative addition of SiMe 3 H to give [Ru 2 (H)(SiMe 3 )(k-CH 2 )(CO) 2 Cp 2 ] equilibration with [Ru 2 (CH 3 )(SiMe 3 )(CO) 2 Cp 2 ] followed by oxidative addition of a second mol of SiMe 3 H and then elimination of CH 4 .967 Silylene–metal and –alkylidene intermediates are implicated in related reactions.829,968–970 The complex [Mo(PMe 3 ) 6 ] reacts with phenols to give [MoH(OR)(PMe 3 ) 4 ] as the thermodynami- 571 Inorganic mechanisms P P P P – Pt Cl Cl Pt Cl Cl Me + 6 7 Pri 2 Pri 2 Pri 2 Pri 2 cally favoured product rather than an orthometallated species which deuteriumlabelling and magnetisation-transfer experiments suggest is kinetically accessible.971 Oxidative addition of 4-RC 6 H 4 I to [Pd0(R)(PPh 3 ) 2 ]~ leads to [PdIIR(R@))(I)- (PPh 3 ) 2 ]~ with t " ca.1–5 ms.972 Reductive elimination of RR@ then follows. The *V8 and other kinetic parameters973 suggest that oxidative addition of PhI to Pd is not the rate-determining step in its coupling with 2,3-dihydrofuran. Aryl coupling with Ni0 complexes has also been studied.974 Cyclometallation processes have also been investigated 975–978 with complex 6976 su§eringC–C fission and MeCl loss on reaction with HCl via a proposed PtIV intermediate 7. ReductiveC–Celimination from [OsIV(C 6 H 3 Me 2 -2,4) 4 ] induced by phosphines and phosphites is an associative process in which P-donor co-ordination and intramolecular C–C elimination are synchronous.979 Reductive elimination of aryl ethers from [Pd(OC 6 H 4 X-4)(OBu5)(dppf)]980 has been described. The varied [CO] dependence of the C–H reductive-elimination process of [Ru 3 (k3 -CX)(k-H) 3 (CO) 9 ] to [Ru 3 (k3 -g2- CHX)(k-H) 2 (CO) 9 ] (X\CO 2 Me) or to [Ru 3 (k3 -g2-CH 2 X)(k-H)(CO) 9 ] (X\SEt)981 are ascribed to anchimeric assistance by X.The unusual oxidative elimination of H 2 from [UIII 2 (k-OH) 2 (g5-C 5 H 3 R 2 -1,3) 4 ] to give [UIV 2 (k-O) 2 (g5-C 5 H 3 R 2 -1,3) 4 ] (R\SiMe 3 ) obeys first-order kinetics with k H /k D \4.1 and is believed to be an intramolecular process.982 Hydrogen and hydrido complexes The rate constant for oxidative addition of H 2 to trans-[IrCl(CO)L 2 ] is 45-fold larger for L\PPh 2 (C 6 H 4 SO 3 K-m) in water than for L\PPh 3 in toluene.983 The activation barrier for the reaction of Fe(CO) 4 andH 2 in the gas phase to give [FeH 2 (CO) 4 ] is \4 kcal mol~1.984 Reaction of [Ru 3 (k-H) 2 (k-PBu5 2 ) 2 (CO) 8 ] with H 2 involves rapid initial CO loss followed by oxidative addition of H 2 to a single Ru atom.985 Parahydrogen- induced polarisation has been used to show that H 2 addition to [PtMPh 2 PCH 2 CH(Me)OPPh 2N] is a concerted process,986 to characterise intermediates and minor isomers in Ir–H and Ru–H complexes,987,988 to establish reversibility in a Rh-catalysed hydrogenation process989 and to characterise related proton-exchange processes.990 Spin-saturation transfer also reveals991 that exchange occurs between Ru-bound- and Si-bound-H in [Ru(H)(SiHPh 2 )(CO)L 2 ] a process which occurs intramolecularly in the absence of SiH 2 Ph 2 .The *G8 value for H 2 loss increases with R H–H for a series of isoelectronic analogous Os complexes [OsH(X)(g2- H 2 )(CO)L 2 ].992 In an independent study on [OsH(Cl)(g2-H 2 )(CO)(PPr* 3 ) 2 ],993 hydrogen exchange between H and H was found to be very slow with k H /k D \4.6.The cation [Os(CO)(g2-H 2 )(PPh 3 ) 2 (bipy)]2` is inert to exchange with D 2 over several weeks at room temperature.994 The cation trans-[Os(g2-H 2 )(NCMe)- (dppe) 2 ]2` with a pK! of [2 is claimed to be the most acidic fully characterised995 572 N.Winterton stableH 2 complex. No evidence has been found that [MH 2 (pp 3 )]` (M\Co Rh or Ir) contains ligated g2-H 2 though a kinetic isotope e§ect k H /k D of 1.3 for M–H site exchange suggests that an g2-H 2 bonded species might be an intermediate.996 Theoretical studies suggest that the di§erence in energy between [Os(H)Cl(CO)(g2-SiH 4 )] and [OsCl(CO)(g2-H 2 )(SiH 3 )] is very small.997 Equilibria involving [Ir(H) 2 X(S)(PPr* 3 ) 2 ] H 2 and [Ir(H) 2 X(g2-H 2 )(PPr* 3 ) 2 ] in hydrocarbons have been characterised.998 The classical polyhydride [ReH 5 (cyttp)] reacts with H` to give [ReH 4 (g2-H 2 )(cyttp)]` which is inert to CO and P(OMe) 3 at room temperature.999 Hydrogen–deuterium exchange catalysed by RuII–porphyrin1000 and Pt–Au clusters1001 has also been studied.Related spectroscopic,1002–1008 structural,1009–1013 theoretical1014–1018 and reactivity1019–1024 studies have also been reported. References 1 J.H. Espenson Chemical Kinetics and Reaction Mechanisms McGraw-Hill New York 2nd edn. 1995. 2 J.D. 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ISSN:0260-1818    

DOI:10.1039/ic093541

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

28 Bioinorganic chemistry By J. D. CRANE School of Chemistry University of Hull Cottingham Road Kingston-upon-Hull HU6 7RX UK 1 Introduction A bioinorganic chemistry textbook presented from a broad natural sciences perspective has been published1 and complete volumes of Chem. Rev. and Adv. Inorg. Biochem. were devoted to aspects of metalloprotein structure and chemistry.2,3 General reviews concerning electron transfer in proteins,4 metalloprotein design,5 the synthesis and study of artificial enzymes,6 bioelectrochemistry,7 the role of nitric oxide as a ligand for metal ions in biology,8 and the bioinorganic chemistry of aluminium9 have been published. The structures and functions of dinuclear and trinuclear metalloenzymes have been discussed with particular regard to how the metal ions at the active site cooperate to achieve e¶cient catalysis.10 The various binding modes of carboxylate and phosphate ligands in dimagnesium(II) complexes have been studied as models for the family of dimagnesium(II) phosphatase enzymes.11 An investigation of the biomimetic growth of fluoroapatite aggregates in a gelatin matrix has been reported.12 2 Vanadium The irreversible deactivation of the vanadium bromoperoxidase from Ascophyllum nodosum at low pH is reported to be due to the formation of a 2-oxohistidine residue near the metal binding site one consequence of which is the release of vanadium from the enzyme.13 The reactivity of some functional models for vanadium halogenoperoxidases has also been described.14 The electrochemical properties of amavadin and some of its model complexes have been studied.15 They have been shown to act as electron-transfer mediators for the oxidation of thiols to disulfides suggesting a possible biological role for this complex.Owing to the interest in vanadium–maltol (HL1) complexes as insulin mimics the speciation of this system under physiological conditions over the range pH 5–10.5 has been investigated by potentiometry 51V NMR and EPR spectroscopy.16 3 Manganese The crystal structures of the apo and manganese(II) complexed forms of inorganic pyrophosphatase from Escherichia coli have been determined at 2.2Å resolution.17 Royal Society of Chemistry–Annual Reports–Book A 593 O O OH CH3 HL1 The manganese(II) ion was found to have a distorted octahedral geometry and is co-ordinated by three monodentate aspartate residues and three water molecules.The catalytic activity of human manganese superoxide dismutase has been studied by stopped-flow spectrophotometry and compared to the similar manganese protein from Thermus thermophilus.18 The reduced dimanganese(II) and superoxidised dimanganese( III–IV) forms of manganese catalase from Lactobacillus plantarum and a selection of model complexes have been studied by manganese L-edge XAS.19 The e§ects on manganese binding strength and enzyme activity caused by the selective sitedirected mutagenesis of the manganese(II) ligating residues of the manganese peroxidase from Phanerochaete chrysosporium have been reported.20 A model compound for the manganese peroxidase biosite comprising a manganese(II) centre tethered proximal to an iron(III) porphyrin has been described.21 Addition of C 6 F 5 IO as oxidising agent resulted in the generation of a manganese(III) centre the formation of which required the presence of the iron(III) porphyrin.The addition of a reducing substrate (phenol) resulted in the regeneration of the manganese(II) centre. Manganese K-edge XAS studies of the manganese cluster of the oxygen-evolving complex of photosystem II have shown that replacement of the active site calcium(II) ion with other metal ions caused little observable change in structure.22 This favours a more distant hydrogen-bonded interaction between the calcium(II) ion and the manganese centres rather than the previously proposed close manganese–calcium interaction at about 3.3Å. A study of manganese(II) substitution in the calcium(II) depleted protein is also consistent with the location of the calcium(II) ion binding outside the manganese first co-ordination sphere.23 Upon irradiation of the S 2 state of photosystem II with near-IR light at 150K the multiline EPR signal is converted into the g\4.1 signal which fully reversibly reverts back to the multiline signal upon warming to 200 K.24 This is consistent with the assignment of these EPR signals to two forms of the S 2 state di§ering in the arrangement of the oxidation states within the manganese cluster.In addition a di§erence IR study has identified protein conformational changes associated with these two EPR signals.25 It has been reported that the g\4.1 EPR signal comprises two signals arising from two magnetically isolated dimanganese units in the cluster both of which are coupled to an additional S\1 2 species.26 An EPR/ESEEM spectroscopic study of the broad EPR signal of photosystem II generated under conditions which inhibit dioxygen evolution has confirmed its assignment as a tyrosyl radical proximal to the manganese cluster.27 The possible role of this residue for the extraction of protons or hydrogen atoms from water molecules bound to the manganese cluster was discussed.The redox chemistry of dimanganese complexes as potential models for the oxygen-evolving complex of photosystem II has also been described. 28 594 J.D. Crane 4 Iron non-haem biosites The role of X-ray crystallographic methods for the elucidation of iron biosite structures has been reviewed.29 The crystal structure determination of the dinuclear iron(III) complex Fe 2 L2 3 of the bis(hydroxamate) macrocyclic siderophore alcaligin (H 2 L2) shows that it has a mono-bridged structure rather than the triple helix structure proposed for the better known bis(hydroxamate) siderophore rhodotorulic acid.30 The dopa co-ordination of bis(catechol) siderophores with iron(III) has also been modelled with the simple ligand H 4 L3.31 In this case a triple helix [Fe 2 (k-L3) 3 ]6~ complex was formed but in addition the complex [Fe 2 (k-OH) 2 (k-L3) 2 ]6~ could be isolated and was proposed as a stable intermediate in the complexation process.The molecular structures of several bacterial siderophores have been determined by spectroscopic methods.32 The inversion and isomerisation mechanisms of gallium(III) complexes of catechol ligands have been studied as models for iron(III) tris(catechol) siderophores.33 Linear and cyclic tris(hydroxamate) ligands have been synthesised and studied as models for the desferrioxamine siderophores.34 The binding of iron(III) to a dopa-rich adhesive protein isolated from marine mussels has shown that at low iron levels the iron is predominantly present as mononuclear tris(catecholate) complexes whereas at higher iron levels dinuclear oxo-bridged iron complexes are formed.35 The crystal structure of human transferrin has been determined with oxalate bound in place of the exogeneous carbonate ligand.36 One oxalate is bound at each of the two iron(III) centres but the larger size of this anion was found to cause significant perturbations of the binding sites.Human transferrin has been reported to bind the bismuth(III) ion relatively strongly (logKB18) in the presence of carbonate compared to other large cations and based on this observation it was proposed that metal binding at this biosite depends predominantly upon its a¶nity for the ligand set rather 595 Bioinorganic chemistry than its ionic radius.37 The crystal structure of the site-specific mutant of human transferrin in which the iron(III) Asp-60 ligand is replaced by Ser has shown that the Ser residue does not co-ordinate to the iron(III) centre and that a water molecule occupies the vacant site.38 The role of the three-fold channels as the major route for iron uptake into the core of ferritin has been studied by the selective substitution of amino acid residues.39 The relevance of polynuclear iron–oxo–hydroxo species and minerals as structural models for the core of ferritin has been assessed using a combination of EXAFS spectroscopy and X-ray crystallography.40 Recent developments towards a better understanding of the activation of dioxygen by non-haem iron proteins have been reviewed.41 The crystal structure of mouse ribonucleotide reductase apo-protein has been determined but interestingly the protein was not iron-free; there was partial occupancy of one of the dinuclear iron sites and the metal co-ordination environment comprised three Glu residues (one bidentate) and one His residue.42 The crystal structure of the reduced diiron(II) form of ribonucleotide reductase from Escherichia coli has been determined and compared to that of the known diiron(III) form.43 The major structural di§erences were the absence of the bridging oxo group and the loss of the two bound water molecules resulting in the generation of two unusual four-co-ordinate iron(II) centres.The assembly of the diiron(III) core and tyrosyl radical of ribonucleotide reductase from E. coli has been studied by complexation of the apo-protein with iron(II) followed by reaction with dioxygen.44ENDORand 57FeMo� ssbauer spectroscopy have been used to investigate the nature of the diiron intermediate that is believed to generate the tyrosyl radical. This intermediate has previously been described as a diiron(III)–(k-O·~) species but these studies indicate that although one of the iron centres is iron(III) the other has considerable iron(IV) character and the bridging ligands are probably oxo and/or hydroxo. EPR spectroscopy studies of the diiron core and the tyrosyl radical of ribonucleotide reductase and of the generation of tryptophan radicals in protein mutants lacking the Tyr residue have been reported.45 Kinetic studies on the reduction of the diiron core and tyrosyl radical have also been reported.46 The mechanism of alkane hydroxylation by the particulate form of methane monooxygenase from Methylococcus capsulatus (Bath) has been probed through studying the hydroxylation of the chiral ethane derivatives CH 3 CHDT and CD 3 CHDT.47 In all cases the hydroxylation proceeded with complete retention of configuration which is inconsistent with even the transient formation of free ethyl radicals (or cations) and strongly favours a concerted mechanism with a five-co-ordinate carbon transition state.However it is not yet known whether the new C–X bond is carbon–iron or carbon–oxygen. The reactivity of methane monooxygenase with methyl cubane as substrate has been studied and hydroxylation found to occur exclusively at the methyl group.48 This is in contrast to the observations that the cubyl positions of the substrate are about forty times more reactive than the methyl group towards Bu5O· radicals and the reaction with cytochrome P-450 resulted in some degree of hydroxylation at all positions. These patterns of reactivity indicate very di§erent transition states for the two enzymes and are consistent with a concerted mechanism for methane monooxygenase with the requirement for the side-on approach of a C–H bond at the active site. The structures of the native and dmso-treated mixed-valence iron(II)–iron(III) forms of methane monooxygenase have been investigated byENDORspectroscopy.49 596 J.D.Crane The dmso is co-ordinated to the iron(III) centre through oxygen and the bridging hydroxide and aqua groups are retained. A theoretical study of the binding mode of dioxygen to a dinuclear iron(II) complex has shown that the most favourable geometry is the FeIII 2 (k-g1 g1-O 2 2~) mode.50 The structures and reactivities of a series of diiron(III)–peroxo complexes have also been reported as potential models for the active site of methane monooxygenase.51 A diiron biosite similar in structure to those of ribonucleotide reductase and methane monooxygenase has been identified in one of the protein components of the toluene-4-monooxygenase from Pseudomonas mendocina.52 The crystal structure of the diiron(II) form of *9-stearoyl desaturase from castor seed has been determined at 2.4Å resolution.53 The diiron core is co-ordinated by two His and four Glu residues and is located adjacent to the proposed substrate (stearic acid) binding site; indeed modelling of substrate binding at this site indicated that the *9 carbon atom of stearic acid is positioned close to one of the iron centres. The regiochemistry of the *9 desaturase from Saccharomyces cerevisiae has been probed with selectively deuteriated substrates.54 The observed deuterium kinetic isotope e§ects are consistent with an initial relatively slow hydrogen atom extraction at the *9 carbon followed by the rapid formation of the desaturated product. The binding of phosphate to the iron(III)–zinc(II) form of uteroferrin has been studied by iron and zinc K-edge XAS.55 Both iron–phosphorus and zinc–phosphorus interactions of about 3.2Å were identified consistent with a bridging phosphate binding mode.The reaction of the iron(II)–iron(III) form of uteroferrin with phosphate and four other tetraoxo anions (MoO 4 2~,WO 4 2~ VO 4 ~or AsO 4 ~) has also been investigated spectrophotometrically.56 This mixed-valence diiron form of uteroferrin has been studied by 57Fe Mo� ssbauer spectroscopy and an exchange constant of about J\35 cm~1 reported.57 Magnetic susceptibility studies on the iron(II)–iron(III) and diiron(III) forms of the purple acid phosphatases from bovine spleen and kidney bean indicate exchange constants of J\[5 to [15 cm~1 for both systems.58 Model compounds for the purple acid phosphatase biosites have also been described.59 The role of the two conserved Gly residues in the two small chelate loops at the Fe(Cys) 4 site of rubredoxins has been probed through the study of six site-specific mutants.60 Replacement of these Gly residues with Ala and/or Val in the rubredoxin from Clostridium pasteurianum perturbed the pattern of NH· · · S interactions near the iron site and altered the reduction potential of the protein by between [16 and [85 mV.The kinetics of the redox reactions of this rubredoxin have also been studied.61 A series of model compounds for the oxidised iron(III) form of rubredoxins with monodentate thiolate ligands has been reported.62 The recently discovered [2Fe-2S] cluster in mammalian ferrochelatase is essential for enzyme activity and both oxidation states of this cluster were characterised by 57Fe Mo� ssbauer and EPR spectroscopy.63 A 57Fe Mo� ssbauer study of the S\9 2 form of the Cys-56-Ser mutant of the [2Fe-2S]` cluster of C.pasteurianum has confirmed that this is a valence delocalised system.64 The structure and spectroscopy of Rieske [2Fe-2S] clusters have been reviewed.65 The crystal structure of the 2[4Fe-4S] ferredoxin from Chromatium vinosum has been determined at 2.1Å resolution.66 The [4Fe-4S] ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus (which thrives at temperatures up to 100 °C) is ligated by three Cys and one Asp residue and although the reduction potential 597 Bioinorganic chemistry ([350mV) is well within the range for normal four Cys ligated clusters the rate of electron self exchange was found to be much slower.67 It was postulated that this Cys to Asp ligand change not only confers high thermal stability on the protein but may o§er advantages for mediating electron transfer at these high temperatures.The Cys-77-Ser mutant of the [4Fe-4S] high potential iron protein (HiPIP) from C. vinosum has been prepared and spectroscopically characterised and only a small change in the reduction potential was observed.68 This is consistent with Cys ligation in these proteins being required for stability rather than electronic reasons. The solvent accessibility to the [4Fe-4S] cluster of the native and mutant forms of this protein has also been studied.69 A computational model using formal protein charges around the [4Fe-4S] cluster has been developed to predict qualitatively the reduction potentials of HiPIPs.70 The crystal structure of a [4Fe-4S]` cluster model compound with an S\1 2 ground state has been described.71 Model compounds which reproduce aspects of the NH· · · S hydrogeg of the cluster protein environment have also been reported.72 The electrochemical reduction of three [3Fe-4S] ferredoxins to their fully reduced all-iron(II) [3Fe-4S]2~ forms and their spectroscopic characterisation have been described. 73 The reduction from the all-iron(III) [3Fe-4S]` form is accompanied by the uptake of three protons and these are proposed to bind at or near the cluster site. A series of model compounds for [3Fe-4S] and [3Fe-4Se] clusters has been described.74 A resonance-Raman study of the [6Fe-6S] proteins from Desulfovibrio vulgaris and D.desulfuricans has indicated the presence of a Fe–O–Fe or Fe––O group although it is not yet known if it forms an intrinsic part of the cluster.75 The electronic structure of the [4Fe-4S]-sirohaem centre of sulfite reductase from E. coli has been reviewed.76 A model compound for this biosite incorporating an iron isobacteriochlorin component has also been reported.77 5 Iron haem biosites The crystal structures of deoxy- met- and CO-myoglobin and deoxy- oxy- and aquomet-cobalt myoglobin have been reported.78 The crystal structure of the fluoridebound form of met-myoglobin determined at 2.5Å resolution shows that the fluoride anion is co-ordinated to the iron(III) centre and is hydrogen bonded to both the distal His residue and a water molecule.79 The crystal structures of the deoxy- and COforms of a dimagnesium(II)–diiron(II) derivative of haemoglobin have also been reported.80 A resonance-Raman study of oxy-myoglobin and a series of haem pocket mutants has shown that unlike the corresponding carbon monoxide derivatives the l(Fe–O 2 ) frequency is largely insensitive to the mutations.81 The resonance-Raman spectra of 15N and 2H derivatives of met-myoglobin and the 13C14N~ 12C15N~ and 13C15N~ isotopomers of cyanomet-myoglobin have been reported and assigned.82 An IR study of the reduced iron(II) form of cyano-myoglobin has shown that the l(CN) frequency is dependent on pH (pH 8 2057 cm~1; pH 5.6 2034 cm~1) and is therefore a potential probe for the haem pocket environment.83 In contrast the l(CN) frequency for the iron(III) cyanomet form appears insensitive to environment (2124^1 cm~1).The proximal His ligand of myoglobin has been replaced with Gly to yield a mutant 598 J.D. Crane which can bind a range of heterocycles as an exogeneous proximal ligand and the e§ect of these ligands upon carbon monoxide binding was investigated.84 The reaction of met-myoglobin with hydrogen peroxide to yield a ferryl haem (FeIV––O) is accompanied by the formation of an unstable protein radical and the probable sites for this radical were identified by a combination of spin trapping and mass spectrometry experiments.85 Interestingly the replacement of the distal His-64 and nearby Lys-29 residues of myoglobin with Lys and His respectively yielded a mutant with highly stereoselective peroxidase activity.86 The role of the distal His-64 residue of myoglobin (and its hydrogen-bonding network) in the inhibition of unwanted electron-transfer processes has been demonstrated with a range of His-64 mutants which were all found to display greatly enhanced electron-transfer kinetics compared to the wild type protein.87 The binding of carbon monoxide at the NO-binding haem of soluble guanylate cyclase has been investigated with IR spectroscopy.88 The binding of nitric oxide to this haem is thought to activate the enzyme through the resulting dissociation of the axial His ligand and this process has been studied in myoglobin haemoglobin and a mutant myoglobin model system in which the proximal His-93 residue was replaced with Gly and imidazole was bound as an exogeneous ligand.89 The formation of an iron(II)haem–NO` intermediate in nitrite reductase has been studied by IR spectroscopy.90 Plant cytochrome P-450 monooxygenases have been reviewed.91 The hydroxylation of phenylcyclohexane by cytochrome P-450#!. and two mutants lacking Tyr-96 occurs exclusively at the 3 and 4 positions of the cyclohexane ring.92 The direct electrochemistry of the iron(II) and iron(III) forms of both camphor-free and camphorbound cytochrome P-450#!. has been reported.93 The binding of the axial water molecule to the low-spin iron(III) camphor-free form has been investigated by pulsed ENDOR and ESEEM spectroscopy.94 Site directed mutagenesis of the proximal His-93 residue of myoglobin to a Cys residue generated a spectroscopic model for cytochrome P-450 which in addition displayed some peroxidase activity.95 Stable structural models for cytochrome P-450 which reproduce the NH· · · S hydrogenbonding interactions at the biosite have also been reported.96 The crystal structure of the peroxidase from the fungus Arthromyces ramosus has been determined at 1.8Å resolution.97 The mechanism of the oxidation of sulfides (R 2 S) by peroxidases has been probed by analysing the products formed; i.e.sulfoxides (R 2 SO) and/or substrate fragmentation.98 For horseradish peroxidase the reaction is consistent with the initial formation of the radical cationR 2 S·` which can react further to give a range of products. However for chloroperoxidase sulfoxide formation was exclusively observed. IR spectroscopy of the carbon monoxide complex of horseradish peroxidase has been used to probe the pattern of hydrogen bonding within the haem pocket.99 The role of the proximal His-170 residue in horseradish peroxidase has been studied in the His-170-Ala mutant.100 In this mutant the distal His now appears to co-ordinate to the metal centre but addition of imidazole as an exogeneous replacement for the proximal His was found to restore the structure and most of the activity of the protein.The 57Fe Mo� ssbauer and EPR spectroscopy of the green allylbenzenedeactivated form of chloroperoxidase has been reported.101 The electron transfer and redox behaviour of cytochrome c peroxidase has been studied.102 A theoretical model for a peroxidase centre has been investigated in order to understand the binding and 599 Bioinorganic chemistry reaction of hydrogen peroxide and the stability of possible intermediates.103 Model compounds for these biosites have also been reported.104 The e§ects of bis(methionine) axial co-ordination on the properties of a haem centre has been investigated with the His-102-Met mutant of cytochrome b 562 .105 Timeresolved resonance-Raman spectroscopy has been used to monitor the reaction of haem a 3 of cytochrome c oxidase with 16O 2 –18O 2 .106 The sequential appearance of l(Fe–O) stretching modes at 571/544 804/764 785/750 and 450/425cm~1 was observed and proposed to correspond to FeIII–O 2 ~ FeV–– O FeIV––O and FeIII–OH~ respectively.Structural model complexes for the haem a 3 –Cu B site of cytochrome c oxidase and similar biosites have also been described.107 6 Cobalt The cobalt-containing nitrile reductase from Rhodococcus rhodochrous has been studied by cobalt K-edge EXAFS spectroscopy; the cobalt(III) centre was found to be co-ordinated by two mutally cis S-donor ligands at 2.2Å and three or four N/O-donor ligands at 1.95Å similar to the site structure of the known iron(III) enzyme.108 The EPR spectrum of the reduced form of the enzyme was characteristic of a low-spin cobalt(II) centre.The dependence of Co–C bond cleavage in adenosylcobinamide on the nature of the axial ligand has been studied for a range of di§erent substituted pyridines.109 Although the rate of Co–C homolysis was approximately constant the rate of Co–C heterolysis increased markedly with electron donating substituents on the pyridine consistent with the hypothesis that the role of the 5,6-dimethylbenzimidazole ligand in vitamin B 12 is to inhibit the heterolysis reaction.A resonance-Raman study of alkyl cobalamins has shown that the l(Co–C) frequencies depend on the nature of the alkyl group but surprisingly appear to be una§ected by the nature of the trans axial ligand.110 EPR spectroscopic studies of the reaction of the adenosylcobalamin cofactor with the ribonucleosi reductase from Lactobacillus leichmannii indicate that the cofactor generates a catalytically active thiyl radical from a Cys residue.111 The conversion of a side chain of cyanocobalamin into a primary amine resulted in the formation of a green derivative the crystal structure of which was determined.112 The solution structures of cyanocobalamin derivatives have also been investigated by 1H NMR spectroscopy.113 7 Nickel The crystal-structure determination of the oxidised form of the [NiFe] hydrogenase from Desulfovibro gigas at 2.5Å resolution has revealed further details of the site structure (Fig.1).114 In this inactive form of the enzyme the iron and nickel centres are bridged by two Cys residues and an O-donor ligand the latter probably an hydroxide and only present in this oxidised form. The co-ordination sphere of the iron centre is completed by three diatomic ligands postulated to be carbon monoxide (but possibly cyanide nitric oxide or a mixture). The characteristic IR spectra of these terminal ligands depend on the redox state of the enzyme indicating a redox role for the iron 600 J.D. Crane Ni S Fe S S S X Cys Cys L L L Cys Cys X = probably OH L = probably CO Fig. 1 Biosite structure of the oxidized inactive [NiFe] hydrogenase from Desulforvibrio gigas centre.A nickel K-edge XAS study of several [NiFe] hydrogenases in di§erent oxidation states has been reported.115 The results for the D. gigas enzyme were consistent with the crystal structure but the nickel co-ordination sphere in the Alcaligenes eutrophus hydrogenase appeared to change from two or three S-donors at 2.35Å and three or four N/O-donors at 2.05Å in the oxidised form to four S-donors at 2.20Å upon reduction. The di§erent oxidation states of these enzymes have also been studied by EPR spectroscopy.116 The relevance of model compounds to the structure and function of [NiFe] hydrogenases has been reviewed.117 The CO-oxidation/CO 2 -reduction site (the C-cluster) of carbon monoxide dehydrogenase from Clostridium thermoaceticum has been studied by EPR and 57Fe Mo� ssbauer spectroscopy.118 The data were consistent with the proposed [Ni–X–Fe 4 S 4 ] structure for this cluster with a distinct iron(II) aconitase-like subsite in the cluster proximal to the nickel centre.The binding of cyanide to the cluster an inhibitor of CO-oxidation was found to occur at this distinct iron site apparently in contrast to previous work which indicated binding at the nickel centre. In fact resonance-Raman and IR spectroscopy studies of the cyanide bound form [l(CN) 2037 cm~1] and its 13C 15N 54Fe or 64Ni isotopomers indicated that the cyanide binds simultaneously to both metal centres; vibrational modelling favoured an Fe–CN–Ni bridging geometry with a Ni–N–C angle of about 140°.119 Based on this arrangement of the metal ions it was postulated that CO-oxidation proceeded by the attack of aNi–OHnucleophile on a proximal Fe–CO group although other studies have indicated that CO/CO 2 binding occurs at the nickel site.120 EPR spectroscopy studies of the acetyl coenzyme A synthesis site (the A-cluster) of carbon monoxide dehydrogenase are consistent with it also having a [Ni–X–Fe 4 S 4 ] structure.121 The binding and reactivity of carbon monoxide at nickel(II) centres has been studied in biologically relevant model compounds.122 Nickel-containing superoxide dismutases have been isolated from Streptomyces seoulensis and S. coelicolor.123 For both proteins EPR spectroscopy indicated the presence of a nickel(III) centre and the activity of the biosites was lost upon removal of the metal. The crystal structure of the inactive His-134-Ala mutant of urease from Klebsiella aerogenes has been determined at 2.0Å resolution.124 Unlike the native enzyme only one nickel(II) ion was bound at the active site and its geometry was octahedral rather than distorted tetrahedral due to the binding of two additional water molecules.XAS studies have shown that the enzyme inhibitor phenylphosphorodiamidate binds directly to the nickel centres of urease.125 Model compounds for the active site of urease have also been reported.126 601 Bioinorganic chemistry 8 Copper The important recent developments in the bioinorganic chemistry of copper have been reviewed.127 The use of resonance-Raman spectroscopy to probe the metal co-ordination geometry in copper–sulfur proteins128 and the redox properties of blue copper proteins129 have also been reviewed.EPRand 14N/15NENDORspectroscopy studies of the azurin from Pseudomonas aeruginoas were consistent with the presence of a weak interaction between the copper centre and the backbone carbonyl group of Gly-45.130 The crystal structure of the Met-121-Ala mutant of this azurin has been determined both with and without azide bound to the copper centre.131 In the azide-free form the copper retains its type I character despite the loss of the Met-121 ligand and the shortening of the Cu–O (Gly-45) distance from 2.97 to 2.74Å. In the bound form the azide occupies the Met-121 site and the copper atom has moved towards the azide anion with a Cu–N distance of 2.04Å. XAFS spectroscopy has been used to study the e§ect of azide binding and pH on the copper site geometry in the similar Met-121-Gly mutant.132 The copper sites of these and other Met-121-X mutants (X\Gln Val Leu or Asp) have been studied by EPR spectroscopy.133 The crystal structures of the Ile-7-Ser and Phe-110-Ser mutants have also been determined.134 The paramagnetic copper(II) site of amicyanin has been studied by 1H NMR spectroscopy.135 The copper binding Cys–X2–His–X2–Met protein loop in the amicyanin from Thiobacillus versutus has been replaced by the longer Cys–X2–His–X4–Met loop from poplar plastocyanin.136 The copper centre of the hybrid protein is stable and redox active but its spectroscopic properties are very similar to pseudoazurin rather than amicyanin or plastocyanin.Studies of the site structure and electron-transfer properties of horseradish umecyanin137 and the voltammetry of plastocyanin from Anabaena variabilis138 have been reported.The marked di§erences between the type I copper site geometries of the nitrite reductase from Achromobacter cycloclastes and poplar plastocyanin have been used to rationalise the unusual spectroscopic properties of the nitrite reductase site.139 The crystal structure of the copper-containing pea amine oxidase has been determined at 2.2Å resolution.140 The copper centre is co-ordinated by three His residues and two water molecules and the shortest distance to the topaquinone cofactor is about 6Å. The formation of radical intermediates in the catalytic cycle of amine oxidase has been investigated with resonance-Raman and EPR spectroscopy.141 A series of model compounds for the copper(II) site of galactose oxidase (Fig.2)142 and the type II copper active site of nitrite reductase143 has been reported. The crystal structure of the [CuZn] superoxide dismutase from Xenopus laevis has been determined at 1.5Å resolution.144 The structure of a new crystal form of yeast superoxide dismutase has revealed a trigonal planar copper centre which is presumably copper(I) and cleavage of the imidazolate bridge although the structure of the zinc(II) site appears to be unperturbed.145A study of the pH dependence of the activity of zinc-free superoxide dismutase has indicated that the role of the zinc centre is to facilitate the rapid dissociation of hydrogen peroxide from the reoxidised copper(II) centre.146 The spectroscopic properties of two mutant forms of the [CuZn] superoxide dismutase from Saccharomyces cerevisiae in which one of the copper His ligands has been replaced by a Cys residue (His-46-Cys or His-120-Cys) have been reported.147 602 J.D.Crane N Cu N O H2O O HN NH Tyr Tyr His His S Cys Fig. 2 Biosite structure of galactose oxidase Cu S Cu S N N S O Cys Cys HN NH CH3 Met His His Fig. 3 Structure of the Cu A biosite of cytochrome c oxidase The oxidation of deoxy-haemocyan from Carcinus maenas with nitrite yields a half-met form of the biosite and one equivalent of nitric oxide.148 The magnetic and spectroscopic properties of the dicobalt(II) substituted form of the haemocyanin from C. maenas have also been reported.149 The binding of dioxygen to the dicopper(I) biosites of haemocyanin and tyrosinase has been the subject of computational modelling studies.150 The reactivity of several model compounds for these biosites has also been described.151 The structure and function of the tricopper site of ascorbate oxidase has been reviewed.152 The binding of dioxygen to the reduced form of the tricopper site of laccase from Rhus vernicifera has been studied for the derivative in which the nearby type I copper site is substituted with a redox-inactive mercury(II) ion.153 Spectroscopic studies were consistent with the dioxygen binding to the type III dicopper(I) centre to form a hydroperoxide-bridged dicopper(II) species with the adjacent type II site remaining reduced.A trinuclear copper model system for the four-electron reduction of dioxygen to water has also been described.154 The recent developments in elucidating the structure and function of the Cu A dicopper site of cytochrome c oxidase (Fig.3) have been reviewed.155 Spectroscopic studies of the mixed-valence form of this biosite have indicated complete delocalisation to a Cu1.5`Cu1.5` state.156 The possibility of a Cu–Cu bond has been investigated with resonance-Raman spectroscopy and a band at about 130cm~1 has been tentatively assigned to the Cu–Cu stretching mode.157 A 1H NMR spectroscopic study of the mixed-valence Cu A site of cytochrome ba 3 from Thermus thermophilus has indicated that this dinuclear site has advantages over mononuclear copper(II) sites for e¶cient electron transfer due to the greatly reduced nuclear reorganisation required. 158 An EPR spectroscopic analysis of the similar mixed-valence Cu A site of 603 Bioinorganic chemistry N N O Zn His His OH His-119 O Glu-117 H N N N Zn His His OH2 His-119 O Gln-117 H H - wild type Glu-117-Gln mutant Fig.4 Biosite structures of carbonic anhydrase II and its Glu-117-Gln mutant nitrous oxide reductase has also been reported.159 A protein model for these Cu A sites has been generated by the site-directed mutagenesis of a second copper-binding site into an azurin protein and the mercury(II) and mercury(II)–silver(I) derivatives of the engineered biosite were prepared.160 A simple model compound for the mixed-valence Cu A site has also been described.161 The sequential binding of copper(I) ions to mammalian metallothionein has been shown initially to be random and non-cooperative but the final structure is the result of an internal migration process of metal ions to form clusters.162 Accurate mass measurements of the binding of zinc(II) ions to metallothioneins has shown that two protons are released for each metal ion bound thus indicating that Cys residues may still act as ligands even if not deprotonated.163 9 Zinc Recent developments in understanding the function of the zinc biosite of carbonic anhydrase II have been reviewed.164 The necessity of the second co-ordination sphere ligand Glu-117 in carbonic anhydrase II for e¶cient enzyme function has been demonstrated by the greatly diminished activity for the Glu-117-Gln mutant.165 The authors postulated that the reversed hydrogen bonding favoured by the Gln residue stabilised the deprotonated imidazolate form of the zinc ligand His-119 (Fig.4). Structural model compounds for the zinc(II) site of carbonic anhydrase have been reported.166 The structure and reaction mechanism for the zinc(II) site of liver alcohol dehydrogenase have been the subjects of computational modelling.167 The spectroscopic properties of the copper(II) substituted form of the biosite have also been reported.168 The crystal structure of the His-412-Gln mutant of the alkaline phosphatase from Escherichia coli reveals substantial perturbation of the active zinc binding site.169 Dinuclear zinc(II) compounds as structural models for the biologically relevant Zn–(H 3 O 2 )–Zn and Zn–(OH)–Zn bridging groups have been described.170 604 J.D.Crane 10 Molybdenum tungsten and the nitrogenases The bioinorganic chemistry of oxomolybdenum enzymes in general171 and of dimethyl sulfoxide reductase in particular172 have been reviewed.The crystal structure of dimethyl sulfoxide reductase from Rhodobacter sphaeroides has been reported and the oxomolybdenum(VI) centre found to be asymmetrically co-ordinated by two pterin moieties bound as chelating dithiolenes and a Ser residue.173 XAS studies of this enzyme in the reduced molydenum(IV) oxidation state indicate the presence of three or four S-donors at 2.33Å and two N/O-donors at 2.16 and 1.92Å but no oxo group.174 A mechanism for the function of the aldehyde oxidoreductase from Desulfovibrio gigas has been proposed based on the crystal structures determined for several di§erent states of the enzyme.175 A 13C/17O ENDOR spectroscopy study of xanthine oxidase has indicated that the oxygen atom in the product originates from a water molecule and not from the Mo–– O group as previously thought.176 Instead the xanthine substrate is proposed formally to add across the Mo––S group to form a reactive intermediate with a molybdenum–carbon bond which is then attacked by a water molecule.A model compound for the molybdenum site of xanthine oxidase has been reported.177 The structures of the molybdopterin sites of human sulfite reductase and the nitrate reductase from Thiosphaera pantotropha have been investigated by XAS.178 The bioinorganic chemistry of tungsten-containing proteins has been comprehensively reviewed.179 The di§erent oxidation states of the tungsten and [4Fe-4S] centres in the aldehyde ferredoxin oxidoreductase from Pyrococcus furiosus have been studied by EPR and MCD spectroscopy.180 Model compounds for this tungsten biosite have also been reported.181 A 57Fe ENDOR spectroscopic study of the carbon monoxide inhibited forms of nitrogenase has shown that CO binds to the FeMo-cofactor and that either one or two carbon monoxide molecules are bound depending on the pressure of CO applied.182 The possibility of a bound diazene (HN––NH) intermediate in dinitrogen reduction has been probed by studying the reactions of the substrates diazirine (CH 2 N––N) and trans-dimethyldiazene (H 3 CN––NCH 3 ) with nitrogenase.183 In the latter case methane ammonia and methylamine were formed in a 1 1 1 ratio consistent with C–N bond cleavage to give a M––N–NHMe intermediate followed by N–N bond cleavage.A theoretical model for dinitrogen binding and reduction at the FeMo-cofactor has been described,184 and the chemistry of model compounds for nitrogenase has been reviewed.185 References 1 R. 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ISSN:0260-1818    

DOI:10.1039/ic093593

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

29 Fullerene chemistry By P. R. BIRKETT School of Chemistry Physics and Environmental Science University of Sussex Brighton BN1 9QJ UK 1 Introduction The 1996 Nobel Prize for chemistry was awarded to Curl Kroto and Smalley1 for their seminal paper reporting both the identification of [60]fullerene using mass spectrometry and the closed cage structure of the fullerene molecules.2 Because of the large number of publications covering fullerene chemistry and physics the following are highlights. A number of special symposia conference proceedings and books have been published.3 2 Synthesis separation and physical properties of fullerenes UV radiation emitted during the production of fullerene soot by the carbon arc method is found to reduce the yield of fullerenes.4 Two isomers of [78]fullerene which have C 2v symmetry have been isolated and their infrared Raman and electronic spectra recorded.5 The isolation and characterisation of a single chiral isomer of [80]fullerene which has D 2 symmetry has been reported.6 The topology of all pentagon isolated fullerenes from [60]fullerene to [90]fullerene is found to correspond with the isolation of the larger amounts of three isomers of [84]fullerene.7 The generation of the boron heterofullerenes,C 59 B and C 69 B by arc evaporation of doped graphite rods and extraction with pyridine opens a viable route for the macroscopic production of these compounds.8 The nitrogen heterofullerenes (C 59 N) 2 and (C 69 N) 2 are produced by heating butylamino adducts of diazabis-(1,6);(1,9)-homofullerenes bearing methoxyethoxymethyl protected imino bridges with p-toulene sulfonic acid (Scheme 1).9 The parent hydroheterofullerene C 59 NH was prepared from the dimer (C 59 N) 2 by reaction with either hydroquinone or tributyltin hydride.10 Density functional theory calculations on azafullerene dimer (C 59 N) 2 indicate that the dimer has a weak intermolecularC–C bond which should account for the missing signal in the sp3 region of the 13CNMRspectrum.11 A remarkable self-assembly of (C 59 N) 2 has been revealed by electron microscopy; it is suggested that the large hollow spherical particles are formed by the rapid evaporation of the solvent.12 Synchotron X-ray powder di§raction data of (C 59 N) 2 points to a monoclinic crystal structure.13 As the pressure increases on (C 59 N) 2 during angle-dispersive X-ray di§raction measurements the Royal Society of Chemistry–Annual Reports–Book A 611 Scheme 1 New method for the synthesis of the nitrogen heterofullerenes (C 59 N) 2 and C 69 N) 2 .(Reproduced by permission from Chem. Commun. 1996 1421.) interdimer distances decrease more rapidly than the intradimer ones leading to a novel high pressure solid structure which appears to be characterised by almost isotropic bonding.14 The unusual solvatochromism of [70]fullerene is found to be due to the formation of clusters of particle sizes ranging from 100 to 1000nm depending upon the solution concentration.15 The droplet model of clusters has been used to explain the formation of fullerene aggregates and the unusual temperature dependence of fullerene solubility. 16 The synthesis and isolation of the first fullerene cation salt [C 76 ]` [CB 11 H 6 Br 6 ]~ is reported.17 Selective reduction of [60]fullerene to C 60 ~ and C 60 2~ using zinc powder in aqueous caustic and tetrahydrofuran solutions reveals that the anions are stable to water under these experimental conditions.18 Photochemically generated C 60 ·~ in a micellar medium is found to have an usually long lifetime in excess of two minutes.19 Ten reduction processes are observed during the cyclic voltammetry of two isomers of [84]fullerene each of which has distinct half-wave reduction potentials.20 The spectral bands in the photoelectron spectra of [86]- and [90]-fullerenes above 4.5 eV are characteristic of individual fullerenes while those below 5 eV resemble each other and are similar to other higher fullerenes.21 The thermal analysis of carbon allotropes has been described as an advanced undergraduate experiment.22 The partial molar volumes of [60]fullerene have been 612 P.R.Birkett determined to be 350 to 440cm3 mol~1 from high precision density measurements in organic solvents these values are far smaller than the estimated molar volume of liquid [60]fullerene.23 Heat capacity anomolies caused by the orientational phase transitions of [70]fullerene are observed at 280 to 340 K.24 The gas phase ionisation potentials (I') of [60]- [70]- [76]-fullerenes two isomers of [78]fullerene [82]- and [84]-fullerenes are experimentally determined to be 7.49 7.26 7.06 6.96 6.85 6.90 and 7.05 eV respectively;25 in good agreement with those calculated by Seifert et al.26 An emission line at j 258 nm which is identified with an electronic transition into the ground state has been observed for the first time by optical radiation of [60]fullerene in the gas phase.27 The very weak fluorescence of [70]fullerene may increase by one or two orders of magnitude through the mechanism of delayed thermal fluorescence the quantum yield of triplet formation /T was determined to be 0.994^0.001.28 The high resolution luminescence spectrum of [70]fullerene has been obtained at low temperature; 29 the spectrum has been analysed in terms of the Herzberg–Teller mechanism combined with spin–orbit interactions.30 The transient absorption of the triplet state of [60]fullerene has been recorded using di§use reflectance laser flash-photolysis.31 The weak infrared active modes observed experimentally in [60]fullerene are argued to be combination modes caused by anharmonicity.32 The prominent features of the fluorescence and fluorescence excitation spectra of [60]fullerene have been assigned; in addition the origins of the two lowest single excited states have been located.33 The optical energy gap of a [70]fullerene film is derived to be 1.66 eV and can be described in terms used for amorphous semiconductors.34 The first observation of the lowest S 1 ]Sn transition band and the absorption edge of 1C 60 * have been made using picosecond time resolved near-IR spectra.35 The rates of fragmentation of fullerene ions in molecular beams are significantly smaller than the evaporative ensemble prediction consistent with an alternative cooling mechanism through emission of electromagnetic radiation.36 Photoexcited [60]fullerene undergoes disproportionation to generate cation and anion radicals; the cation radical reacts with solvent molecules and leads to the formation of brown solutions and precipitates.37 Flash photolysis studies of [60]fullerene photoprocesses confirm that [60]fullerene acts as a catalyst for electron-transfer processes.38 Fullerene anions made by photoreduction in a lipid bilayer produce the largest trans-membrane steady state photo-currents yet observed,B6.0 kAcm~2.39 Thin film field e§ect transistors made of [70]fullerene and a-sexithiophene exhibit markedly di§erent performances to those consisting of [60] fullerene.40 [60]Fullerene works as an e¶cient mediator for preparative-scale indirect cathodic reduction.41 [60]Fullerene is found to give rise to drag reduction or lubricity on fairly thick liquid films suggesting that it may be used as an additive to conventional liquid lubricants.42 Neutral fragments formed during mass spectrometry of fast moving mass-selected ions can be identified and provide an invaluable insight into the mechanism of decomposition of precursor ions.43 The fusion of fullerenes occurs for energies above a sharp barrier which lies in the region of 60 to 80 eV and increases with increasing number of atoms participating in the collision.44 Reviews of ab initio calculations of fullerene structures and the use of potential energy surfaces in rationalising the structure dynamics and thermodynamics of clusters have been published.45 Semiempirical quantum chemical calculations predict the most stable structure of odd-numbered fullerenes [119]fullerene to have C 2 symmetry; three bonds are formed between the C 59 and C 60 moieties.46 The cost of pentagon 613 Fullerene chemistry Fig.1 The structures of the fullerene fragment C 36 H 12 1 and the first corannulene cyclophane 2. adjacency in fullerene cages is found to grow from 72 kJ mol~1 for [30]fullerene to 111 kJ mol~1 for [60]fullerene by quantum consistent force field/pi (QCFF/PI) model calculations.47 The isolated pentagon rule is shown to be applicable for large fullerenes such as [84]fullerene.48 QCFF/PI and density functional tight binding (DFTB) approaches agree in predicting that no cage with one or more heptagons is of lower energy than the best classical [40]fullerene; in addition when heptagons are present the structures of lowest energy are those that maximise the number of pentagon–heptagon contacts.49 Using the same theoretical approaches it is shown that no structure with four-membered rings can better the classical [40]fullerene structure.50 Total energy calculations suggest that atomic carbon present during fullerene formation may act as a catalyst which allows lower activation barriers for fullerene annealing.51 Other theoretical calculations show that the energy barriers of pyracyclene rearrangements from the penultimate intermediates to [60]- and [70]-fullerenes are 118– 125 kcal mol~1.52 A ‘cycloaddition model’ is proposed to explain the mechanism of formation of the fullerenes.53 High resolution transmission electron microscopy has allowed the observation of single layer fullerene shells forming at the surface of previously untreated graphite particles.54 Gas phase ion mobility measurements show that the growth sequence for pure carbon clusters and carbon clusters containing one metal atom are altered by the presence of an additional metal atom.55 A number of new approaches to the chemical synthesis of fullerenes have been reported this year.A review of polynuclear aromatic hydrocarbons related to the structure of [60]fullerene has been published.56 The flash vacuum pyrolysis of decacyclene at 1200–1300 °C produces the fullerene fragment triacenaphthotriphenylene C 36 H 12 1 (Fig. 1) comprising 60% of the fullerene carbon cage.57 The same author reports the characterisation of three novel isomeric dicyclopentapyrenes;58 the analysis of the combustion products of ethylene shows the presence of the same three dicyclopentapyrene isomers59 and the use of high temperature gas phase cyclisation reactions for constructing bowl-shaped polycyclic aromatic hydrocarbons related to [60]fullerene.60 The introduction of an additional five-membered ring to the framework of corannulene increases the curvature and rigidity of the system significantly.61 [60]Fullerene is produced in low yield by the pyrolysis of corannulene under a range of conditions62 pyrolysis of cyclopentadiene triindane fluoranthene triphenylene de- 614 P.R.Birkett cacyclenene biphenylene perylene and pyrene also produces fullerenes in low yield.63 Other approaches have seen the synthesis of a ‘buckybowl’ C 30 H 12 from 1,2,5,6- tetraketopyracene,64 benzo[c]naphtho[2,1-p]chrysene,65 trifluorehemifullerene,66 [7]circulene,67 and hepta[5][5]circulene.68 The first corannulene cyclophane 2 has been synthesised (Fig.1).69 Corannulene itself has been prepared by the pyrolysis of silylvinyl ethers70 and pyracyclene by the thermolysis of 5-(1-chloroethenyl)acenaphthene. 71 Routes to precursors of endohedral fullerenes have been developed by two groups. The synthesis and X-ray structure of a flexible acetylenic cyclophane C 60 H 18 has been reported; however no peak for [60]fullerene was detected upon its fragmentation by mass spectrometry.72 A stable pentaethynylcyclopentadienyl radical has also been reported73 and penta(cyclopentadienyl)cyclopentadienyl compounds have been prepared by multiple Pd-catalysed cyclopentadienylations.74 Separation of the fullerenes by chromatography continues to be the subject of much attention.Copper phthalcocyanine stationary phases have been evaluated for fullerene separation in microcolumn liquid chromatography.75 The separation of fullerenes is found to be strongly dependent upon the central metal ion and substituents of attached phthalocyanines.76 Similarly a maximum retention temperature exists with long alkyl bonded stationary phases.77 The use of functionalised porphyrins has also been evaluated in the chromatographic separation of fullerenes.78 An alternative approach involves the reversible attachment of fullerenes to thermally stable silicasupported dienes by covalent bonding.79 A chemically bonded [60]fullerene silica stationary phase has been used to separate calixarenes.80 Laser pyrolysis of oil shale residues obtained at 400 °C produce fullerenes in low yield.81 Further natural deposits of fullerenes have been reported; global wildfires following the Chixclub impact 65 million years ago are suggested as the most likely source of their formation.82 No fullerenes were detected in lunar samples leading to the conclusion that formation of fullerenes from extra lunar carbon on the moon had very low yields or that fullerenes were formed with detectable yields but were subsequently removed from the surface of the moon in a very short time.83 The measured profiles of three narrow di§use interstellar bands (DIB) are consistent with the theory that gas phase molecules are some of the DIB carriers; these are suggested to be due to large polycyclic aromatic hydrocarbons with more than 40 carbon atoms chains of 12–18 carbons 30 carbon rings or [60]fullerene derivatives.84 Two gas-phase chemical models have been formulated for the production of large abundances of fullerenes and fulleranes which may be candidates for the carriers of the DIBs.85 The shape dependence of the UV absorption of spherical and tubular carbon particles show that these materials are possible components of interstellar dust.86 A study of the 3He/4He ratio trapped within the fullerenes from the Sudbury impact structure reveals that the ratio exceeds the accepted solar wind value by 20 to 30% and is higher by an order of magnitude than the maximum mantle value reported.87 The implication is that the helium within the molecules is of extraterrestrial origin.3 Endohedral fullerenes A review of the synthesis characterisation and properties of endohedral fullerenes has been published.88 A new method for the production of endohedral fullerenes contain- 615 Fullerene chemistry ing alkali metal ions has been reported; monolayers of [60]fullerene are exposed to an intense beam of alkali metal ions at an energy chosen so that the ions can penetrate the cage but do not destroy it (6 eV for Li` increasing to 40 eV for K`).89 Improvements have been made in the extraction of a range of endohedral [60]fullerenes by using pyridine90 and a study of the chromatographic behaviour of endohedral fullerenes has been made.91 Three air-stable isomers of Tm@C 82 are found to exist; the thulium is present as Tm2`.92 The dimetallofullerene Ce 2 @C 80 has also been isolated and characterised; the Ce atoms are present as Ce3` implying that one valence electron remains on each metal atom.93 A simple procedure using solid phase extraction is reported to remove 22–30% of empty fullerenes from a mixture also containing metallofullerenes.94 Other metallofullerenes which have been isolated using a variety of methods include Ca@C 82 ,95 Ca@C 84 ,95 Pr@C 82 ,96 Pr 2 @C 82 ,96 Ho@C 82 ,97 Ho 2 @C 82 ,97 Ho 3 @C 82 ,97 and La@C 74 .98 13CNMR studies of Sc 2 @C 84 show that the two Sc atoms are encapsulated along the D 2d axis of the D 2d[84]fullerene in a symmetric position.99 Ab initio molecular dynamics show that the metal adsorption sites depend on the cage structure in a simple way.100 La@C 82 is found to have overall paramagnetic behaviour.101 X-Ray photoelectron spectroscopy of Ce@C 82 shows that this material is analogous to La@C 82 and that the Ce atom donates three valence electrons to the cage.102 The fullerenolanthanides have been shown to be both strong electron donors and acceptors compared to empty fullerenes by cyclic voltammetry.103 Three types of fragment ions lanthanum containing carbon clusters which have either gained or lost two carbon atoms with respect to the parent fullerene and empty fullerenes are detected by surface collision experiments on La@C 82 .104 A review of the endohedral noble gas fullerenes has been published.105 Semiempirical calculations show that the incorporation of helium into [60]fullerene must overcome a barrier of more than 200 kcal mol~1 (relative to non-interacting groundstate [60] fullerene and helium).106 Krypton has also been incorporated into fullerenes.107 4 Chemistry of the fullerenes Organic chemistry The covalent chemistry of the fullerenes has been reviewed.108 Nucleophilic Bingel cyclopropanation has been used to produce three isomeric pairs of diastereoisomeric mono-adducts of [76]fullerene two possessing C 1 symmetry and the other C 2 symmetry.109 A number of crystal structures of monocycloadducts of [60]fullerene have been reported including isoxazoline,110 triazolinyl,111 methano112 and pyrrolo113 addended [60]fullerene. Dibromomethanofullerene has been synthesised and found to produce two dimeric species C 121 and C 122 formed during mass spectrometry which are predicted to be dumbell shaped molecular carbon allotropes.114 Methanofullerenes have also been synthesised utilising an electrosynthetic approach.115 New pyrrolidine cycloadducts are formed by the photoinduced reactions of tertiary amines with [60]fullerene;116 similarly [60]fullerene reacts with alklaldehydes in the presence of ammonia producing dialkyl substituted pyrrolidine derivatives.117 Chiral azomethine ylides react with [60]fullerene producing diastereomeric fulleropyrrolidines which can be separated by flash chromatography.118 The parent aziridino[ 60]fullerene C 60 NH has been synthesised119 and used to prepare urethano- 616 P.R.Birkett amido- and sulfonamido-[60]fullerenes by nucleophilic substitution.119a [60] Fullerene dimers linked by double donor spacers have been reported; the electrochemical data indicates that the two fullerene moieties behave independently.120 There has been considerable interest in the production of biologically active fullerene derivatives and a review has been published.121 Water soluble fulleronicotine derivatives have been prepared by adapting the azomethine synthesis using triethylene glycol chains as the solubilising appendage; the new derivatives were found to be active against a variety of microorganisms.122 An acridine adduct of [60]fullerene has been synthesised and found to have enhanced photoinduced DNA cleaving activity.123 A series of fullerene derivatives which have been solubilized by an organic linker (‘detergent type’) and those to which a number of polar groups (‘sphere type’) have been attached were evaluated for biological activity; the detergent-type derivatives were found to be potent HIV protease inhibitors.124 Further studies of the bis(succinimide) methanofullerene derivative which has been found to have anti-viral activity against human immunodeficiency virus types 1 and 2 have been reported; the new data report that the compound is fatal to rats at a dose of 25 mgkg~1.125 A range of ‘ball-and-chain’ systems incorporating [60]fullerene and donor substituted aryl chromophores linked via a norbornylogous hydrocarbon bridge has been prepared and characterised.126 Photophysical studies indicate that rapid long-range intermolecular electron transfer processes take place and that the electronic coupling between the [60]fullerene and the hydrocarbon bridge is unusually strong.126a Similarly [60]fullerene linked to zinc porphyrins has been shown to have photoinduced charge separation and subsequent charge recombination by picosecond fluorescence lifetime measurements.127 A porphyrin linked to [60]fullerene by a pyrrolidine spacer has also been prepared.128 Fullerene derivatives bearing a 2,2@ 6@2A-terpyridine moiety have been prepared and used to synthesise ruthenium(II) diads and triads in metallosupramolecular chemistry.129 Silver ions (Ag`) have been shown to interact with the surface of the fullerene cage and the oligo(ethylene oxide) side chain attached to the fullerene by 5,6-bridging nitrogen.130 Quinone-type methanofullerenes have also been synthesised and are reported to have better electron-accepting properties than [60] fullerene.131 The mono- and di-anions of a benzoquinone linked [60]fullerene are synthesised by alkali metal reduction.132 A methanofullerene with two cholesterol derivatives attached has liquid crystalline properties and high thermal stability.133 Cycloadditions to [70]fullerene have also been the subject of some attention.1,3-Dipolar addition of nitrile oxides takes place at one of the radialene-like [6,6] bonds of the polar five-membered ring generating two orientational isomers. The crystal structure of one is reported a third cycloaddition takes place in the neighbourhood of the pole.134 A chemoselective route to [1,9]methano[70]fullerene derivatives has been reported.135 Tetraalkoxyethylenes react e¶ciently with [70]fullerene; the cycloadduct reverts to [70]fullerene on irradiation with UV or visible light.136 Cycloaddition of dipolar trimethylenemethane to [70]fullerene is found to be promoted by a trace amount of water and produces a mixture of an acetal and a carboxylic acid ester; the former undergoes isomerisation to the latter at 120 to 160 °C.137 Benzyne addition to [70]fullerene produces a mixture of four mono-adducts one of which is triptycene homologue.138 Two isomers of [70]fullerene epoxide have been characterised as a mixture by techniques including 3HeNMR spectroscopy.139 The study of the regiochemistry of multiple additions to [60]- and [70]-fullerenes 617 Fullerene chemistry Scheme 2 Synthesis of the tetrakis-adducts of [60]fullerene using the tether-directed remote functionalisation method.(Reproduced by permission from Chem. Commun. 1996 797.) has continued. A highly uniform topochemically controlled fullerene refunctionalisation based on a thermal intermolecular anthracene transfer reaction from the monoadduct of [60]fullerene with anthracene produces just [60]fullerene and the antipodal 6,6-bisanthracene adduct.140 The addition of chirally tethered bismalonate enables the enantioselective synthesis of optically active [60]fullerene bisadducts; the crystal structure of one such compound is described.141 Similarly the tether-directed remote functionalisation of [60]fullerene followed by a further addition and removal of the tether-reactive group conjugate leads to the first D 2h-symmetrical tetrakis adduct (Scheme 2).142 The X-ray crystal structure of a hexakis adduct of [60]fullerene synthesised by the tether-directed approach has also been reported.143 Bisimino[60]fullerenes are the first examples of [60]fullerene derivatives with open transannular [6,6] bonds.144 A general bond-labelling algorithm for multiple additions to [60]fullerene has been developed which enables unambiguous bond labelling of both [6,6] and [6,5] bonds as well as the assignment of the absolute configuration of any fullerene derivative.145 3HeNMR spectroscopy has also been used to study the isomers resulting from 618 P.R.Birkett bis-addition in three types of reaction cyclopropanation (five isomers found) addition of azomethne ylides (four isomers detected) and reduction with [60]fullerene.146 Oxidative homocoupling of 2-functionalised 1-ethynyl[60]fullerenes provide access to a new class of dumbell-shaped bisfullerenes linked by butadienyl moieties;147 dumbells linked by either a butadiyne or an acetylene linker have also been prepared.148 1,2-Dihydro[60]fullerenes possessing functional groups have been prepared by reacting the fulleren-1-ide with carbon electrophiles149 The crystal structure of a sterically hindered bis-silylated [60]fullerene adduct has been reported.150 Addition of [2-(trimethylsilyl)ethoxy]carbene to [60]fullerene produces 1,2- and 1,4-dihydrofullerene adducts rather than the expected (dialkoxymethano)fullerene.151 A further example of the ene type of reaction has been reported between [60]fullerene and 3-methylene-2,3-dihydrofuran.152 Lithium fluorenide reacts with [60]fullerene to produce 1-fluorenyl-1,2-dihydro[60]fullerene after protonation; prolonged reaction time produces the 1,4-bis-adduct with two fluorenyl groups attached.153 Similarly addition of [60]fullerene to a suspension of the Collman reagent produces a 1,4-adduct bearing two benzyl groups after quenching with benzyl bromide.154 [60]Fullerene reacts in the solid state with zinc and ethyl bromoacetate producing 1-ethoxycarbonylmethyl-1,2- dihydro[60]fullerene after quenching with acid.155 The first fully characterised [60]- and [70]-fullerenols have been prepared156 and further details of the one-pot synthesis of water-soluble [60]fullerenols reported.157 Up to four oxygen atoms were found to be present by mass spectrometry following the ozonolysis of [60]fullerene.158 X-Ray di§raction data show that there are two epoxides present in the complex [Ir(CO)Cl(PPh 3 ) 2 (g2-C 70 O)]·5C 6 H 6 ; there are also two di§erent orientations of the long fullerene axis.159 The synthesis of one isomer of C 60 H 6 with three-fold symmetry has been achieved using a zinc–copper couple in toluene.160 C 60 H 18 has been confirmed to have C 3v symmetry by 1HNMR spectroscopy (Fig.2).161 Three low energy structures have been identified for C 70 H 36 by MNDO calculations.162 C 60 F 18 has been synthesised by reaction between [60] fullerene and potassium hexafluoroplatinate and confirmed to have the structure analogous to that of C 60 H 18 by 19FNMR spectroscopy.163 Other fluorinated fullerenes have been reported viz. C 60 F 24 ,164C 60 F 54 ,165C 60 F 27–52 166 and C 60 F 46 .167 Near edge X-ray absorption fine structure spectroscopy and UV photoemission spectroscopy have been used to derive an energy diagram for C 60 Fx.168 The electron a¶nity of C 70 F 54 is estimated to be 4.42 eV 1.76 eV larger than that of [70] fullerene.169 Fluorinated fullerenes C 60 Fx and C 70 Fx have been used as the cathodes in solid-state lithium cells.170MNDOcalculations predict that the most stable isomer of C 60 F 48 has D 3 symmetry and the 13CNMR spectrum should have ten lines if fully 19F-decoupled.171 Only the alkoxy radicals RO· of the three radicals produced by photolysis of ROSSOR (dialkoxy disulfides) add to [60]fullerene and yield adducts which can be detected by EPR spectroscopy.172 The alkoxy radical derivative (Bu5O)nC 60 (n[2) is persistant under nitrogen but reacts with ·NO over several minutes producing fullerene dimer aminoxyl radicals.173 Five regioisomers have been detected for the first time in the addition of methoxy radicals to [70]fullerene.174 In contrast simple alkyl radical addition results in only three of the possible five isomers; more reactive alkyl and aryl radicals produce four out of five isomers and trifluoromethyl radicals produce all five.175 Radio frequency muon spin resonance experiments have allowed the 619 Fullerene chemistry Fig.2 Schlegel diagram of C 60 H 18 (filled circles) and C 60 H 36 (filled plus open circles); encirclement indicates the constant 6H addition pattern that can produce both structures. (Reproduced by permission from J. Chem. Soc. Perkin Trans. 2 1996 2051.) identification of both endo- and exo-hedral adducts.176 There is no evidence for the formation of the radical cation C 60 `· during laser flash photolysis and pulse radiolysis studies on [60]fullerene and halocarbon radicals.177 Polymers A methanofullerene-based ‘charm bracelet’ conjugated polymer containing an electroconducting polythiophene moiety retains the characteristic electrochemical behaviour of both acceptor and donor groups.178 A polystyrene star polymer with six arms terminating with amino groups has been reacted with [60]fullerene producing a fullerene end-capped polymer containing a precise number of [60]fullerene units (Fig.3).179 Soluble pendant [60]fullerene–polystyrene polymers have been prepared by fullerenating polystyrenes in Friedel–Crafts-type reactions.180 It has been proposed that under the reaction conditions reported [60]fullerene retards the free-radical polymerisation of styrene and that each [60]fullerene molecule terminates either one or possibly two polymer chains;181 the co-polymerisation of [60]fullerene and styrene has also been reported.182 A highly water soluble pendant methanofullerene polymer has been synthesised by reaction between methano[60]fullerene dicarboxylate and a copolymer poly(propionylethylenimine-co-ethylenimine).183 Fullerenated poly(N-vinylcarbazole) (PVK) has intriguing temperature sensitivity and an unusual temperature dependence of the EPR spectrum has been observed.184 Subpicosecond photoinduced absorption spectra of poly(p-phenylene vinylene) and methanofullerene blends show that electron transfer from the donor polymer to the fullerene acceptor occurs within a picosecond of photoexcitation of the polymer.185 Organometallic chemistry A cobalt(III) complex with a fifteen-membered ring results from triple scission of a six-membered ring on the surface of [60]fullerene via consecutive pericyclic reactions 620 P.R.Birkett Fig. 3 The structure of the [60]fullerene end-capped star polymer 3. (Reproduced by permission from Chem. Commun. 1996 1565.) and oxidative cobalt insertion.186 The first pentahapto metal complex of [60]fullerene has been formed by quenching the pentaphenyl cyclopentadienyl anion of [60] fullerene with thallium ethoxide; the crystal structure of the product shows that the thallium atom is situated centrally 2.60Å above the cyclopentadiene ring.187 Similarly the first hexahapto[60]fullerene complex in which [60]fullerene displays arene like co-ordination to the open face of a triruthenium cluster has been synthesised;188 a tungsten hexahapto complex has also been reported but not fully characterised.189 The Fe 2 S 2 (CO) 6 adduct of [60]fullerene has been characterised by X-ray crystallography.190 Organo-palladium or -platinum [60]fullerene complexes C 60 Mn react with isonitriles to produce the complexes C 60 M(CNR) 2 ; in addition the platinum complexes C 60 Pt(CNR) 2 react with additional isonitrile to form C 60 Pt(CNR) 4 .191 X-Ray photoelectron spectroscopy demonstrates that there is electron transfer from palladium to [60]fullerene in complexes of the form C 60 Pdn; there is both magnetic and ferrimagnetic ordering in the system.192 Addition of [Pt(PPh 3 ) 2 ] to [70]fullerene ceases after the formation of the tetra-adduct (C 70 )MPt(PPh 3 ) 2N4 (Fig. 4) due to steric considerations and the low reactivity of the remaining exposed double bonds.193 Picosecond and nanosecond laser photolysis were used to obtain the absorption 621 Fullerene chemistry Fig.4 A perspective drawing of the structure of (C 70 )MPt(PPh 3 ) 2N4 4. (Reproduced by permission from Angew. Chem. Int. Ed. Engl. 1996 35 188.) spectra of the photoexcited [Pd(PPh 3 ) 2 C 60 ] complex.194 No bands corresponding to the charge transfer from 3d (Fe) to [60]fullerene orbitals are observed in the visible region of the electronic spectrum of [Fe(CO) 4 C 60 ].195 57Fe Mossbauer spectra of [Fe(CO) 4 C 60 ] are consistent with a bonding interaction between the metal centre and the ligand a carbon–carbon double bond of [60]fullerene; there is no significant temperature dependent vibrational anisotropy evident in the experimental temperature range.196 The complex [PtMP(OPh) 3N2 C 60 ] displays [60]fullerene n–n* intraligand bands in the UV–VIS region and a long wavelength absorption at j 770 nm which is assigned to a metal–ligand charge transfer transition from platinum to fullerene.197 Inclusion and charge-transfer complexes The inclusion of [60]fullerene in cyclotriveratrylene (CTV) results in the formation of micelle-like aggregation and a one-dimensional zigzag polymeric array of [60] fullerenes in the solid state in the cavity of a CTV molecule;198 similarly the 1 1 p-Bu5-calix[8]arene complex of [60]fullerene is micelle-like.199 Calix[6]arenes bearing N,N-dialkylaniline groups have also been found to form host–guest complexes with [60]fullerene.200 Molecular dynamics and molecular mechanics reveal that the [60] fullerene molecule is encapsulated within the cavity of p-Bu5-calix[8]arene with six phenyl groups orientated upwards and the remaining two outward.201 Molecular 622 P.R.Birkett mechanics have also been used to study the [60]fullerene·c-cyclodextrin inclusion complex.202 The transient spectrum obtained by di§use reflectance laser flash photolysis of the solid [60]fullerene·c-cyclodextrin complex is similar to those recorded in aqueous solution; however there is an absence of triplet–triplet annihilation and molecular oxygen quenching processes.203 A one-electron reversible and three irreversible adsorptive electroreductions of the (1 2) [60]fullerene·c-cyclodextrin complex have been observed by hanging mercury drop electrode.204 The incorporation of [60]fullerene and two [60]fullerene adducts into micelles and liposomes has been achieved; the fullerenes are also found to be able to enter into photo-redox chemistry with biologically relevant redox partners in micellar solution.205 [60]Fullerene molecules have been incorporated into layered double hydroxide (LDH) by boiling the dodecyl sulfate LDHpowders in an organic solution of [60]fullerene; the [60]fullerene molecules were found to rotate less freely by 13CNMR spectroscopy after incorporation into the LDH layers.206 The first charge-transfer complexes of covalently bound [60]fullerene and tetrathiofulvalene (TTF) systems show semi-conducting behaviour.207 The mixed fullerene radical-anion salt [PPh 4 ] 2 [C 60 ·Cl 1~xIx] prepared by electrocrystallisation has been investigated and found to contain chlorine which makes the previous concept of partial charges in the salt invalid;208 the analogous [70]fullerene salt [PPh 4 ] 2 [C 70 ·I] has also been prepared.209 A large downshift and strong enhancement of linewidth of the F 16 mode is observed by electronic and vibrational spectroscopy in the complexes [MPh 4 ] 2 [C 60 Y] (M\P or As; Y\Cl Br or I).210 The Sherrington–Kirkpatrick model has been used to account for the temperature dependence of the magnetisation found in the TDAE-[60]fullerene complex.211 The synthesis and single-crystal structure of the complex between N,N,N@,N@- tetramethyl-p-phenylenediamine (TMPD) and [60]fullerene have been reported;212 freezing of the motion of the [60]fullerene molecules in the solid state has also been detected by optical transmission spectroscopy.213 The inorganic halide cluster Pd 6 Cl 12 co-crystallises with [60]fullerene from benzene; the crystal structure contains a complex array of molecular components with face-to-face contacts between the benzene molecules and both [60]fullerene and Pd 6 Cl 12 .214 Hypothetical molecular solids consisting of transition metal clusters and [60]fullerene of this type have also been suggested to possess metallic properties.215 Metal fullerides Monomethylamine has been employed as the solvent for the synthesis of alkali-metal doped [60]fullerenes; the technique works particularly well for the more reactive metals potassium and rubidium and the high volume of superconductivity observed in Rb 3 C 60 reflects the accuracy of the end-product stoichiometry.216 Twelve lithium atoms per [60]fullerene modecule can be inserted into the crystal lattice at a pressure of up to 2.3GPa at room temperature; the number of lithium atoms can rise to 30 per [60]fullerene molecule at the same pressure but at 456.5 K.217 There has been further development of the microwave-induced argon plasma method for the rapid synthesis of alkali-metal fullerides; potassium rubidium and caesium fullerides are reported to be readily synthesised by this method.218 There is no evidence for superconductivity down to 0.5K in the fullerides Ba 2 MC 60 (M\K Rb Cs); these fullerides have a face-centred cubic (fcc) symmetry.219 Similarly Ba 2 CsC 60 has been synthesised by the reaction of the metals with [60]fullerene in liquid ammonia; the ternary metal fulleride 623 Fullerene chemistry Fig.5 The average structure of Ba 2 CsC 60 viewed along the [001] direction of the unit cell.The two ‘standard’ orientations are present with fractions of 83% (carbon atoms shown as white) and 17% (black). The Ba cations on the tetrahedral sites (large white) and the Cs cations (large black) on the octahedral sites are also shown. (Reproduced by permission from Chem. Commun. 1996 1191.) translational symmetry in this case is fcc; however a new type of orientational ordering of the fulleride anions Fm3 is adopted (Fig. 5).220 A Na–NH 3 cluster occupies the octahedral interstice and the remaining K or Rb occupies the tetrahedral sites in the fulleride (NH 3 )xNaA 2 C 60 (0.5\x\1; A\K or Rb); the sodium ion is displaced by 0.4 to 0.6Å from the centre of the octahedral site.221 The first example of post-transition metal mercury intercalation into a fullerene host Na 2 C 60 has been reported; the Hg present does not transfer charge to the [60]fullerene and acts as a neutral spacer;222a the 13CNMR chemical shift of the mercury doped sample is very close to that of Na 2 C 60 .222 Similar potassium amalgam doped fullerenes K 3 HgxC 60 (x\1 2 or 3) have now also been reported.223 Two di§erent samarium-based fullerides of nominal composition Sm 3 C 60 and Sm 6 C 60 have been synthesised; the latter shows diamagnetic behaviour.224 Significantly increased air-stability of (dibenzo-18-crown-6)KC 60 is attributed to the intermolecular interactions between the two aryl rings of the crown ether and the [60]fulleride anion; a strong 13CNMR signal which has a negative chemical shift [182 ppm due to the fulleride anion is observed below 200 K.225 The [60]fullerene monoanion salt [N(CH 3 ) 4 ]C 60 ·1.5thf has been synthesised by reaction of NaC 60 ·5thf with N(CH 3 ) 4 F 624 P.R.Birkett Fig. 6 Schematic diagram of the structure of CsC 60 in the orthorhombic polymer phase showing the covalently bonded chains. The chains are projected on the plane defined by the chain axis (a) and a diagonal of the bc face of the unit cell; consecutive chains are related by 90° rotations about the chain axis. (Reproduced by permission from Chem. Commun. 1996 2465.) in acetonitrile at [30 °C; the cell is hexagonal with layers of close-packed[60]fullerene molecules with the cations and solvent molecules located in trigonal-prismatic sites.226 The first azafullerene salt,K 6 C 59 N has a body-centered cubic structure and is isostructural with K 6 C 60 ; the position of the nitrogen atom of the azafulleride is currently unknown.227 Electron energy loss spectroscopy has shown that Cs and C layered structures can be fabricated at low temperatures because of the high mobility of Cs adatoms.228 At temperatures\370 K neutron inelastic scattering measurements show that the CsC 60 fulleride comprises of linear chains of [60]fulleride monoanions presumably because of [2]2] cycloadditions (Fig.6);229 13C solid state NMR spectra have provided evidence for sp3 hybridised carbon atoms in linear chains.230 In addition the chainlike structure has been confirmed by high-resolution transmission electron microscopy and electron-energy-loss spectroscopy (EELS) of polymerised KC 60 where the fine structure of the EELS spectrum shows the sp2 hybridised carbon atoms along with features which distinguish the bonding in this fulleride from that of other fullerenes.231 The polymer-like phase of the fullerides AC 60 (A\K Rb) has been analysed by using phenomenological theory to describe orientational ordering of the fullerene molecules.232 [60]- and [70]-Fullerenes have been studied at high pressures and temperatures; polymerised linear chains of [60]fullerene molecules are produced although [70]fullerene does not undergo polymerisation of this type under these conditions.233 The lowest energy neutral [60]fullerene dimer is determined to be the [2]2] cycloadduct; however the most stable mono-anionic [60]fullerene dimer is the singly bonded isomer formed by direct covalent bonding between [60]fullerene molecules.234 The Gibbs free energy of the phase separated ‘intermediate state’ at low temperatures where polymerised KC 60 is the stable form is determined to be signifi- cantly higher compared to the other various phases of this fulleride material.235 Evidence for a metal–insulator phase transition to low temperatures in RbC 60 and CsC 60 has been found; KC 60 remains metallic at all temperatures.236 A review of nuclear magnetic resonance investigations of alkali metal fullerides has 625 Fullerene chemistry been published.237 The phase transition of Na 2 C 60 from the low-temperature orientationally ordered simple cubic (sc) structure into the high-temperature orientationally disordered one occurs at 339 K.238 The superconducting transition temperature T# of Na 2 RbC 60 Na 2 KC 60 Li 2 CsC 60 and ammoniated A 3 C 60 is significantly lower than that expected from the BCS relation based on the spin susceptibilities at room temperature determined from the isotropic 13CNMRhyperfine coupling constant.239 A quasi-elastic feature arising from the di§usive motion of the C 60 3~ anions with a width of 1.03(6)meV at an average scattering vector of 1.09Å is observed at a temperature of 350K in the low energy inelastic neutron scattering spectra of Na 2 CsC 60 .240 87Rb and 13CNMRspectroscopy spin–spin relaxation and two-dimensional exchange experiments on Rb 3 C 60 allow an estimation of the timescale of vortex fluctuations.241 Room-temperature 87RbNMR spectra of Rb 3 C 60 K 2 RbC 60 and Rb 2 CsC 60 have an additional broadened line indicating that for samples prepared from sublimed [60]fullerene a considerable number of the Rb ions are located on sites with symmetry lower than cubic.242 Tunnelling and optical transmission studies of Rb 3 C 60 indicate that this material is an s-wave superconductor but the superconductivity cannot be described in the weak coupling limit.243 The functional dependence of T# on the electronic density of states has been determined for Rb 3 C 60 ; the results are consistent with weak-coupling BCS theory.244 A normal-state resistivity dominated by electron–phonon interactions is implied by following the Elliot–Yafet theory of electronic relaxation in metals from the temperature dependence of the EPR linewidths and g values of Rb 3 C 60 K 3 C 60 and Na 2 CsC 60 .245 The first example of a C 60 3~ metal to undergo a metal–insulator transition at 40K in (NH 3 )K 3 C 60 has been identified by EPR and 13CNMR measurements.246 Spin–lattice relaxation and line shift in the NMR spectra of A 3 C 60 and A 4 C 60 depend strongly on the electronic properties and di§er considerably for the various alkali metal concentrations in A 3 C 60 and A 4 C 60 .247 The 1CNMR spin–lattice relaxation rate is temperature dependent as pressure increases;248 this behaviour is interpreted in terms of one channel owing to excited intermolecular triplet states above the Jahn–Teller ground state plus another one related to electron–hole excitations through an indirect band gap.High resolution 13CNMR spectra of A 6 C 60 (A\K Rb and Cs) and Ba 3 C 60 reveals isotropic lines in each of the saturated alkali metal compounds consistent with orientationally ordered [60]fullerene molecules leading to three non-equivalent carbon sites; in addition an isotropic line at around 156ppm in each of the compounds investigated was identifi- ed.249 Electron energy-loss spectra of A 4 C 60 (A\Na K and Cs) reveals a strong splitting of the electronic states near the Fermi level in comparison with K 3 C 60 or K 6 C 60 which indicates a lowering of the high degeneracy of the [60]fullerene molecular orbitals because of broken symmetry ground states and/or the opening of a correlation gap in these systems.250 Ab initio calculations and EELS suggest the origin of the anomalous trend in the charge transfer from sodium to [60]fullerene in NaxC 60 (x[6) lies in the formation of interstitial states that trap part of the excess electrons and that are neither [60] fullerene or sodium derived.251 A system incorporating electron–electron repulsion and phonon mediated attraction in the adiabatic region at half-filling has been used to explain the anomalous trend in fulleride superconductors.252 The non-adiabatic polaron theory of superconductivity taking into account the polaron band narrowing and realistic electron–phonon and Coulomb interactions has been applied to 626 P.R.Birkett AxC 60 .253 A pairing mechanism mediated by high-frequency intramolecular phonon modes is suggested to explain the superconductivity ofK 3 C 60 or Rb 3 C 60 by analysis of the electrodynamic response in the superconducting ground state within the Eliashberg electron–phonon theory of superconductivity.254 A simplified BCS-like theory is used to understand phonon-mediated superconductivity in A 3 C 60 highlighting the higher T# caused by intramolecular vibrations and which is related to the existance of an alkali metal cation on the tetrahedral site surrounded by four nearest neighbour [60]fullerene molecules.255 Sodium-reduced [60]fullerene incorporated into interlayers of hydrocalcite exhibits strong photoluminescence; the phenomenon is caused by the interaction between the alternating layers of clay and fulleride ions which alters the photophysical properties of [60]fullerene and relaxes the electron transition inhibition.256 Solid-state electrochemical cells have been prepared in situ by the intercalation of Li` or K` into vacuum evaporated thin films of [60]fullerene under constant current conditions up to a maximum stoichiometry of Li 12 C 60 and K 6 C 60 .257 Fullerene films and scanning-tunnelling microscopy studies Stable Langmuir monolayers of the methanofullerene,C 60 [C(CO 2 H) 2 ] are formed on water and solutions containing Ca2` or Cd2` ions also multilayers up to ten monolayers thick are formed by transfer onto quartz or silicon substrates.258 Self-organised multibilayer membrane films of a [60]fullerene derivative bearing a triple-chain lipid moiety have been prepared; the films possess phase transitions which influence fullerene electronic properties.259 The alternative approach of employing interfacial hydrogen bonding has been successfully employed to prepare self-assembled monolayers of a [60]fullerene–crown ether derivative on a gold surface functionalised with ammonium-terminated alkanethiolate.260 Despite their lack of distinct amphiphilic character polyamino derivatives of [60]fullerene form homogeneous monomolecular layers at air/water interfaces.261 Langmuir–Blodgett films of [60]fullerene and cadmium arachidate have been exposed to hydrogen sulfide gas; some disruption of the film occurs on conversion of the cadmium present to cadmium sulfide with the planes of the Cd2` being drawn into CdS nanoparticles.262 Thin films of [60]fullerene derivatives containing silicon alkoxide functionalities have been prepared via the sol–gel method and show that highly homogeneous glasses can be obtained with large fullerene concentrations.263 There has been considerable interest in the use of scanning-tunnelling microscopy (STM) to analyse fullerene film growth and adsorption on various semiconductor and metal surfaces.264 A Si(111) 7]7 surface can be terminated with a monolayer of [60]fullerene;265 individual [60]fullerene molecules can be displaced using the tip of an ST microscope at room temperature.266 Similarly [60]fullerene can be chemisorbed on Si (100) 2]1267 and the intramolecular features of individual [60]fullerene molecules observed.268 [60]Fullerene molecules have also been studied by STM on Ge (111) 2]8,269 GeS (001),270 Al (111) 6]6,271 Cu (111) 1]1,272 Ag (111),272 TiO 2 (100) 1]3273 and Au (111) 23]I3 where the adsorption of [60]fullerene at three temperatures 295 80 and 30 K was studied.274 At 295K the [60]fullerene molecules only absorb at the step edges at 80K the fullerene molecules absorb on the terraces but are not stable and at 30K the [60]fullerene molecule are trapped especially at the elbow positions of the herringbone structure of the Au (111) sur- 627 Fullerene chemistry face.274 A theoretical simulation of STM images of [60]fullerene on Si (100) 2]1 surface reproduced excellently the internal stripe pattern observed experimentally;275 similarly the images of [60]fullerenes on Si (111) 7]7 are reproduced well.276 STMstudies of the endohedral metallofullereneNd@C 82 have also been completed; small crystalline islands are formed which can be broken up by the STM tip when the tip to island bias voltage exceeds a critical value.277 Electron microscopy has been applied to the study of isolated molecules of Gd@C 82 on MgO(001) films; the molecules are detected as elliptic rings with some contrast due to a Gd atom inside.278 Electron di§raction studies of [60]fullerene thin films on mica (001) substrates reveal ordered fullerene layers.279 Niobium and silver films have been prepared with [60] fullerene as substrate; resistance increase is found for the Ag/C 60 system.280 It has been shown that photoelectron di§raction patterns from monolayer [60]fullerene films are directly related to the intramolecular structure of [60]fullerene allowing a direct and unambiguous identification of the molecular orientation of the adsorped fullerenes with respect to the substrate.281 Atomic force microscopy reveals that there is lamellar growth clearly evident on the surfaces of the single crystals of [70]fullerene.282 5 Carbon nanotubes A review of carbon nanotubes has been published.283 Aligned carbon nanotubes 50 km long with spacings between the tubes of 100nm have been synthesised by chemical vapour deposition catalysed by iron nanoparticles embedded in mesoporous silica.284 Yields of more than 70% of single-wall nanotubes which self-organise into metallic ‘ropes’ are produced by condensation of a laser-vaporised carbon–nickel– cobalt mixture the formation of the (10 10) tube with C 5v symmetry is strongly favoured.285 Transition metals supported on zeolites have also been used as catalysts in the decomposition of unsaturated hydrocarbons for the production of nanotubes.286 Methods for the production of nanotubes single- and multi-walled and other carbon particles include the evaporation of carbon in the presence of helium and other elements.287 Fullerene soots formed by the evaporation of carbon have been analysed and found to contain networks of encapsulated aggregates of defective carbon ‘onions’ which do not have all valencies satisfied.288 A range of fullerenerelated graphitic sheet structures have been prepared and good agreement is found with molecular simulation studies based on such graphene surfaces.289 Solid spheres of layered graphitic flakes are formed by using a mixed-valent oxide catalytic carbonisation and natural gas.290 Composite graphite and metal rods (Co Fe Ni and Pt) catalyse the production of single-walled tubes when vaporised.291 Hexagaonal boron nitride and tantalum produce new graphite-like materials which have a B:N ratio of approximately 1 1.292 Only open carbon nanotubes with a diameter of four nm or more are filled with silver nitrate utilising capillary forces; chains of silver nanobeads separated by highpressure gas pockets are formed upon the decomposition of the silver nitrate within the tubes.293 Material deposited on the outside of tubes during filling is removed from the outside of rhodium trichloride filled carbon nanotubes by washing with the reversed micelle mixture dodecylammonium propionate (dap)-solubilised water in benzene (Fig.7).294 Treatment of the tubes with rhodium trichloride and H 2 produces 628 P.R. Birkett Fig. 7 TEM micrograph of rhodium-filled nanotubes after the removal of the externally located rhodium particles by treatment with a reverse micellar solution. (Reproduced by permission from Chem. Commun. 1996 2673.) nanotubes filled with discrete crystals of rhodium metal.294 Continuous and crystallographically single crystals of molybdenum oxide (MoO 2 ) inside carbon nanotubes are formed by filling opened tubes with molten MoO 3 which it is proposed is reduced to MoO 2 .295 Similar nanoscale encapsulation of molybdenum carbide in carbon clusters has been reported,296 as has encapsulation of cobalt particles.297 The functionalisation of carbon nanotubes by reaction with H 2 SO 4 – HNO 3 is reported to produce highly functionalised tubes with SO 3 H OH and other groups present.298 Individual carbon nanotubes are estimated to have an exceptionally high Young’s modulus in the terapascal (TPa) range by measuring the amplitude of their intrinsic thermal vibrations in a transmission electron microscope.299 A realistic many-body potential continuum shell model predicts that nanotubes subjected to large deformations reversibly switch into di§erent morphological patterns.300 Four-probe measurements on single nanotubes show that each multi-shell tube has unique conductivity properties; both metallic and non-metallic behaviour is observed.301 Sharp jumps in conductivity are found as the temperature is varied and there is a much greater di§erence between the electrical properties of each nanotube than expected.301 Another study of the resistivities of carbon nanotubes shows that the most structurally perfect tubes have resistivities an order of magnitude lower than those previously found and that defects in the structure of the tube cause substantial increases in resistivity.302A strong negative magneto-resistance and a fall in resistance with increasing temperature has been reported in a two-probe measurement of conductance of a non-annealed tube.303 A metallic carbon nanotube (5,5) with a diameter of 7Å has been estimated to have a T# value of 1.5]104 K.304 A computer model of a nanotube which has a metallic section in which electrons can move around freely and a semi-conductor like section has been developed.305 Heating of carbon ‘onions’ to 700 °C and irradiating with electrons transforms the core of the onion to diamond.306 629 Fullerene chemistry A single carbon nanotube has been attached to the tip of the scanning force microscope allowing access to deeper recesses of surface structure;307 similarly a [60] fullerene adsorbed STM tip has been used to image the threefold symmetric electron scattering from point defects on a graphite surface which is not possible with a bare metal tip.308 References 1 (a) P.Ball Nature (London) 1996 383 561; (b) P. 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ISSN:0260-1818    

DOI:10.1039/ic093611

出版商:RSC    

年代:1997

数据来源: RSC