7 Transition-metal Chemistry By J. R. DILWORTH G. J. LEIGH and R. L. RICHARDS ARC Unit of Nitrogen Fixation University of Sussex Falmer Sussex BN 1 9QJ 1 Groups IV and V Titanium Zirconium and Hafnium;Vanadium Niobium and Tantalum In contrast to the explosion of organometallic and complex chemistry which has been concentrated at the right-hand end of the Transition Series the left-hand end has been somewhat neglected and much of its inorganic chemistry has proceeded along well-tried classical paths involving such areas as phase-studies and formation of polymeric 0x0-species in solution. Of late however there has been greater emphasis on extending to Groups IV and V the techniques and ideas established with for example the platinum metals and consequently new insights are being obtained.The elements of Groups IV and V are electron-deficient in their complexes as judged by the 18-electron rule. Their chemistry which shows a preference for higher rather than lower oxidation states often belies this. Co-ordination number 5 (as in [TiClJ and [TiBrJ) is not usual for titanium(Iv) but has been observed in a dichloromethane solution containing TiC1 or TiBr and the appropriate halide.' In the solid state these ions are dimerized through halogen bridges. Co-ordination number 6 is of course common but is not without surprises. The anion in [AsPh,] [Ta(benzenedithiolate),l exhibits trigonal-prismatic co-ordination,2 but whereas the niobium analogue is regular one of the dithiolates in the tantalum complex is twisted about the two-fold axis by ca.12",and the other two are bent about the S-S axis. No explanation is forthcoming. Co-ordination number 7 has been reported3 in complexes such as [TaCl,L2] (L =NN'-dicyclohexylacetamidinate),which form distorted pentagonal bipyramids with halides in the axial position. However co-ordination number 8 is apparently much more common but several stereochemistries appear possible. The complex K,[Nb(CN),],2H20 is apparently isomorphous with its molyb- denum analogue and thus [Nb(CN),I4- is probably dodecahedral. This is confirmed by the e.s.r. spectrum which also is interpreted as showing that in solution in glycerol the ion takes on an antiprismatic c~nfiguration.~ The antiprismatic structure has now been identified for the first time in a solid of the type [M(LL'),] for M = Nb and LL' =2,2,6,6-tetrame thylheptane-3,5 -dionate .5 Another variant has been recog- nized in the complex [{Nb(C,H,0,),}20(HCl),],MeCN.6The tropolonate ligand is C.S. Creaser and J. A. Creighton J.C.S. Dalton 1975 1402. J. L. Martin and J. Takats Inorg. Chem. 1975 14 1358. M. G. B. Drew and J. D. Wilkins J.C.S. Dalton 1975 261 1. P. M. Kiernan and W. P. Griffith J.C.S. Dalton 1975 2489. T. J. hnnavaia B. L. Barnett G. Podolsky and A. Tulinsky J. Amer. Chem. Soc. 1975,97 2712. 149 J. R.Dilworth G.J. Leigh and R. L. Richards found to have a very short bite [2.43(2) A compared with a previous shortest bite of 2.490(6)A]. This may be associated with the very short interligand distances observed but the 'NbOs' nucleus is best described as 'an irregular bicapped trigonal It is likely however that the dodecahedral structure as evidenced by [NbX,(diarsine),] (X = C1 or Br)7 and [MCl,(diar~ine),]',~ is the most common.It is evident that the various structures available for eight co-ordination are not widely separated in energy so that non-rigidity is common. The complex [Zr(acac),(NO,),] (acac =acetylacetonate) exhibits a distorted dodecahedral co- ordination in the solid state,' and the reasons for the particular stereoisomer which is observed have been discussed. In solution the number of co-ordinating groups does not change but the complex is stereochemically non-rigid on the n.m.r. timescale down to -130 "C with a coalescence temperature of -144 "C.The complex ion [Ta(S,CNMe,),]' is also dodecahedral in the solid state." However in solution the coalescence temperature is -62 "C whereas the isoelectric [M(S2CNEt2),] (M = Ti Zr or Nb) is non-rigid down to at least -140 "C. The reasons for these differences are not evident. There have also been developments in classes of compounds which have been known for some time. Thus tetrahedral titanium has been resolved for the first time in e.g. [(n-CsH4CHMePh)(?r-CSHs)PhTiCI]"and [(?r-CSH4CMe2Ph)-(rr-C5Hs)TiC1(OPh)]12 (see also Chapter 8 p. 192 Scheme 5). The complexes [M(BHJ4] (M =Zr or Hf) can be considered as having the metal in a co-ordination number of 12 since each borohydride ion is bonded to the metal uia a triple- hydrogen bridge.It was suggested some time ago that the borohydride group in these systems can be regarded as a three-electron donor (cf.allyl) and thus the compounds are 16-electron species. However a simple group-theoretical argument has been held to show that the borohydride group is in fact a 3.5-electron donor.13 It is not easy to envisage what this means particularly as there are no geometrical conse- quences (cf.NO) of this assignment. However the arguments are based upon B-H bonding electrons being involved in linkage to the metal and no others. A Raman and i.r. study of [Hf(BH,),] and [Hf(BD,),] suggests that these molecules contain a significant amount of direct Hf-B b~nding.'~ This alone casts doubt on the concept of a ligand donating specific numbers of electrons to an acceptor.A. R. Davis and F. B. Einstein Inorg. Chem. 1975,14 3030. 'D. L. Kepert and K. R. Trigwell J.C.S. Dulron 1975 1903. * J. C. Dewan D. L. Kepert C. L. Raston and A. H. White J.C.S. Dulron 1975 2031. V. W. Day and R. C. Fay J. Amer. Chem. SOC.,1975,97,5136. lo R. C. Fay D. F. Lewis and J. R. Weir J. Amer. Chem. SOC.,1975,97,7179. l1 C. Moise J. C. Leblanc and J. Tirouflet J. Amer. Chem. Soc. 1975,97,6272. 12 A Dormond J. Tironflet and F. Le Moigne J. Orgunometallic Chem. 1975,101 71. 13 A. Davison and S. S. Wreford Znorg. Chem. 1975 14 703. l4 T. A. Keiderling W.T. Wozniak R.S. Gay D. Jarkowitz E. R. Bernstein S. J. Lippard andT. G. Spiro Znorg. Chem. 1975,14 576. Transition -metal Chemistry 151 Complexes such as [(n-C,H,),Zr(BH,),] contain double-hydrogen bridges.However there is a rapid exchange of hydrogens between the B-H bonds and the C5H5 rings. Methylene intermediates such as (1) and (2) have been proposed to explain this e~change.'~ It is likely that this kind of H-transfer from the ring is common in (n-cyclopentadieny1)-complexesof Groups IV and V. An X-ray photoelectron spectroscopic study of volatile vanadium compounds has been reported.'6a It is calculated that even in VF the positive charge on the vanadium is not much greater than one unit and in VCl it is considerably less. In [v(co),] the carbon monoxide is a nett electron acceptor. All this accords with a growing amount of data gathered from other elements. A study of some complexes of pyridine-2,6-dicarboxylicacid17 gave no indication of the formal oxidation state of the vanadium.This also accords with more general experience. A U.V. photoelectron study of [M(NMe,),] (M =Ti Zr Hf V or a typical element of Group IV) as well as of [W(NMe,),] has shown that except for M = V the first band($ arises from MO's which are linear combinations of nitrogen lone-pair AO's.16* StudiesI8 of vanadium tetraphenylporphyrin derivatives have shown that sub- stituents in the phenyl ring have little effect on equilibrium constants or e.s.r. parameters. This is in contrast to the nickel analogues and the reason is not clear. A detailed preparative investigation of vanadium nitrosyls has been reported. l9 This is not a trivial matter because NO tends to attack both metal and certain ligands (such as phosphines) yielding 0x0-complexes and ligand oxides.Van-adium(1v) chloride reacts with NO in carbon tetrachloride at 20°C to yield [V(NO),Cl,] possibly via [V(NO)Cl,]. The tris(nitrosy1) reacts with hard nitrogen- and oxygen-bases L to yield [V(NO)Cl,L,] which have v(N0) at ca. 1650cm-'. Triphenylphosphine oxide in benzene yields [V(NO)(PPh,O),CI]Cl and triphenyl- phosphine in chloroform produces [VOCl,(PPh,O),]. Bis(dipheny1phos-phino)ethane does not react with [V(NO),Cl,] because it is suggested it is too soft. Groups IV and *V show a distinct preference for harder rather than softer ligands and this can even affect reaction mechanisms. Thus in the equilibrium (1)(M=Nb or Ta; X =F C1 or Br) it is found that the mechanism is primarily a dissociative one for L =Me,O or Et,O and associative for L =Me,S Me,Se or Me,Te.,' [Mx,L]+L* * [Mx,L*]+L (1) The compounds MCl (M =Ti or V) and MCl (M =Nb or Ta) form simple adducts with C6HSCN.With acetonitrile however NbCl and TaCl yield amongst other CI 2-I C1/&] Me C=N-Nb-NCMe (3) l5 T. J. Marks and J. R. Kolb J. Amer. Chem. SOC.,1975,97 3397. l6 (a)R. R. Rietz T. F. Schaaf and W. L. Jolly Inorg. Chem. 1975 14 2818; (6) S. G. Gibbins M. F. Lappert J. B. Pedley and G. J. Sharp J.C.S. Dalton 1975 72. D. L. Hoof and R. A. Walton Inorg. Chim. Acta 1975 12 71. 1s F. A. Walker E. Hui and J. M. Walker J. Amer. Chem. SOC.,1975,97 2390. 19 W. Beck H. G. Fick K. Lottes and K. H. Schrnidtner 2.anorg. Chem. 1975,416,97.20 R. Good and E. Merbach Znorg. Chem. 1975,97 1030. 152 J. R. Dilworth G.J. Leigh and R.L. Richards compounds dinuclear complexes (3)., This is presumably a consequence of the strong Lewis acid character of the halides. Methyl isocyanide has been found to insert into the metal-chlorine bonds of MCl (M =V or Ti) and MCl (M =Ti Zr or Hf) to yield derivatives containing the -CCl=NR grouping., Titanium(1v) chloride behaves as a Lewis acid with the base [Pt(PPh,),] which forms [(TiCl,),{TiCl,(PPh,)),Pt] and then this reacts further with triphenylphosphine to yield [(TiCl, PPh,),Pt]. The adduct [(TiCl, PPh,),Pt] was also described., Titanium(1v) nitrate is a strong oxidizing agent which can nitrate aromatics at room temperature. Its electronic structure has been correlated with its observed electron deficiency., Pentakis(NN-dimethylcarbamato)niobium(v) undergoes stepwise and facile exchange with gaseous carbon dioxide.26 The complex itself is eight-co-ordinate with two unidentate and three bidentate dithiocarbamates and it is also formed rapidly from [Nb(NMe,),] and CO,.The COz exchange is believed to be due to the extrusion of CO to form a tetrakis(carbamato)amido-complex which has been detected in solution and which reacts with CO to reform the pentakis(carbamat0)- complex. The mechanisms of rearrangement of chelate complexes have received attention during the past year. Complexes [Ti(dik),(NCO),] and [Ti(dik),(NCS),] (dik = RCOCHCOR; R=Me or But) have been described in considerable detail; they invariably have a cis-c~nfiguration.~~ N.m.r.studies indicate that the alkyl groups of the diketonate are exchanging by a process which does not involve rupture of any of the metal-ligand bonds. A similar exchange has been reported to occur in cis-[Ti(a~etylacetonate),(OPh),].~~ This is consistent with a body of older data. It has now been pointed that because the rates for methyl interchange of the acetylacetonate groups of [Ti(a~etylacetonate)~(OCHMe,),]are the same as those for the methyl interchange of the isopropoxy-groups then the mechanism of interchange must involve inversion of the helicity of the chelate rings. This inference- concerning mechanism is confirmed by another n.m.r. study2” which also agrees with earlier work in suggesting that bond rupture should not be involved in the interchanges.The structures of some compounds have also been determined by less usual methods. Thus comparison of the diffusion coefficients of NbC1 and NbCl and of ZrC1 and NbCl leads to the conclusion that NbCl is a dimer in the gas phase.,’ An n.q.r. study of MX (M =Nb or Ta; X = F C1 or Br) shows that NbF is a tetramer.31 21 J. D. Wilkins J. Organometallic Chem. 1975,92 27. 22 P. A. Finn M. S. King P. A. Kilty and R. E. McCarley J. Amer. Chem. Soc. 1975 97 220. 23 B.Crociani M. Nicolini and R. L. Richards J. Organometallic Chem. 1975,101 c1. 24 J. F. Plummer and E. P. Schram Inorg. Chem. 1975,14 1505. 25 C. D. Garner I. H. Hillier and M. F. Guest J.C.S. Daffon,1975 1934. 26 M. H. Chisholm and M.Extine J. Amer. Chem. Soc. 1975,97 1623. 27 A. F. Lindmark and R. C. Fay Inorg. Chem. 1975,14 282. 28 J. F. Harrod and K. R. Taylor Inorg. Chem. 1975,14 1541. 29 (a)P. Finocchiaro J. Amer. Chem. Soc. 1975 97 4443; (b) N. Baggett D. S. P. Poolton and W. B. Johnson J.CS. Chem. Comm 1975 239. 30 A. D. Westland 2.anorg. Chem 1975,414 284. 31 G. K. Sernin S. L. Kuznetsov I. M. Alimov T. L. Khotsianova E. V. Bryukhova L. A. Nisselson and K. Tretyakova Inorg. aim. Acta 1975 13 181. Transition -metal Chemistry 153 An electron diffraction study of VOC13 shows it to be roughly tetrahedral.32 The penta(thi0cyanates) of niobium and tantalum are dimer~.~~ The structures of metal alkoxides have long been intriguing and a considerable amount of work has been carried out omnew alkoxides.Thus the isopropoxides [MM’(OPr’),] and [MM‘,(OPT~)~~] (M =Nb or Ta; M’ =Ga or Al)have been charac- terized and the structures (4) and (5) A mixed alkoxide containing two different transition elements has been reported for the first time it is “bTa(OMe),ol(6). Me Me Me RR 6 8 OR Me0 0 OMe O O\ ,/OR Ro\ ,/ \I/ \ / \l/\l/ Ro \I/ MM M M M‘ Nb Ta /I\ / \ / \/I\ / \ /!\/I\ ROoO OR RO OR Me0 0 OMe R R Me Me $e The potentially novel allyloxides of titanium niobium and tantalum have been reported but they appear to be normal alkoxide dimer~.~~ Alkoxide dimers such as [Nb,(OMe),,] form adducts [Nb(OMe),L] with a variety of bases L where L can be a primary or secondary amine ammonia an amine oxide or a phosphine ~xide.~’ Compounds L such as tertiary amines ethers sulphides phosphates nitriles or sulphoxides do not form adducts and the overall pattern of behaviour of the various ligands L does not correlate with hard-soft classifications or ideas concerning 7r-bonding.Steric factors may be overriding. The tantalum complexes have higher formation constants than their niobium analogue^.^' There has been considerable activity in the classical areas of 0x0-derivatives and 0x0-polyanions. Vanadium bis(metaphosphate) and tris(metaph0sphate) have been prepared as crystals and their structures inferred.38 The former probably contains vanadium(1v) co-ordinated by oxygen atoms in a distorted octahedron aligned along a crystal axis and with the chains of octahedra linked by metaphosphate chains.The tris(metaphosphate) also has octahedrally co-ordinated vanadium. New polyvana- dates for example Ba,VSi,O, Ba,V,O, Ba6V6014 and Sr6V6011 have been ~haracterized.~~ The preparation and thermal decomposition of Nb3O7Cl have been de~cribed.~’ The constitution and interconversions of the polyvanadates have received considerable attention. The nature of decavanadates in the solid state and in acidic and basic conditions has been studied. The i.r. spectra of solid state and acidic solutions show no sign of hexavanadate formation and are consistent with the presence of [v100~8]6-only or of some protonated species immediately derivable from it. 41 The n.m.r. data can also be rationalized on this basis. Base titration and 32 T.Karakida and K. Kuchitsu Znorg. Chimica Am 1975 13 113. 33 H. Bohland and E. Harke Z. unorg. Chem. 1975 413 102. 34 S.Govil P. N. Kapoor and R. C. Mehrotra Inorg. Chim. Actu 1975,15,43. 3s L.G. Hubert-Pfalzgraf and J. G. Riess Znorg. Chem. 1975,14,2854. P. N. Kapoor S. K. Mehrotra R. B. King and K. C. Nainan Znorg. Chim. Acta 1975 12 273. 37 L. G. Hubert-F’falzgaf,Znorg. Chim. Acta 1975 12 229. 38 B. C.Tofield G. R. Crane G. A. Pasteur and R. C. Sherwood J.C.S. Dalton 1975 1806. 39 A.Feltz S. Schmalfuss H. Langbein and M. Tietz 2. anorg. Chem. 1975,‘417, 125. 40 H. Kodama and M. Goto Z. anorg. Chem. 1975,415 185. 41 F.Corigliani and S. Di Pasquale Inorg. Chim. Acta 1975 12 99. 154 J. R. Dilworth G. J. Leigh and R.L.Richards extraction into non-aqueous solution allowed4' the identification of a series of protonated decavanadates [H3V10028]3- [HzVlo02,]4- and [HVlo028]5- as well as [VloO,,]"-. In basic solution decavanadate decomposes to form [VO,]'- and this decomposition has been shown to proceed via both base-dependent and base- independent paths.43 The base-dependent path involves a reactive alkali-metal cation decavanadate species. Basic solutions of [VO]" have been studied by a variety of techniq~es.~~ The ion [VO(OH),]- of uncertain degree of aquation is predominant and higher oligomers are unimportant. In fact the aquated ion is likely to be [VO(OH),(H,O),]- on the basis of optical and e.s.r. spectra and related to [VO(H,O),]" by simple protonation. 2 Group VI Chromium Molybdenum and Tungsten Complexes with Metal-Metal Bonds.-A recurrent theme in the chemistry of this triad has been the chemistry of derivatives with metal-metal multiple bonds.A comprehensive review of the subject to the end of 1974 has been published.45 Calculations by the SCF scattered-wave Xa method for [Mo2Cls14- ion4" provide striking confirmation for Cotton's original proposals on the nature of the metal- metal quadruple bond. A set of Mo-Mo bonding orbitals of predominantly metal d-character in the order (of increasing energy) a,T,S were found as required for the a,T,and 6 overlap model. An empty 8" orbital lies just above the S orbital and the peak observed at ca. 19 000 cm-' in the electronic spectrum of [Mo,C~,]~- ions can now be assigned to a dipole-allowed S +S" transition.Improved resonance Raman spectra of [Mo~CI,]~-ions with a range of counter-ions have been together with their electronic spectra. The technique involves irradiation with an exciting frequency within the contour of an allowed electronic transition and causes enormous enhancement of bands due to the fundamental metal-metal stretching vibration vl at 346 cm-'. Overtones of up to 11vl are observed for the dicaesium salt. Solution of K4[Mo2Cl,] in O.1M-HSO,CF and addition of K2S04 gives pink K4[Mo,(S04),],2H20 which on attempted recrystallization also forms lavender crystals of K,[Mo,(SO~)~],~.~H,O.~~ Both complexes have the s!me structural skeleton (7) with Mo-Mo bond lengths of 2.1 ll(1) and 2.164(2) A respectively and are related by an ElI2redox potential of 0.22V us.SCE. The complex [Mo,{PhC(NPh),},] prepared by heating [Mo(CO),] with NN'-diphenyl-benzamidine has a structure analogous to the acetates [Mo,(O,CR),] with the amidine groups bridging the two rn01ybdenums.~~ Attempts to prepare the chromium and tungsten analogues gave uncharacterized products appearing to contain arene- tricarbonyl moieties. 42 F. Corigliani and S. Di Pasquale Znorg. Chim. Acta 1975 12 102. 43 D. M. Druskovich and D. L. Kepert J. C. S. Dalton 1975 947. 44 M. M. Iannuzzi and P. H. Rieger Znorg. Chem. 1975,14 2895. 45 F. A. Cotton Chem. SOC.Rev. 1975,4 27. 46 J. G. Norman and H. J. Kolari J. Amer. Chem. SOC.,1975 97 33. 47 R. J. H. Clark and M. L. Franks J. Amer.Chem. SOC.,1975,97 2691. 48 A. R. Bowen and H. Taube Znorg. Chern.,1974,13,2245; F. A. Cotton B. A. Frenz E. Pedersen and T. R. Webb Znorg. Chem. 1975,14,391. 49 F. A. Cotton T. Inglis M. Kilner and T. R. Webb Znorg. Chem. 1975 14 2023. Transition -metal Chemistry Following last year's report" of the preparation of [CrMo(O,CMe),] [MoW(O,CBu'),I]'(8) has been prepared by iodination of benzene solutions of an inseparable mixture of [Mo,(O,CBu'),] and [MOW(O,CBU'),].~~ A crystal structure of an acetonitrile solvate of cation (8)showed that the iodine is bonded exclusively to the tungsten. Reduction with zinc in acetonitrile gave pure samples of the uncharged derivative [MoW(O,CBu'),]. 0' s 'o (7) The reaction between [WCl,(OEt),] and three equivalents of LiNMe gives an inseparable 1:2 mixture of [W(NMe,),] and dimeric [W,(NMe2)6].52 An X-ray crystal structure of this mixture confirmed the presence of a triple bond between the tungstens of the dimer with an W-W bond-length of 2.294 A.The structure and preparation of the molybdenum analogue from MoCl and LiNMe were reported during 1974.53Use of the sterically more demanding LiNEt favours the formation of dimer relative to monomer and permits preparation of pure [W,(NEt2)6],54 which serves as a useful starting point for the synthesis of a range of metal-metal bonded derivatives (Scheme 1). [W,(OSiMe,) J,2Et,NH % [W2(OSiMe3),I Me,SiOHy cs [W(S,CNEt,)3] [W2(NEt2),l ,plOH co [W,(OBu'),] -$ [W,(~B~'),(~,C~BU')~] Scheme 1 Two groups of worker^^^*^^ have studied the interesting equilibrium between a formally triple and a formally single metal-metal bond (see Chapter 8 p.196). A surprising feature of the X-ray crystal structure of [Cp,Mo,(CO),] is the near linearity of the Cp-Mo-Mo-Cp system; the complexes [Cp2M,(CO),] (M = Cr or Mo) have pronounced M-M-Cp angles.57 Nitrogen Fixation and other Reactions of Biological Significance.-Although the role of molybdenum in biological systems is still largely unknown there is great current interest in reactions catalysed by molybdoenzymes particularly nitrogen 50 C. D. Garner and R. G. Senior J.C.S. Chem. Comm. 1974 586. s1 V. Katoric J. L. Templeton R. J. Hexmeier and R. E. McCarley J. Amer. Gem. SOC.,1975,97,5300. 52 F. A. Cotton B.R. Stults J. M. Troup M. H. Chisholm and M. Extine J. Amer. Chem. Soc. 1975,97 1242. 53 F. A. Cotton B. A. Frenz L. Shive M. H. Chisholm and W. Reichert J.C.S. Chem. Comm. 1974,480. 54 M. H. Chisholm and M. Extine J. Amer. Chem. Soc. 1975,97,5625. 55 D. S. Ginley and M. S. Wrighton J. Amer. Chem. Soc. 1975,97 3535. 56 R. J. Klinger W. Butler and M. D. Curtis J. Amer. Chem. Soc. 1975,97 3534. 57 R. D. Adams D. M. Collins andF. A. Cotton Inorg. Chem. 1974,13,1086; J. Amer. Chem. Soc. 1974 96.749. J.R. Dilworth G. J.Leigh and R. L. Richards fixation. This last subject has been exhaustively reviewed in recent and coverage is here restricted to the 1975 literature. A significant advance in abiological nitrogen fixation has been the of the formation of ammonia from dinitrogen terminally co-ordinated to molybdenum or tungsten.The complex trans-[Mo(N,),(dpe),] (dpe =Ph2PCH2CH2PPh2) under- goes up to 37% conversion of one dinitrogen ligand into ammonia when treated with HBr in N-methylpyrollidone (NMP).59 It was proposed that [MoBr,(NNH,) (dpe),] is formed initially and then converted into a nitride via a dinuclear Mo-N-N-Mo system. However protonation of the nitride complexes [MoNX(dpe),] gives the nitrene complexes [MoX,(NH)(dpe),] and no ammonia,61 and it seems possible that the temperatures required to remove the NMP cause partial replacement of the diphosphines by solvent and degradation of the NNH derivative to ammonia. Treatment of the complexes cis-[M(N,),(PMe,Ph),] (M =Moor W) with sulphuric acid in methanol gives ca.36% conversion of one dinitrogen ligand into ammonia for M =Mo and up to 90% conversion for M = W.60,62 In each case ca. 1mole of dinitrogen is evolved. Although no intermediates could be isolated the use of other acids and tertiary phosphines permits the isolation of intermediates containing NNH and NHNH groups bound to the metal [equations (3) and (4)].63 However the mechanism by which these intermediates produce NH is not yet clear. Since base treatment of [MX,(NNH,)(PMe,Ph),] gives 1.6 moles of NH3 for M = W and only half as much for M =Mo there may well be different mechanisms operative for the two metals. These results clearly suggest that the active site of nitrogenase could comprise a single molybdenum binding site for dinitrogen which is then protonated to NH via NNH and NHNH intermediates with little or no hydrazine formation.However the information currently available about the enzyme does not rule out a two-site mechanism (two Mo’s or one Mo and one Fe) with bridged dinitrogen complexes reducing via bridged di-imide and hydrazine intermediates and work continues on the chemistry.of these types of complex ligand prepared by oxidation of bridging hydrazine derivatives. The N-H protons of [p-N2H2{Cr(C0)5}2] undergo rapid H-D exchange and in the presence of catalytic amounts of base rapid and irreversible disproportionation to N2 and [~-N,H,{CT(CO)~}~] occurs.64 A crystal structure of the THF solvate of [p-N2H2{Cr(C0)5}2] shows the di-imide ligand to have a trans configuration with an N-N bond length of 1.25 A.65[Mo(CO),N,H,] is 58 D.Sellmann Angew. Chem. Znternut. Ed. 1974,13,639. A. D. Allen R. 0.Harris B. R. Loescher J. R. Stevens and R. N. Whiteley Chem. Reu. 1973,73 11. 59 C. R. BrQlet and E. E. Van Tamelen J. Amer. Chem. SOC.,1975 Y7 911. 6o J. Chatt A. J. Pearman and R. L. Richards Nature 1975,253 39. 61 J. Chatt and J. R. Dilworth J.C.S. Chem. Comm. 1975 983. 62 J. Chatt J. Orgunometullic Chem. 1975 100 17. J. Chatt A. J. Pearman and R. L. Richards J. Orgunometullic Chem. 1975,101 C45. 64 D. Sellmann A. Brandl and R. Endell J. Orgunometullic Chem. 1975 90 309. G. Muttner W. Gartzke and K. Allinger J. Orgunometullic Chem. 1975,91,47. Transition -metal Chemistry 157 prepared from [MO(co),] and N2H4 and controlled oxidation yields [p-N,H,{Mo(CO),},] which is less stable than its Cr or W analogues and readily disproportionates to the p-hydrazine derivative and N2.66 The formation of nitrogen-carbon bonds directly from dinitrogen is potentially a reaction of industrial importance and continues to be studied.Photochemical reaction of the dinitrogen complexes [M(N2)2(dpe)2] (M=Mo or W) with alkyl halides RX has been independently studied by two groups68769 and shown to give alkyldiazenido (9) and alkylhydrazido(2 -) (10) complexes interconvertible by acid and base. X-Ray crystal structures of (9; R = C6H1,,X =I M =Mo)~~ and (10; R=Me X=Br M=W)68 both show essentially linear M-N-N systems with M-N bond lengths of 1.95(1) and 1.768(14)& respectively.If the alkylation reaction is carried out in tetrahydrofuran as solvent the tetrahydropyridazine complex (11) is formed and can be isolated at its hydrobromide salt.” The four carbons of the pyridazine ring presumably originate from the THF and the role of the methyl bromide is not clear; it may participate in removal of the THF oxygen as an alcohol. R HR + n / \/ v Y I I (9) Efforts continue to be directed towards the simulation of the action of nitrogenase using systems containing a dimeric molybdenum(v) cysteine complex (12) and a reducing agent and the area has been reviewed.71 Such systems reduce acetylene to ethylene and dinitrogen to ammonia in low yields. Ferredoxin model compounds such as [Fe,S,(SR),]”- (n= 2-4) apparently accelerate the transfer of electrons NC 0 CN \II / /YO\ NC 0 CN fS N = -0ZCCHNH2 I \O CH2S-66 D.SelLnann A. Brandl and R. Endell J. Organometallic Chem. 1975,97 229. 67 J. Chatt G. A. Heath and G. J. Leigh J.C.S. Chem. Comm. 1972 444. 68 A. A. Diamantis,J. Chatt G.J. Leigh and G.A. Heath J. Organometallic Chem. 1975,84 C11; F. F. March R. Mason and K. M. Thomas J. Organometallic Chem. 1975,96 C43. 69 V. W. Day T. A. George and S. D. A. Isbe J. Amer. Chem. SOC.,1975,97,4127. ’0 A. A. Diamantis J. Chatt G. A. Heath and G. J. Leigh J.CS. Chem. Comm. 1975 27. 71 G. N.Schrauzer Angew. Chem. Znternat. Edn. 1975 14 514. J. R.Dilworth G.J. Leigh and R. L. Richards from reductant to molybdenum and improve ethylene yields.72 Adenosine-5- triphosphate (ATP) is a requirement for the enzyme and also stimulates the model system.The ATP is postulated to facilitate removal of OH groups from the monomer (13)[formed from (12) in base] as phosphates and thereby increases the concentration of the active reduced Mo'" species (14). Use of the tetracyano-0x0- Mo complex (15) as an alternative to (12) increases the nitrogen-fixing ability of the model system considerably and yields of up to ca. 0.3 moles of ammonia per mole of molybdenum are Electrochemical reduction of the Mo" cysteine dimer shows that it undergoes a single four-electron reduction step to Mo"' products and it is suggested that a monomeric Mo'" species is the catalytically active species.75 A dextran-bound cysteine polymer (1.05 mmol cysteine per g dextran) forms a molybdenum complex analogous to (12) which shows no evidence for alkaline dissociation into monomers analogous to (13).76 However in the presence of borohydride the supposed complex reduces acetylene ca.30 times faster than (12) possibly because the inert support serves to keep apart reduced catalytically-active species analogous to (14),prevent-ing formation of inactive 0x0-bridged dimers. The nitrate ion is+ quantitatively converted into NO and subsequently nit- rogen(m) (probably NO) by reaction with [MoOCl,L,] or [MoOCl,L]- [L = Yh,PO or (Me,N),PO] or [MoOC~,]~- and this has been advanced as a model for the molybdoenzyme nitrate reducta~e.~' The kinetics of the overall reaction were studied by stopped-flow techniques and are consistent with the mechanism in Scheme 2.77 However unlike the enzyme the system is not catalytic and reduces nitrite quantitatively to NO.Since 0-bonding appears to be a prerequisite for the reduction of nitrate or nitrite it is suggested that the protein stabilizes N-bonding of nitrite by hydrogen-bonding with the oxygens preventing its red~ction.~' c1 0 c1 c1 0 c1 c1 0 c1 \II / L i" L c1 0 c1 \II / LL 0 0 0 I 0 0 Scheme 2 Isocyanides nitriles or acetylenes are also substrates for nitrogenase and the chemistry of these ligands bound to molybdenum has been studied. Both dinitrogen ligands of frans-[Mo(N,>,(dpe),] are displaced by methyl isocyanide to give trans-[Mo(MeNC),(dpe),]. One or both of the ligating isocyanides can be protonated at 72 K.Tan0 and G. N. Schrauzer J. Amer. Chem. SOC.,1975,97 3404. 73 G. N. Schrauzer G. W. Kiefer K. Tano and P. R. Robinson J. Amer. Chem. SOC.,1975,97,6088. 74 G. N. Schrauzer P. R. Robinson E. L. Moorehead and T. M. Vickney J. Amer. Chem. SOC.,1975,97 7069. 75 D. A. Ledwith and F. A. Schultz J. Amer. Chem. SOC.,1975 97 6591. 76 H. Susuki S. Meshitsuka T. Tabashima and M. Ichikawa Chem. Letters 1975 4 285. 77 C. D. Garner M. R. Hyde F. E. Mabbs and V. I. Routledge Nature 1974 252 580. 78 C. D. Garner M. R. Hyde and F. E. Mabbs Nature 1975,253,623. Transition -metal Chemistry nitrogen to give complexes containing the carbyne-like ligands -C-NHR.79 Sub-stituted benzonitriles displace only one dinitrogen from [Mo(N,),(dpe),] to give [Mo(N,)(p-XC,H,CN)(dpe),] (X =NH, MeO Me H C1 or COMe) the latter being useful precursors for the synthesis of derivatives such as [MoCl(N,COPh)- (dpe),] with N-C bonds." Acetylene readily displaces the carbonyl ligands from the complexes [Mo(C0),(S2PPri2),] to give [MoCO(C,H,)(S,PP~~,)~]; but reaction with acid gives only ca.20% yields of ethane from the acetylene." 3 Group VII Manganese Technetium and Rhenium Manganese Porphyrin and Related Complexes.-Although the dioxygen-carrying capabilities of iron and cobalt complexes have been extensively investigated (see pp. 162 and 167) analogous manganese derivatives have only recently been studied in detail. Manganese porphyrin complexes were comprehensively reviewed in 1972.* Manganese haemoglobin (MnHb) does not bind dioxygen reversibly and is irrevers- ibly oxidized to Mn"'Hb,83 and the manganese tetraphenylporphyrin (TPP) system also shows significant differences from comparable iron derivatives.Reduction of [Mn"'Cl(TPP)] with [Cr,(acac),] in toluene gives the purple four-co-ordinate species [Mn(TPP),2toluene]( 16).84 A partial X-ray crystal structure and magnetic measure- ments (peR=6.2 BM) suggest a high-spin configuration with the manganese lying out of the plane of the porphyrin ring. [Mn(TPP)] reacts with an excess of a base such as 2-methylimidazole (2-MeIm) to give [Mn(TPP)(2-MeIm)] with no evidence for a six-co-ordinate species.84 In contrast to its iron analogue [Mn(TPP)( 1-MeIm)] does not react with dioxygen except to undergo very slow oxidation.It is suggested that the six co-ordination required for an 0,-adduct cannot occur unless a transition to a low-spin state occurs which is not possible with the porphyrin ligand system. However [Mn(TPP)] does reversibly form an 0,-adduct at -90 "Cin toluene-THF although this has not yet been fully characterized. 79 J. Chatt A. J. L. Pompeiro R. L. Richards G.M. D. Royston K. W. Muir and R. Walser J.C.S. Chem. Comm. 1975,512. 8o T. Tatsumi M. Hidai and Y. Uchida Inorg. Chem. 1975 14 2530. 81 J. W. McDonald J. L. Corben and W. E. Newton J. Amer. Chem. SOC.,1975,97 1970. 82 L. J. Boucher Coordination Chem. Rev. 1972,7 289. 85 C. Ball R. C. Fisher and B. M. Hoffmann Biochern. Biophys. Res. Comm. 1974,59 146. 84 B.Gonzalez J. Kouba S. Yee C. A. Reed J. K. Kirner and W. R. Scheidt J. Amer. Chem. SOC.,1975 97 3247. J. R. Dilworth G.J. Leigh and R. L. Richards Reaction of [Mn(TPP)(py)] in toluene with 0,at -80 "C produces an apparently identical 0,-adduct of stoicheiometry [Mn(TPP)O,] with displacement of ~yridine.~'E.s.r. spectra of the 0 adduct were interpreted in terms of a manganese with three unpaired electrons suggesting that the complex can formally be rep- resented as [Mn'V(TPP)(0,2-)] with a 'sideways bound' 0,2-ligand.85 The structural distortions exhibited by five- or six-co-ordinate high-spin mangan- ese (111) complexes provide the theme for several X-ray crystal structure determina- tions. The structures of [Mn(N,)(TPP)] and [Mn(N,)(MeOH)(TPP)]86 show that the manganese ion is displaced ca.0.18 Afurther out of the plane in the five-co-ordinate derivative. A comparison of the structures of a number of related five-co-ordinate derivatives suggests that manganese(II1) is displaced by ca. 0.25 Afrom the plane in complexes of sterically non-hindered porphyrins. An even larger displacement (0.343A)occurs in the case of (17) with its less constrained quadridentate ligand system.86 The structure of [Mn(acac),N,] shows that the manganese is in fact six co-ordinate as the azide ligands bridge adjacent [Mn(acac),]+ units producing infinite chains of pseudo-octahedral manganese ions.87 The products of oxidation by dioxygen of five-co-ordinate manganese(rI1) Schiff base complexes such as [Mn(salen)(H,O)]' (18; R = H) have not in the past been well characterized p -peroxo di-p -hydroxo and di-p -ox0 structures having been proposed.88 The situation has been clarified by detailed studies of the oxidation of the more soluble [Mn(Busalen)H,O]ClO (18; R = s-b~tyl).~~ In the presence of base and dioxygen this gives a complex formulated as the manganese(1v) [(Busalen)Mn(p-O),Mn(Busalen)]H,O (19).This is dimeric in CHCI and the low magnetic moment (2.5 BM) is attributed to antiferromagnetic exchange between the manganese ions. A characteristic i.r. band in the region 640-4550 cm-'is assigned to the di-p-oxo bridging system. Protonation of (19) with perchloric acid provides further evidence for the structure as hydrogen peroxide is formed via the di-p- hydroxo species [(Busalen]Mn(p -OH),Mn(B~salen)].~~ The preparation of manganese or iron tetraphenvlporphyrin nitrosyl complexes by direct reaction with nitric oxide is dependent on the presence of secondary amine.Thus [(NO)(amine)Mn(TPP)] is prepared in 80% yield by reaction of [ClMn(TPP)] ~35 C. J. Weschler B. M. Hoffmann and F. Basolo J. Amer. Chem. SOC., 1975,97,5278. 86 V. W. Day B. R. Stults E. L. Tasset R. S. Maranelli and L. J. Boucher Inorg. Nuclear Gem. Letters 1975 11 505. 8' B. R. Stults R. S. Maranelli and V. W. Day Inorg. Chem. 1975 14 722. H. S. Maslen and T. N. Wates J.CS. Chem. Comm. 1973 760; T. Matushita T. Yarno I. Masuda T. Shomo and K. Shinra Bull. Chem. SOC.Japan 1973,46 1712. 89 L. J. Boucher and C. G. Coe Inorg. Chem. 1975,14 1289.Transition -metal Chemistry 161 with NO in CHCI with piperidine present." Since it has now been shown that the same reaction can be achieved with the so-called N202 adducts in reality the N-nitrosohydroxylamines [R'R'NNH,][R'R'NNONO] their intermediate forma- tion is advanced as an explanation for the amine specificity of the direct nitrosation reaction." Complexes with Metal-Metal Multiple Bonds.-The chemistry of complexes with metal-metal bonds has also been a feature of the manganese group and as mentioned previously the literature to the end of 1974 has been comprehensively reviewed.45 The electrochemistry of the octahalogenodimetallates [M2C18]"- (M = Tc or Re; n = 2 or 3) has been reported,92 and the processes [Tc2Cls12-+e -+[Tc,C~,]~- and [Re~C18]~-+e -b [Re2C1,]3- shown to be quasi-reversible with El, potentials (rela- tive to SCE) of 0.140 and -0.840 V respectively.Paramagnetic [Tc,C~,]~- has an e.s.r. spectrum corresponding to one unpaired electron coupled to two 99Tc nuclei with I = 9/2; the corresponding [Re2Cl8l3- anion is too unstable for e.s.r. spectra to be However tertiary phosphine derivatives of both [Re2X813- and [Re2Xs14- anions have been prepared.93 Rhenium(II1) chloride or the anions [Re2Xs12- (X = C1 or Br) react with alkyl or mixed alkyl-aryl tertiary phosphines to give species of stoicheiometry [Re,X,(PR,),] or [Re,X,(PR,),] the extent of reduction depending on the degree of alkyl substitution of the phosphine. Thus MePh,P and EtPh,P give [Re,X,(RPh,P),] and PEt gives [Re,X,(PEt,),].The ditertiary phosphine Ph,PCH,CH,PPh (dpe) does not reduce the [Re2C1,]2- ion and [Re,Cl,(dpe),] is formed,94 and shown by a crystal structure det$rminationg5 to have a di-p-chloro bridge with an Re-Re distance of 3.809(1)A7 too large for any metal-metal interaction. The complex [Re,Cl,(PEt,),] is quasi-reversibly oxidizable electrochemically to [Re,Cl,(PEt,),]+ and [Re2C1,(PEt,),l2' and [Re2(PhC0,),l2' can be reduced to [Re2(PhC02),]+.96 The ex. spectra of the monocations can be qualitatively corre- lated with one-electron energy diagrams based on MO and scattered wave Xa calculations. The sulphato-bridged Na,[Re,(SO,),] can be prepared in 90% yield by reaction of [ReCl8I2- with sulphuric acid and sodium sulphate in diglyme.The structure is essentially similar to that of [Mo2(SO4),I4- (7) with an Re-Re bond length of 2.214(1) k9'Partial replacement of halide ions occurs when [Bu,N],[Re,Cl,] is fused with NN'-diphenylbenzamidine with formation of [Re,Cl,(N,CPh,),]. The amidine ligands bridge the two rheniums and the Re-Re bond length of 2.177 A is the shortest yet 9O P. L. Piciulo G. Rupprecht and W. R. Scheidt J. Amer. Chem. Soc. 1974,% 5293. 91 P. L. Piciulo and W. R. Scheidt Znorg. Nuclear Chem. Letters 1975 11 309. 92 F. A. Cotton and E. Pedersen Znorg. Chem. 1975,14 383. 93 J. R. Ebner and R. A. Walton Znorg. Chem. 1975,14 1987. 94 J. A. Jaecker D. P. Murtha and R. A. Walton Znorg. Chem. Acra 1975,13 21. 95 J. A. Jaecker W. R. Robinson and R.A. Walton J.C.S. Dalton 1975 698. 96 F. A. Cotton and E. Pedersen J. Amer. Chem. SOC. 1975,97 303. 97 F. A. Cotton B. A. Frenz and L. W. Shive Inorg. Chem. 1975,14,649. F. A. Cotton and L. W. Shive Znorg. Chem. 1975 14 2027. J. R. Dilworth,G.J. Leigh,and R.L. Richards 4 Group VIIIA, Iron Ruthenium and Osmium Synthetic Dioxygen Carriers.-A feature of the non-organometallic chemistry of iron has been the development of complexes that have dioxygen-carrying capabilities similar to those of haem proteins. Most of the model systems comprise ferrous iron surrounded by four co-planar nitrogens with a variety of nitrogeneous axial ligands and progress to the end of 1974 has been described in two A major problem with the model systems is the irreversible formation of p-0x0- bridged dimers on treatment with dioxygen.This can be avoided by use of bulky Iigands disposed to one side of the N ligand which prevents the irons getting close enough to form a dimer. Thus the ferrous complex of the 'picket-fence' porphyrin [rneso-tetra(aacua -0-pivalamidophenyl)porphyrin,TpivP] (20) adds two molecules of bases such as 1-methylimidazole (1-MeIm) to give diamagnetic six-co-ordinate derivatives which bind 0 reversibly with displacement of one axial ligand."' Although there were disorder problems an X-ray crystal structure of [Fe(O,) TpivP)(l-MeIm)] showed that the 0 is bound 'end-on' with an Fe-0-0 angle of ca. 136O."' The enthalpy of 0,-binding in the solid state has been determined manometrically102 (AH" =-15.6 kcal mol-') and comparison with natural haem proteins (ox myoglobin AW = -15 kcal mol-'; human myoglobin AW = -13.4 kcal mol-') suggests the protein chains in the natural system do not contribute signific- antly to 0,-binding.C Me,I CMe,I NH-CO CO-NH An alternative to the 'picket-fence' approach has been to use substituted haem derivatives such as (21) where the axial base is attached to a pyrrohaem system with a similar geometry to the proximal histidine in rnyogl~bin.'~~~'~~ These systems bind 0,reversibly and the kinetics of oxygenation can be studied as a function of the axial base and solvent polarity which do not interfere. The rates of oxygenation closely resemble those of myoglobin and the dependence of the 0 'on'-rate on the axial base suggests that the natural system could control the rate of binding of 0 by variation of the basicity of the proximal histidine.'@' Not all systems are based on 99 F.Basolo B. M. Hoffmann and J. A. Ibers Accounts Chem. Res. 1975,8,384. loo T.H.Maugh Science 1975,187 154. J. P. Collman R. R. Gagne C. A. Reed T. R. Halbert G. Lang and W. T. Robinson J. Amer. Chem. Soc. 1975,97 1427 and references therein. lo2 J. P. Collman J. I. Brauman and K. S. Suslick J. Amer. Chem. SOC. 1975,97 7185. lo3 C. K.Chang and T. G. Traylor J. Amer. Chem. SOC.,1973,95,5810. 104 C. K. Chang and T. G. Traylor Roc. Naf. Acad. Sci. U.S. A.,1975,72 1166. Transition -metal Chemistry Me porphyrin ligands and the substituted octa-aza[14]annulene complex (22) also undergoes reversible oxygenation at low temperature.The X-ray crystal structure of (22) shows that the iron lies in a 4.5A deep pocket between the 9,lO- dihydroanthracene rings; the octyl groups are directed away from the iron.lo5 0x0-bridge formation can also be inhibited by oxygenation at low temperatures (-50 to -80 "C). [Fe(TPP)(l-MeIm),] is irreversibly oxidized at 25 "C but functions as an 0,-carrier at -80 "C. Oxygenation is almost complete in methylene chloride but minimal in toluene and the stabilization in polar solvents supports the formal representation of the 0,-adduct as [Fe"'(O,-)(TPP)(l-MeIm)]. Studies of the kinetics of oxygenation show that [Fe(TPP)(l-MeIm)] reacts at about the same rate with 0,and 1-MeIm and the stability of the 0,-adduct is dependent on axial base as for complex (20).1°' Attachment of [Fe"(TPP)] to an inert solid support holds the irons apart and prevents dimer formation.The iron perphyrin is bound to silica via 3-imidazoylpropyl groups and binds 0,reversibly at low temperature. However the 0 is only weakly chemisorbed with a pl/ value of 0.4 Torr at -80 "C,compared with an extrapolated value of 0.14 Torr for human myoglobin at 0 0C.99 Cytochrome P450 contains a haem iron prosthetic group but is functionally more complex than haemoglobin acting as a dioxygen and electron-transport agent and as an oxidation catalyst. Four distinct reaction states can be identified correspond- ing to binding of the oxidizable substrate (sub) adjacent to the iron reduction of the ferric cytochrome Fe"'(cyt) binding of 02,and further reduction with loS R.G. Little J. A. Ibers and J. E. Baldwin J. Amer. Chem. SOC.,1975,97,7049. C. J. Weschler D. L. Anderson and F. Basolo J.C.S. Chem. Comm. 1974,757. lo' C. J. Weschler D. L. Anderson and F. Basolo J. Amer. Gem. Soc. 1975 W,6707. lo* I. C. Gunsalus S. G. Sligar and P. G. Debrinner Biochem. SOC.Trans. 1975,3 821; and preceding papers. 164 J. R. Dilworth G.J. Leigh,and R. L.Richards formation of hydroxylated substrate (sub-OH) and water and regeneration of Fe"'(cyt) (Scheme 3). E.s.r. spectra of the biological system suggest axial ligation by sulphur and studies on model systems have accordingly been based on iron complexes with macrocyclic N,-donor and axial thiolate ligands.[Fe"'(cyt)] 9[Fe"'(sub)(cyt)] 4; [Fe"(sub)(cyt)] 3 [Fe"(sub) (c yt)(02)] 5 [Fe"'(cyt)] A B C D + Sub-OH + H2O Scheme 3 The five-co-ordinate species [Fe(SPh)(TPP)] can be prepared from [{Fe(TPP)},O] and thiophenol and has an e.s.r. spectrum with g values close to those of species B in Scheme 3.l" In the presence of a base such as methylamine or ammonia at low temperatures an e.s.r. spectrum resembling that of species A is observed. An iron protoporphyrin dimethyl ester (PPDME) system forms the analogous [Fe(SC,H4po- NO,)PPDME)] and the X-ray crystal structure shows the iron displaced ca. 0.43 A out of the plane of the ligating porphyrin. Models for species C afid D in Scheme 3 were synthesized by reaction of benzene solutions of [Fe(PPDME)]"' or [Fe(TpivP)]"' with an excess of crown-ether-solubilized thiol which effects reduc- tion to the ferrous state.Under CO a U.V. band at ca. 450nm appears which corresponds to the anomalous Soret band which gives the cytochrome its name. If mercaptan rather than mercaptide is used the Soret band appears at ca. 420 nm. In combination these studies on model systems while not conclusive do offer confir- mation that the ferric species A and B of cytochrome P450 do contain an axial thiolate ligand. However the nature of the diamagnetic 0,-binding species D is not yet clear. Iron-Sulphur Cluster Systems.-Non-haem iron-sulphur proteins are implicated in biological processes as diverse as photosynthesis and nitrogen fixation. They can be classified according to the number of iron atoms present Fe 2Fe 4Fe and 8Fe proteins.X-Ray crystal structures of the 4Fe protein from Chrumatium and the 8Fe protein from P.aerugenes show them to contain one and two [Fe,S,(S-cys),] clusters (S-cys =cysteinyl sulphur) respectively. Redox and spectral measurements show that the 4Fe proteins are predominantly of two types the high potential protein (HiPIP; Eb -+0.35 V) from photosynthetic bacteria and the non-photosynthetic bacterial ferredoxins (Fd;.Eb- -0.4 V). The complexes [Fe,S4(SR),l2- (R =alkyl or aryl) containing structurally and spectrally analogous clusters were synthesized some time ago and work in this area up to the end of 1974 has been re~iewed."~ The synthetic clusters are prepared in high yields from FeCI, thiol and NaHS in the presence of base and an X-ray crystal structure (23) indicates that they are somewhat distorted from cubic symmetry.' l4 Polarography shows that the [Fe,S4(SR),l2- clusters are members of the electron-transfer series [Fe,S,(SR),]"- lo9 J.P. Collman T. N. Sorrell and B. M. Hoffmann J. Amer. Chem. Soc. 1975,97 913. 110 S. Koch S. C. Tang R. H. Holm R. B. Frankel and J. A. Ibers J. Amer. Chem. Soc. 1975,97 917. C. K. Chang and D. Dolphin J. Amer. Chem. Soc. 1975,97 5948. 11* J. P. Collman and T. N. SorrelldJ. Amer. Chem. Soc. 1975 97,4133. 113 R. H. Holm Endeavour 1975,34,1. 114 B. A. Averill T. Herskovitz R. H. Holm and J. A. Ibers J. Amer. Chem. Soc. 1973,95 3523. Transition -metal Chemistry R where n = 1,2,3 or 4.'15 Magnetic and spectral studies indicate a close similarity between these states and those found in the biological systems as indicated by the columns in Scheme 4.However the 3-+2- reduction step for the synthetic clusters is thermodynamically irreversible with E,,2 values considerably larger than for one-electron reduction of the ferredoxins. The water-soluble cluster [Fe,S,(SCH,CH,CO,),]"-can however be reversibly reduced and the potential associated with the one-electron step (-0.58 V us. hydrogen electrode) is close to that of ferredoxins (Fd)."" [Fe,S4(SR),l4-e[Fe4S4(SR),l3-e[Fe,S4(SR),l2-[Fe,S,(SR),]-2 A Fd, -Fdox -Fds-ox A HiPIP,-,, -HIPIP,, S HIPIP, Scheme 4 The structural unit of the Fe-S moiety of the 2Fe proteins has not been established by X-ray diffraction but the synthetic model (24) prepared according to equation (9 has similar spectroscopic properties to the biological system.The [Fe4S,(SR),I2- clusters undergo facile thiol exchange reactions and this provides the basis of a method for the removal of intact iron-sulphur clusters from the protein^."^ Treatment of a 4:l DMSO:H,O solution of the 8Fe ferredoxin protein from Clostridium pasteurianum with a 35-fold excess of thiophenol gives a 95% recovery of the non-haem iron as [Fe,S4(SPh),12-. The high DMSO content of the solution facilitates reaction by unfolding the protein chains. The Fe,S cores of 2Fe-ferredoxins can be similarly extruded using o-xylylenedithiol. However condi- tions must be carefully controlled as the dinuclear complex readily dimerizes to a four-iron cluster particularly at high pH and in the absence of an excess of thiol."* Although Mossbauer e.s.r.and structural studies indicate that the synthetic clusters are good models for ferredoxins the redox potentials are not comparable. These are clearly dependent on the peripheral groups on the clusters in the model systems and probably on the configuration of the peptide chains in the proteins. The B. V. Pamphilis B. A. Averill T. Herskovitz L. Que and R. H. Holm J. Amer. Chem. SOC. 1974,96 4159. 116 R. G.Job and T. C. Bruice Proc. Nat. Acad. Sci. U.S.A. 1975,72,2478. J. J. Mayerle R. B. Frankel R. H. Holm J. A. Ibers W. D. Phillips and J. F. Weiker Roc. Nat. Acad. Sci. U.S.A. 1973,70 2429. 118 L. Que R. H.Holm and L. E. Mortensen J. Amer. Chem. SOC.,1975,97,463. 166 J. R. Dilworth G.J. Leigh and R. L. Richards latter is certainly suggested by the accessibility of an additional super-reduced state HiPIPs-,d on denaturation of the HiPIP protein with DMSO.'l9 Any subtle distor- tions within the clusters caused by variation of the peripheral ligands should be reflected in the Fe-S stretching frequencies within the cluster. These can be enhanced in intensity by use of resonance Raman spectroscopy and some recent results'20 suggest that the overall symmetry of the biological clusters is lower than that of the models. However these differences are apparently too small to detect by X-ray diffraction as the structures of the biological and synthetic clusters are not significantly different.FeCI3 -+ 2 oCH2SH NaHS NaOMe CH2SH Ruthenium Ammine Complexes.-Ruthenium ammine complexes continue to be a source of unusual and interesting chemistry displacement of the water ligand of [RuA5(H20)I2' (A =NH,) providing a readily accessible co-ordination site. It has been known for some time that [RuA,(H20)I2' reacts with N20 to give [RuA5(N2)I2+ via an unstable N20 complex.12' The latter has now been isolated using high pressures of N20 and isotopic labelling using 15NN0 and ",NO permits assign- ment of anion-dependent i.r. bands in the regions 1945-1975 and 869-880 cm-' to the N20 ligand.122 Force-constant calculations are consistent with an 0-bonded N20 ligand. Nitrogen-metal to carbon-metal bonding isomerization has been observed for imidazole bound to [RuA~]~+.'~~ HCN initially binds via nitrogen and then rapidly isomerizes to the carbon-bound species [RuA,{C(H)N}I2'.In the presence of base a proton is lost generating a cyano-complex which expels an NH ligand to give the product ultimately isolated polymeric [{A,Ru(CN)},]. Xanthine derivatives (25) 124 can also bind to [RuA5I2+ or [RuA,],' via N-7 or C-8 the bonding mode being a function of the substitution pattern of the xanthine and pH; C-bonding is favoured at low pH. 125 Electrochemical studies indicate that both N-and C-bonding stabilize Ru" relative to Ru'" probably because both bonding modes transfer more T-electron density from metal to ligand in the Ru" state. 119 R. Cummack Biochem.Biophys. Res. Comm. 1974,58 974. S.-P. W. Tang T. G. Spiro C. Antaraitis T. H. Moss R. H. Holm T. Herskovitz and L. E. Mortensen Biochem. Biophys. Res. Comm. 1975,62,1. lz1 J. N. Armor and H. Taube J. Amer. Chem. SOC.,1971,93,6476. A. A.Diamantis G. J. Sparrow M. R. Snow and T. R. Norman Austral. J. Chem. 1975,28,1231. 123 R. J. Sundberg R. E. Shepherd and H. Taube J. Amer. Chem. Soc. 1972,94,6558. lZ4 S.S.Isred and H. Taube Znorg. Chem. 1975 14 2561. lZs M. J. Clarke and H. Taube J Amer. Chem. SOC. 1975,97 1397. Transition -metal Chemistry One NH ligand of the Ru"' hexammines [Ru&]"can be deprotonated to NH (pK ca. 12.4) which reacts with dioxygen at pH 13 to give [RuA5(N0)l3+ identified by a strong i.r. band at 1908 cm-' [v(NO)]."~ The NH ligand of [RUA,(NH,)]~' is sufficiently nucleophilic to attack carbonyl carbons and reaction with a-diketones such as diacetyl gives di-imine complexes (26).lZ7Aldehydes RCHO react with [Ru&I3' to give high yields of the nitrile complexes [RuA~(NCR)]~+,~~~ previously prepared from [RUA,(H,O)]~+ and 11itri1e.l~~ The mechanism is not yet known but probably does not involve the Ru'" complexes [RuA,(NCR)I3' as these hydrolyse rapidly in base to amide complexes.13o Treatment of [RuA,(N0)I3' with base produces [RUA,(N,)]~' (25%) and cis-and trans-[Ru(OH)A,(NO)]" (19%);the mechanism is believed to involve nucleophilic attack of an NH ligand on the co-ordinated NO as in Scheme 5.I3l Radiolysis of [RuA,(NO)I3' in Bu'OH generates the Ru" alkylnitroso-complex [RUA,{N(O)CH,C(OH)M~,}]~+; the reaction probably proceeds via attack of the 13' radical CH,C(OH)Me on [RuA,(NO)]*' generated by reaction of H atoms and eiq with the Ru"' nitrosyl complex.133 + [RuA5(N0)13 OH- [RuA4(NH 2)(NO)]z' + [A~Ru{ N(O)NH2)RuA4(NO)] + 4 [RuA5(N2)12++ [RuA4(0H)(NO)l2' Scheme 5 5 Group VIIIB Cobalt Rhodium and Iridium Major areas which have been explored this year are dioxygen complexes new structural properties of chelated complexes and catalytic behaviour.Dioxygen as a Ligand.-An understanding of the binding of dioxygen to cobalt in its complex compounds is of importance with respect to biological dioxygen carriers. 134 Of major interest is the charge distribution within the cobalt-0 moiety. The apportioning of this charge has been the subject of recent controversy but this year Iz6 S.D. Pel1 and J. N. Armor J. Amer. Chem. SOC.,1975,97 5012. lZ7 I. P. Evans G. W. Everett and A. M. Sargeson J.C.S. Chem. Comm. 1975 319. 128 K. Schug and G. P. Guengerich J. Amer. Chem. SOC.,1975,97,4135. Iz9 R. E. Clarke and P. C. Ford Inorg. Chem. 1970,9 227. 130 A. W. Zarella and P. C. Ford Inorg. Chem. 1975,14 42. 13' F. Bottomley E. M. R. Kuremire and S. G. Clarkson J.C.S. Dulron 1975 1909. 132 J. N. Armor R. Furman and M. Z. Hoffman,J. Amer. Chem. SOC.,1975,97 1737. 133 J. N. Armor and M. Z. Hoffman Inorg. Chem. 1975.14.444. 134 F. Basolo B. M. Hoffman and J. A. Ibers Accounts Chem. Res. 1975,8 384. J. R. Dilworth G.J. Leigh and R. L. Richards workers from three independent laboratorie~'~~*'~~ have interpreted e.s.r.spectra of mononuclear dioxygen adducts from Schiff -base or amine complexes of cobalt(I1) in terms of almost complete transfer of an electron from cobalt to dioxygen giving a formally Co"'-O,-linkage. These results convincingly reinforce earlier similar interpretations of such charge transfer in substituted porphyrin complexes of cobalt. An X-ray photoelectron spectroscopic study of dioxygen adducts of Schiff base complexes of cobalt showed that the Co2p3, binding energies increase by 0.9- 1.9 eV when the Co" complexes take up dioxygen again indicating considerable electron transfer to dioxygen.136 0 [L5-cO-0 / 3 [L5Co-0 /0-coL5 1 (271 (28) The reaction of O2with low-spin cobalt(I1) complexes commonly gives terminally- bonded mononuclear (27) or dioxygen-bridged dinuclear (28) adducts and physical studies on both classes of compound have been carried out.The Co-0-0 bond angle of 153.4' for (NEt4)3[Co(CN)5(02)]137 is interpreted in terms of the Col"-superoxide linkage (27). The structure of the dinuclear analogue [(CN)5$02Co(CN)5]5- shows the 'staggered' structure (28) with an 0-0 distance of 1.26 A typical of p-superoxide binding.138 Examination of the electronic spectra of this Complex and its ammine analogue [(NH3)5Co02Co(NH3)5]5+ has established low-spin d6 Co"' centres for both compounds and ligand field ligand-to-metal charge transfer and superoxide-localized transitions have been identified.'39 In particular the bands due to metal-to-ligand charge transfer terminating in the out- of-plane .rr(02-) orbital have been assigned (486 and 672 nm respectively) and a resonance Raman study confirms these assignments the electronic transitions being coupled to the 02-bands (at 1104 and 1135 cm-' respectively) in the Raman spectra.14' The structure of the five-co-ordinate precursor of the above dioxygen compounds [Co(CN),I3+ shows that the yellow solid form of this complex is a truly five-co- ordinate square-pyramidal compound but possibly the green form observed in aqueous solution may have an apical H20 molecule completing octahedral co- ~rdination.'~~ If tertiary phosphines replace cyanide as ligands to cobalt a different mode of co-ordination of dioxygen results.Thus reaction of [Co(CN),(PMe,Ph),] with dioxygen gives the unsymmetrical dinuclear complex [Co2(CN),(PMe2Ph),(02)] (29).14* While this mode of binding of dioxygen is unusual for dinuclear cobalt complexes the 0-0 distance (1.44 A)is in the range of such distances in analogous 135 R.F. Howe and J. H. Lunsford J. Amer. Chem. Soc. 1975,97,5156;D. Getz E. Melamud B. L. Silver and Z. Dori ibid. p. 3847. 136 J. H.Burness J. G. Dillard and L. T. Taylor J. Amer. Chem. SOC.,1975,97,6080. 137 L. D. Brown and K. N. Raymond Znorg. Chem. 1975,14,2595. 138 F. R. Franczek W. P. Schaefer and R. E. Marsh Znorg. Chem. 1975,14 611. 139 V. M. Miskowski J. L. Robbins I. M. Treitel and H. B. Gray Znorg. Chem. 1975,14 2318. 140 T. C. StrekasandT. G. Spiro Znorg.Chem. 1975,14 1421. 141 L.D. Brown and K. N. Raymond Znorg. Chem. 1975,14 2590. 14* J. Halpern B. L. Goodall G. P. Khane H. S. Lin and J. J. Pluth J. Amer. Chem.SOC.,1975,97,2301. Transition -metal Chemistry PhMe,P c PMe,Ph N mononuclear complexes e.g. [IrC1(CO)(02)(PPh3)2] (1.51A) and (Co(Ph,PCH=CHPPh,),(O,)]+ (1.42 A). It is suggested that in (29) both cobalt atoms are formally Co"' two electrons being transferred to the dioxygen via a ligand- bridged inner-sphere mechanism. The complex oxidizes PMe,Ph in methanol solution [equation (6)]. +3PMe2Ph +~[CO(CN)~(PM~,P~)~] [CO,(CN)~(PM~~P~)=,(O,)] +2Me2PhP0 (6) The dioxygen adducts of iridium [IrX(O,)(CO)(PPh,),] (X =C1 Br or I) although quite stable under normal conditions lose dioxygen on irradiation even at 77 K.It is suggested that the photoinitiation process is triggered in an electronic state possessing iridium-to-phosphine charge-transfer chara~ter.'~~ The X-ray structure of one such adduct [Iro2{Ph2PCH2P~h2}2]PF6, has been re-determined.'44 The 0-0 bond distance now found (1.52A) is cpsiderably shorter than that previously obtained and in the range (1.41-1.52A) found in a variety of such complexes of cobalt rhodium and iridium; thus an apparent anomaly is resolved. Dioxygen adducts of a number of other cobalt complexes have been prepared and investigated. Equilibrium studies on some chelating polyamine complexes of cobalt which give w-peroxo-w-hydroxo-complexes with dioxygen and act as reversible dioxygen carriers have shown a linear relationship between the logarithm of the stability constant of the dioxygen adduct and the sum of the pK's of the atoms ligating the cobalt This relationship allows prediction of the tendency of cobalt complexes to form stable dioxygen adducts.In particular it shows that the symmetrical and unsymmetrical ethylenediaminediacetic acids form quite stable dioxygen complexes in an appropriate pH range although they contain only two basic nitrogen atoms rather than the three which had previously been considered necessary. Chelate Complexes of Biological Relevance. In a study of the interactions of metal ions and nucleotides cytosine 5'-monophosphate (CMP) gave a polynuclear cobalt complex [Co(CMP)(H,O)],H,O which has tetrahedral Co" bound to two oxygen atoms of a bridging phosphate group and N-3 of the ~yrimidine.'~~ Cobalt(II1) complexes of azophenols have been prepared and their relevance to azotyrosine- modified enzymes which may form exchange-inert Co"' complexes is noted.14' The X-ray structure of [Co(tren)(gly)]'+ [tren = tris-(2-aminoethyl)amine,gly =glycine] formed by the hydrolysis of a glycine ester with [Co(tren)(OH)(H20)]2' shows that 143 G. L. Geoffroy G. S. Hammond and H. B. Gray J. Amer. Chem. SOC. 1975,97,3933. w4 M. Laing M. J. Nolte and E. Singleton J. Amer. Chem. Soc. 1975,97 6396. 145 G. McLendon and A. E. Martell J.C.S. Chem. Comm. 1975 223. 146 G. R. Clark and J. D. Orbell J.C.S. Chem. Comm. 1975 697. 147 W. I. White and J. I. Legg J. Amer. Chem. SOC. 1975,97 3937.J. R. Dilworth G.J. Leigh and R. L. Richards the glycine is co-ordinated with its nitrogen atom trans to the tertiary amine of tren and the tren ligand forms three diamine rings with two K and one K' conformations. This structure relates to specific hydrolysis of peptides by cobalt(II1) complexes. '48 In a cobalt(Iz1) carboxypeptidase A complex the magnetic moment (p,* =4.77 BM) and intensity of the principal visible absorption band (555.5nm E = 150) are considered compatible with a five-co-ordinate cobalt centre.14' Cobalt complexes containing the corrin inner ring structure have been obtained by abstraction of hydride with a quinone or other reagents from bis(p-iminoamine)cobalt(II) com-plexes containing 14- 15 or 16-membered rings.'" Complexes of Sulphur Ligands.-Some novel polynuclear cobalt complexes with bridging sulphur groups have been characterized.The tetrameric cluster compound (30)has a Co4S10 framework of effectively 7'' symmetry with bridging angles at S of 113".15' A monosulphur-bridged dinuclear complex [(CN),CoSCo(CN)J- which is easily hydrolysed has been prepared from [Co(CN),I3- and sulphur. 152 Dinuclear thio- bridged complexes with di thiocarbamato- or NN'-e thylene-bis(thiosalicyla1diminato)-ligands have been prepared and structural spec-troscopic and magnetic properties determined. 153 Oxidation of the complex [Co(en),(CH,CH,NH,)]'' with Npv' gives a novel cobalt(II1) disulphide complex [CO(~~)~(S(SCH,CH,NH,)CH~CH,NH,}]~'. A radical dimer intermediate is pro- posed.154 Ph A novel terdentate ligand is formed by the condensation of two benzoyl isothiocyanate molecules in the presence of [RhCI(PPh,),] (3l).15 The triply-bridged p-chloro-p-phenylthio-complex [Ir,H,Cl(SPh),(PPh,),] has been prepared by reaction of [IrHCI(SPh)(PPh,),] with AgCIO in acetone.156 Thioformato-complexes [Ir(S,CH)X(PPh,),](X = C1 or Br) have been prepared and the diagnostic spectral properties of this ligand attached to various metals given.15' 148 Y.Mitsui J. Watanabe Y. Iitaka and E. Kimura J.C.S. Chem. Comm. 1975 280. 149 R. C. Rosenberg C. A. Root and H. B. Gray J. Amer. Chem. SOC. 1975,97,21. 150 S. C. Tang and R. H. Holm J. Amer. Chem. Soc. 197.5,97,3359. 151 1. G. Dance and J. C. Calabrese J.C.S. Chem. Comm. 1975,762. Is* P.S. Poskozim J. Znorg. Nuclear Chem. 1975,37 2342. 153 A. R. Hendrickson R. L. Martin and D. Taylor J.C.S. Dalton 197.5 2182; M. F. Corrigan K. S. Murray R. M. Sheahan B. 0.West G. D. Fallon and B. M. Gatehouse Znorg. Nuclear Chem. Letters 1975,11,62.5. lS4 M. Woods J. C. Sullivan and E. Deutsch J.C.S. Chem. Comm. 1975 749. 155 C. M. Lowie J. A. Ibers Y. Ishii K. Itoh I. Matsuda and F. Ueda J. Amer. Chem. Soc. 1975,95,4748. 156 P. J. Roberts G. Ferguson and C. V. Senoff J. Organometallic Chem. 1975,94 C26. 157 S. D. Robinson and A. Sahajpal J. Organometallic Chem. 1975 99 665. 15' conformational properties of the rings are discussed. Transition -metal Chemistry 171 Chelating Phosphine Complexes.-This work involves primarily rhodium and iridium.Large-ring co-ordination complexes with trans-chelate phosphine ligands have recently been synthesized and structural parameters have now been determined for trans-[IrCl(CO){Bu',P(CH,),,PBu',}] and [RhCl(CO){Bu',P(CH,),,PBu'~}]~. The former has a 13-atom ring and the latter two pdiphosphines forming a 26-membered ring; the phosphorus atoms are trans at the metal centres. The Some o-metallated complexes [~r(~~){~~u',(~6~4~}{~~u'~(~~~~~-~}] (X = H or Me) give (X =H) a blood-red paramagnetic (peR= 1.73 BM) iridium@) complex trUnS-[rr(pBUt2(c6H4o)),l on exposure to air. A hydrido-complex [irH{PBu',(C6H46)},] is formed from this and dihydrogen. which reverts to its congener on exposure to air. A benzene solution of trans-[fr{PBu',(c6H40)}~] slowly gives in air the purple C-metallated compound [~~{PBu'~(C~H~O)}{PBU'(C~H~O)(CM~,~H,}].'~~ Another paramagnetic IrI' com- plex [Ir(0,CR),(AsPh,)(CNC6H4Me-p)] (/lefi= 1.67 BM e.s.r.g values at 2.039 and 2.015) has been prepared by treatment of [IrH,(AsPh3),(CNC6H4Me-p)] with p-chlorobenzoic acid. 160 Reactions of Co-ordinated Ligands and Catalysis.-When co-ordinated to [M(NH3),I3+(M=Co Rh or Ir) organic nitriles are activated towards nucleophilic attack. Thus reduction of nitriles by BH4- Michael addition of carbanions to acrylonitrile (M =Co) or base hydrolysis of acetonitrile of benzonitrile (M =Co Rh or Ir) are all greatly accelerated by co-ordination to these metal centres.'61 The complex [Co(NHJnNO]'+ bound within a Y-type zeolite is a catalyst for the conversion of NO and NH into N2and H,O at temperatures above 50 0C.162 This effect relates to the catalysed reduction of oxides of nitrogen from effluent gas streams by ammonia.The compounds [IrC1X(NO)(CO)(PPh,)2] undergo an appar- ent electrophilic attack on the 'NO-' ligand by dioxygen to give nitrato-complexes [IrClX(N0,)(CO)(PPh,)2]; the rate of oxidation decreases with X in the order X = I >Br >C1> NCS >NCO >N3.163 Asymmetric hydrogenation using rhodium complex catalysts continue to receive attention. The catalyst [Rh(cyclo-octa-1,5-diene){1,2-bis[o-anisyl(phenyl)phosphino}ethane)]+ induced optical purity in excess of 95-96'/0 in the reduction of a-acylamidoacrylic acids. 164 Similarly high optical yields have been obtained in the reduction of a-ethylstyrene and N-acetamidoacrylic acid derivatives with a rhodium complex of trans-1,2-bis(diphenylphosphinoxy)cyclohexane,and related reductions occur using isopropylidene- 2,3-di hydroxy- 1,4-bis(dipheny1phosphino)butane as a ligand.165 F. C. March R. Mason B. L. Shaw and K. M. Thomas J.CS. Chem. Comm. 1975,584. 159 H. D. Empsall E. M. Hyde and B. L. Shaw J.C.S. Dalton 1975 1690. 160 A. Araneo F. Morazzoni and T. Napoletano J.C.S. Dalton 1975,2039. I. I. Creaser and A. M. Sargeson J.C.S. Chem. Comm. 1975,974;A. W.Zanella and P. C. Ford Inorg. Chem. 1975,1442,700. 162 K. A. Windhorst and J. H. Lunsford J.C.S. Chem. Comm. 1975,852. 163 M. Kubota and D. A. Philips J. Amer. Chem. SOC. 1975 97 5638. W. S. Knowles M. J. Sabachy B.D. Vineyard and J. Weinkauf J. Amer. Chem. SOC. 1975,97,2567. 165 M. Tanaka and I. Ogata J.C.S. Chem. Comm. 1975,735;T. P. Dang J. C. Poulin and H. B. Kagan J. Organometallic Chem. 1975,91 105. J. R.Dilworth G.J. Leigh and R.L.Richards Rhodium(II1) chloride acts as a homogeneous catalyst for isotopic hydrogen exchange in deuteriation of aromatic compounds or alkanes. "' Reduction of dinitrogen to ammonia has been catalysed by aqueous acidic solutions of rhodium(II1) or iridium(II1) chloride. The reaction of a 1:3mixture of dinitrogen and dihydrogen in presence of a reducing agent (TiCI or SnCI in 10molar excess) gives yields of ammonia in the range 0.1-0.4 moles per mole of 6 Group VIIIC Nickel Palladium and Platinum Complexes of polydentate often macrocyclic ligands continue to be a feature of the chemistry of nickel.Complexes of Multidentate Ligands.-Square-pyramidal complexes with for the first time nickel-bismuth bonds have been prepared from tris-(0-dimethylarsinopheny1)bismuthine (bitas) [NiX(bitas)] (X = halide) and [Ni,(bifa~)~]~+. In the latter complex bitas functions as a terdentate and a quadri- dentate ligand (32). 1,3-Bis(dimethyIstibino)propane (dmsp) gives square-pyramidal complexes [NiX(dmsp),]ClO,. '" Trigonal-bipyramidal stereochemistry is shown by [NiI(NCH2CH2NMe2)3]I.'69 The complex [NiI(nas)JBPh [nas = tris-(2-diphenylarsinoethyl)amine] reacts with NaBH in ethanol to give a dimeric com- pound of nickel(1) (33) with a linear Ni-I-Ni bridge. This unit allows antifer- romagnetic interaction between metal atoms.''O A novel linear Ni-S-Ni system occurs in (34) prepared by reaction of [Ni(H2O)6l2+ H2S and 1 1,1-tris(diphenylphosphinomethy1)ethane. The short Ni-S (2.034A) distance indicates a high r-component in the Ni-S bonds in keeping with the linear Ni-S-Ni system and diamagnetism of the ~omplex.'~' Five-co-ordinate complexes particularly of nickel can have square-pyramidal or trigonal-bipyramidal structure depending on the ligating atoms. The ligand (Ph,PCH,CH,),N however having the NP3 donor set appears to confer exclusively 166 M. R. Blake J. L. Garnett I. K. Gregor W. Hannan K. Hoa and M. A. Long J.C.S. Chem. Comm. 1975,930. 167 M. T. Khan and A. E. Martell Inorg. Chem. 1975 14 938. W. Levason C. A. McAuliffe and S.G. Murray J.C.S. Chem. Comm. 1975,164;R. J. Dickenson W. Levason C. A. McAuliffe and R. V. Parish ibid.,p. 272. 169 P. L. Orioli and N. Nardi J.C.S. Chem. Comm. 1975 229. 170 L. Sacconi P. Dapporto and P. Stoppioni J. Amer. Chem.Soc. 1975,97 5595. 171 c.Mealli S. Midollini and L. Sacconi J.C.S. Chem. Comm. 1975 765. Transition -metal Chemistry trigonal-pyramidal geometry upon nickel(11). 172 This geometry is assigned to the complexes [Ni(CN),L,] and [NiLJ2+ (L =tertiary phosphine or phosphite) which in common with many five-co-ordinate complexes are labile in solution giving an equilibrium of the type (7). The factors determining the equilibrium position are discussed in terms of the electronic spectra of the complexes.173 The ligand tetars ms-Me2As(CH2)3As(Ph)CH,As(Ph)(CH2)3AsMe,, gives square-pyramidal com- plexes of type [MX(tetars)]' (M =Ni" Pd" or Pt") whose absorption and circular dichroism spectra are interpreted in terms of the orbital energy ordering dxy>d, > d, >d,2 >dX2- ,2.174 [NiX2Lgln+ $ [NiX2LJ"* +L (X =CN n =0; X =L n =2) (7) Synthesis of macrocyclic-ligand complexes of ten involves a condensation reaction between a carbonyl compound and an amine function in the presence of or in the co-ordination sphere of the metal. Many further examples of this type of system have appeared this year.'75 A Schiff-base complex of this type with a planar N3 donor set (39 also has both uni- and bi-dentate nitrato-gro~ps.'~~ Some new examples of dimeric Schiff-base complexes of nickel(I1) have been prepared (36) and show intermolecular anti-ferromagnetic beha~i0ur.l~~ I+ Ph\ Me C1 (35) + 172 M.D. Vaira and L. Sacconi J.C.S. Dalton 1975 493. '73 E. J. Lukosius and K. J. Coskran Znorg. Chem. 1975,14 1922. izB. Bosnich W. G. Jackson and S. T. D. Lo,Inorg. Chem. 1975,14 2998. D. B. Bonfoey and G. A. Melson Znorg. Chem. 1975,14,304; N. F. Curtis,J.C.S. Dalton 1975,87.91; R. Cheney L. E. Heyman and E. L. Blinn Znorg. Gem. 1975,14,441; P. Domiano A. Musatti and N. Nardelli,J. C.S. Dalton 1975,295;J. C. DabrowiakandD. H. Busch Znorg. Chem. l975,14,1881;M. J. Mocella F. Wagner E. K. Barefield and I. C. Paul J. Amer. Chem. Soc. 1975,97 192. 1'6 E. C. Alyen G. Ferguson and R. J. Restivo Znorg. em. 1975,14,2491; P.H. Merrell J. C. S. Chem. Comm. 1975,269. 177 R. J. Butcher and E. Sinn J.C.S. Chem. Comm. 1975,832. 174 J.R. Dilworth G. J.Leigh and R. L. Richards Macrocyclic bis( p-iminoamine) complexes of nickel(I1) (see also copper and cobalt) containing 14-membered rings are converted into the delocalized radical cation (37) by oxidative dehydr~genation.'~' This has an e.s.r. signal of more than 80 lines which has been analysed in terms of a 2Bluground state with an essentially Nil'-L-(15m) system; that is extensive ligand delocalization of the unpaired electron. This formalism is similar to that used to describe certain porphyrin cation radicals one of which [Ni"(TPP)]' undergoes reversible electron transfer to give [Ni"'(TPP)]'. This process has been likened to a suggested mechanism of electron transfer in cytochromes [equation @)I."' [Fe"'(cyt)] +[Fe"(cyt)]' $[Fe"'(cyt)]+ (8) The above oxidative dehydrogenation method has been used extensively to convert other larger ring complexes into less saturated derivatives.17' The base- promoted reduction of nickel(I1) complexes of macrocyclic ligands gives nickel- macrocycle radical species which undergo a number of other reactions and are suggested as intermediates in macrocyclic amine-complex reactions. Zerovalent Complexes.-Complexes of nickel palladium and platinum in the zero oxidation state particularly with tertiary phosphine ligands continue to be prepared and their reactions studied. New examples are [Pt{P(CF,)Ph,},],'81 bis-[ 1,2- bis(difluorophosphino)cyclohexane]nickel(0),182 and [Pt(MeC(CH,PPh,),}(PR,)] R =alkyl aryl F NMe, or OPh).lS3 Heating [Pt(PPh,),] in benzene gives the cluster compounds (38) and (39).lS4The reactions of ethane- 1,2-di t hiol 2-(methyl t hio)e t hane thiol and 2-(methy1thio)ethane disulphide with [M(PPh,),] (M =Ni Pt or Pd) have been studied and [Pt(SCH2CH2S),(PPh3),] [Pd,(SCH,CH,S),(PPh,),] and [Ni(SCH,CH,S)(Ph,PCH2CH2PPh2)] prepared.By using an excess of halogen and short reaction times the addition of halogens to [Pt(PPh,),] has been shown to give exclusively trans-[PtX,(PPh,),] (X =C1 Br or I) as the initially formed The commonly observed products from such reactions are the cis- complexes formed by isomerization in the presence of free phosphine avoided by the p2 pPh2 /\ PhP Pt-/ \pt/PPh3 Ph3P -Pt -Pt-PPh I\ /I \/ Ph? P-Pt-PPhz P I Ph 2 Ph (38) (39) 178 M.Millar and R. H. Holm J.C.S. Chem. Comm. 1975,169; S. C. Tang and R. H. Holm J. Amer. Chem SOC.,1975,97 3351. 179 D. Dolphin and R. H. Felton Accounts Chem. Res. 1974,7,26; T. Nien and I. Fujita J. Amer. Chem. Soc.,1975,M 5288. 180 E. K. Barefield and M. T. Mocella J. Amer. Chem. SOC.,1975,97 4238. 181 T. G. Attig M. A. A. Beg and H. C. Clark Znorg. Chem. 1975,14 2986. 18* N. R. Zack K. W. Morse and J. G. Morse Inorg. Chem. 1975,14 3131. J. Chatt R. Mason. and D. W. Meek J. Amer. Chem. SOC.,1975,97 3826. N. J. Taylor P. C. Chieh and A. J. Carty J.C.S. Chem. Comm. 1975,448. 185 (a) B. Ranchfuss and D. M. Roundhill J.Amer. Chem. SOC.,1975,97,3386; (b)R. C. Stonfer ibid. p. 195; (c) K. B. Dillon T. C. Waddington and D. Younger J.C.S. Dalton 1975,790; (d)J. F. Plummer and E. P. Schram Znorg. Chem. 1975 14 1505; (e) K. Maeda 1. Moritani Y. Hosokawa and S. I. Murakashi J.C.S. Chem. Comm 1975,689; (f)M. Foa and L. Cassar J.C.S. Dalton 1975 2572. Transition -metal Chemistry 175 above reaction conditions. [M(PPh,),] (M =Ni Pt or Pd) react with liquid HCl to give cis-[MC1,(PPh,),],'85c with TiCl (M =Pt) to give an adduct with Pt-Ti bonds,"5d and with ketoximes in the presence of dioxygen (M =Pd) to give nitriles and a1deh~des.l'~~ The mechanism of oxidative addition of aryl halides to [Ni(PPh3)4] has been in~estigated."~~ Oxidation StateOne.-Examples of platinum(1) and palladium(1) complexes are still rare (see Chapter 8 p.207).186n*b Electrochemical and e.s.r. studies of dithiolene complexes of palladium and platinum indicate that paramagnetic and presumably monomeric complexes of Pd' and Pt' are formed with these ligand~."~ Nudear Magnetic Resonance Studies.-N.m.r. spectroscopy continues to be a powerful tool in the study of palladium and platinum complexes. The platinum(0) complexes [Pt(triphos)(PR,)] [triphos =MeC(CH,PPh,),] mentioned above have 1fi'"Pt3''Y(R3)] values larger than corresponding values in truns-platinum(11) (twice) and cis-platinum(I1) (ca. 55% greater) complexes. J(31P,31P) values are also higher than in platinum(r1) complexes. These differences are considered to be caused by the steric constraint of the triphos ligand which confers relatively low s-character in the Pt-triphos bonds and correspondingly high s-character in the Pt-PR bonds.lg3 The 1J('95Pt31P) values for cis-[PtCl,(R,PCH,CH,PPh,)] (R = CF or Ph) show a strong dependence on substituents at phosphorus.'s8a The values of J('95Pt 13C)and J(195Pt,'9F) for platinum complexes of carbon monoxide188b or SCF3188C have been discussed in terms of trans -influence of hydride and halide co-ligands. The first values of J('95Pt,77Se) and J('95Pt,125Te) have been reported. They decrease markedly in the order C1> Br >I in the compounds [PtX,(SeMe,)]- [PtX,(SeMe,)]-and [PtX,(TeMe,)]- (X =C1 Br or I).'88d Values of 2J(195Pt 195Pt) have been determined for the complexes [PtCI,(PBu,),] and [Pt214(PBu3),].The mode of bonding of SCN groups (whether Nor Sligation) has been a subject of interest and controversy for many years. Recently n.m.r. spectroscopy and X-ray crystallography has been used to determine the binding of the SCN group to platinum and palladium. In the platinum case the linkage isomers of [Pt(CNS),(SMe,),] (no implied N or S bonding) have been identified from coupling patterns of the 'H_(195pt}INDOR spectra and it is thought likely that 195Pt chemical shifts will provide a means of distinguishing isomers.'89a The ,'Pn.m.r. spectra of cis-[PtX,{P(OPh),},] (X = CNS or C"NS) show the independent existence of linkage isomers in solution. lS9' An X-ray study of the complexes [Pd(CNS),{Ph,P- (CH,),PPh,)] has shown that in the solid state the thiocyanate co-ordination changes from S (n = 1)to SN (n = 2) and N2(n= 3).It is concluded that the bonding mode is controlled primarily by steric effects.lS9' 186 (a) A. Modinos and P. Woodward J.C.S. Dalton 1975 1516; (b)D. J. Doonan A. L. Balch S. Z. Goldberg R. Eisenberg and J. S. Miller J. Amer. Chem. Soc. 1975,97 1961. lS7 F. C. Seuftleber and W. E. Geiger J. Amer. Chem. SOC. 1975,97 5018. 188 (a)T. McLeod Lj. Manojlovit-Muir D. Millington K. W. Muir D. W. A. Sharp and R. Walker J. Organometulfic Chem. 1975,97 C7;(6)W. J. Chewinski B. F. G. Johnson J. Lewis and J. R. Norton J.C.S. Dalton 1975 1156; (c) K. R. Dixon K. L. Moss and M. A. R. Smith ibid. p. 990. (d)P. L. Goggin R. J Goodfellow and S. R. Haddock J.C.S. Chem. Comm 1975 176; (e)A.A. Kiffer C. Masters and J. P. Visser J.C.S. Dalton 1975 1311. lE9 (a)S. J. Anderson and R. J. Goodfellow J.C.S. Chem. Comm. 1975 443; (6) A. J. arty and S. E. Jacobson ibid. p. 175; (c) G. J. Palenik M. Mathew W. L. Stiffen and G. Berau J. Amer. Chem. SOC. 1975,97 1059. J.R. Dilworth G. J.Leigh and R. L. Richards Hydrocarbon Activation.-Important in the search for a catalyst for activation of saturated hydrocarbons is the observation of hydrogen-deuterium exchange in alkanes catalysed by platinum complexes.'90a Further work has shown that by use of H,PtCl in aqueous trifluoroacetic acid at 120"C benzene has been oxidized to chlorobenzene and hexane to chlorohexanes. 1906 Hydrogen-deuterium exchange at alkyl groups of L moieties occurs in the complexes [Pt,Cl,L,] (L=PPr, PBu, PBu'Pr, PBu',Pr PPrPh, PPr,Ph or PBu'Ph,) in aqueous (D,O) acetic acid (CH,COOD) medium.Exchange also occurs at the saturated (C-5) carbon of RCMe,CH=CH (R =Et Pr or Bu) under similar conditions. It may occur through dimeric complexes of the type [Pt,Cl,(RCMe,CH=CH,)] isolated from the reac- tion medium or related dimeric compounds. 19' Diazene Ligands.-Understanding of the properties of diazene (N,R) ligands has progressed from study of their platinum complexes. The bridging diazenido-ligand N,H occurs in the complexes [Pt(N2H)2(PR3)2]22+ [PR = PPh, PPh,Me or P(C,H,Me),] 192 prepared by hydrazine reduction of cis-[PtCl,(PR,),]. In the complex [PtCl(N,C,H,F)(PEt,),] the diazenido-group has the 'doubly-bent' struc- N-/ ture -N and is readily protonated at both nitrogen atoms.193 7 Group IB Copper Silver and Gold Principal activity this year has involved study of structural and magnetic properties of polynuclear complexes and biologically relevant copper complexes.Structural and Magnetic Studies.-The tetrameric species [MXPEt,],(M = Cu or Ag; X =C1 Br or I) have a distorted 'cubane' structure with p,-halide bridges. Only for large halogens (Br or I) together with bulky phosphines (PPh,) does the structure change to 'step-like'. The complex [(CuI),(Ph,PCH,PPh,),] has a triangle of copper atoms connected by iodide and diphosphine bridges (40).194 Polynuclear copper complexes bridged by a variety of groups show magnetic interaction of the antiferromagnetic type and a number of further examples have been investigated this year.An unusual example is the trinuclear complex (41) the first example of a linear array of three oxygen-bridged copper(I1) atoms. Magnetic l90 (a)M. B. Typbin A. E. Shilov and A. A. Shteinman Doklady Akad. Nauk S.S.S.R. 1971,198,380;R. J. Hodges D. E. Webster and P. B. Wells J. Chem. SOC(A),1971 3230; (b)J. R. Sanders D. E. Webster and P. B. Wells J. C. S. Dalton 1975 1191. 191 A. A. Kiffen C. Masters and L. Raymond J.C.S. Dalton 1975 853; P. A. Kramer and C. Masters ibid. p. 849. 192 M. Kembler S. Cenini F. Conti and R. Ugo J.C.S. Dalton 1975 1081. 193 S. D. Ittel and J. A. Ibers Znorg. Chem. 1975 14 636; S. Krogsrund and J. A. Ibers ibid. p. 2298. 194 M. R. Churchill and B.G. DeBoer Znorg. Chem. 1975,14,2402;with S. J. Merdak ibid.,pp. 2041,2496. Transition-metal Chemistry studies show that only two of the three electrons of the trimer are paired only the spin-doublet state being pop~lated.'~~ Antiferromagnetic behaviour has been studied in the complexes diquinoline tetra-p -trifluoroacetate( O,O)-dicopper(~~),'~~ [Cu(tren),X,l2' (tren = 2,2',2"-triaminotriethylamine X = C1 NCO or NCS),19' [{CuLX,},,] (X =C1 or Br L = nicotinamide or is~nicotinamide),'~~ and bis(nicotinato)silver(~r).'~~A correlation between metal environment and antiferromagnetic interaction in oxygen-bridged copper(I1) dimers has been demonstratedzo0 and the magnetic properties of ternary oxides of copper in oxidation states (I) to (IV) have been investigated.,Oi Unusual Valence States and Biologically Related Complexes.-A mixed-valence cluster compound [Cu,(S,CNEt,),Cl,] consists of elongated square-pyramidal Cu" linked by chloride and sulphur bridges to a tetrahedral Cu' unit.It is formed by reduction of polynuclear chloride-bridged [Cu2(S2CNEt2)C12] and is associated with [Cu2(S2NEt2),C12] dimers in the The mixed Ad1'-Au' compounds [Au(mnt)][Au(MPh,),J (mnt =maleonitrile dithiolate M =P or As) are converted into the Au" anion [Au(mnt),12- on treatment with Na,(mnt) probably by an electron-transfer mechanism.203 A Cu"' complex [Cu(G,)]- (G2-=deprotonated tetraglycine) is unusual in that it is reasonably stable in aqueous solution in contrast to the few other examples of Cu"' complexes.The Cu"'-Cu" potential 0.140 V is low enough to suggest that further similar compounds should be obtainable. [Cu(G,)]- is formed by reaction of dioxygen with Cu" tetraglycine complex pro- vided that photochemical inhibition of the reaction is avoided. It is suggested that Cd-Cu"' couples could occur in biological systems such as galactose oxidase thus avoiding high-energy free-radical intermediate^."^ Study of copper-sulphur interactions in relation to copper-containing proteins such as oxidases has been an active field this year. The results of a X-ray photoelec- tron spectroscopic study of bean plastocyanin has been interpreted in terms of delocalized Cu"-cysteine-sulphur binding.205 Comparison has been made between copper-sulphur binding in various complexes and the environment of copper in 195 W.A. Baker and F. T. Helm J. Amer. Chem. SOC. 1975,97 2295 references therein. 196 J. A. Moneland and R. J. Doedens J. Amer. Chem. SOC.,1975,97 508. 19' E. J. Laskowski D. M. Duggan and D. N. Hendrickson Znorg. Chem. 1975,14 2449. 198 R. P. Eckberg and W. E. Hatfield J.C.S. Dalton 1975 1364. 199 R. P. Eckberg and W. E. Hatfield Znorg. Chem. 1975 14 1205. 2oo E. Sinn J.CS. Chem. Comm. 1975,665. 201 M. Arjomand and D. J. Machin J.C.S. Dalton 1975,14 1205. 202 A. R. Hendrickson R. C. Martin and D. Taylor J.C.S. Chem. Comm. 1975,843. 203 T. J. Bergendahl and J. H. Waters Znorg. Chem. 1975,14 2556. 204 F. P. Bossu G. L. Burce K. L. Chellappa and D. W. Margerum J. Amer. Chem. SOC., 1975,97,68%;G.L. Burce D. W. Margerum and E. B. Paningo J.C.S. Chem. Comm. 1975,261. 205 P. J. Clendening H. B. Gray F. J. Grunthaner and E. I. Solomon J.Amer. Chem. SOC.,1975,97,3878. 178 J. R. Dilworth,G.J. Leigh,and R.L.Richards 'blue' proteins as is shown by its spectral parameters. In [bis-(2-pyridyl)disulphide]copper(~)perchlorate the copper selects N3S co-ordination pos- sibly its environment in natural systems.206 A dinuclear complex of copper(r1) with oxidized glutathione is considered to be dimeric with Cu" bridged by a disulphide unit. The Cu" atoms interact as shown by e.s.r. measurements and the compound provides the basis for a scheme to account for the properties of the Cu pair in 'blue' oxidases which accepts two Copper complexes of cyclic or linear poly-thioethers show an absorption (600nm) similar to that of 'blue' proteins assigned in both complexes and proteins to an S+Cu" charge-transfer band.Of this group the complex of the ligand (42) has a planar arrangement about copper suggesting that distorted symmetry about copper need not occur in the proteins. A further extrapolation from this work is that thioether sulphurs of methionine groups could be the copper-binding site of 'blue' proteins.208 Thee.s.r. and visible spectra of a Cur' a,-mercaptopropionylglycine complex are similar to the corresponding parameters for 'blue' A kinetic study of the reduction of 'blue' proteins by [Fe(edta)12- has been discussed in terms of an outer-sphere mechanism for all such proteins but laccase which requires a specific protein activation to accept reductant.,lo In other studies of biological relevance comparison of e.s.r.and electronic spectra of Cu" carboxypeptidase A and model Cu" complexes suggests a protein co-ordination significantly distorted from planar to tetrahedral symmetry.211 Other ligands of the amino-acid type whose interactions with copper have been studied include ace tyl histamine ace tyl his tidine [with Cu'] ;L-hist idine gl y cylgl ycerine and D-penicillamine histidine peptides simple dipeptides epinephrine ~-3,4-dihydroxyphenylalanine and other catechols [with CU"].~~~ Steric and electronic effects in copper Schiff -base complexes have been reviewed.213 Interaction of hydrazines and triazenes with copper have been studied.1,l-Dimethylhydrazine reacts with copper(r1) chloride to give a purple complex of 1,l-dimethyldiazene [Cu3Cl,{Me,N=N},]. Copper(I1) bromide gives the salt [Cu,Br,][Me,N,CHNMe,] in which the central carbon of the cation appears to be derived from formaldehyde formed by hydrolysis of 1 l-dimethyldia~ene.,~~ The metal-metal bonded compounds [L,(CO)MCu(RNNNR)X] (M = Rh' or 18; L =AsR or PR,; R = Me or aryl; and X = C1 Br or I) have been prepared and contain an M'+Cu' donor bond bridged by the triazenido group.215 M. M. Kadooka L. G. Warner and K. Seff J.C.S. Chem. Comm. 1975,990 and references therein. 207 P. Kroneck J. Amer. Chem. SOC.,1975,97 3840. 208 T. E. Jones D. B. Rorabacker and L. A. Schrymowycz J. Amer. Chem. SOC.,1975,97 7485; L. C. Zimmer and L.L. Diaddario ibid. 7163 and references therein. 209 V. Sugiura Y. Hirayama H. Tamaka and K. Ishizu J. Amer. Chem. SOC.,1975,97,5577. 210 S. Wherland R. A. Holmerda R. C. Rosenberg and H. B. Gray J. Amer. Chem. SOC.,1975,97,5260. 211 R. C. Rosenberg C. A. Rost P. K. Bernstein and H. B. Gray J. Amer. Chem. SOC.,1975,97,2092. z12 P. A. Terinissi and A. Vitagliano J. Amer. Chem. SOC.,1975,97 1572; S. H. Laurie T.Lund and J. B. Brynor. LCS. Dalton 1975 1389; R. P. Agarwal and D. D. Perrin ibid. p. 268; G. Brookes and L. D. Pettit ibid. p. 2302; R. K. Boggess and R. B. Martin J. Amer. Chem. SOC.,1975,97 3076. 213 H. S. Moslea and T. N. Waters Coordination Chem. Rev. 1975 17 137. 214 J. R. Boehm A. L. Balch K. F. Bizot and J. H. Enemark J. Amer. Chem. SOC.,1975,97,501.215 J. Kyper P. I. Van Wet and K. Vrieze J. Organometallic Chem. 1975,96 289.