13 Iron, cobalt and nickel S. A. Cotton Uppingham School, Uppingham, Rutland, UK LE15 9QE 1 Introduction This report follows the pattern of recent years in covering the available 1998 literature, together with some late 1997 papers. A review has appeared of polyiron oxides, oxyhydroxides and hydroxides as models for biomineralisation1 whilst the structure and function of nickel sites in metalloproteins have been reviewed.2 Reviews have appeared covering the co-ordination chemistry of iron,3 cobalt4 and nickel5 for 1995, for nickel over the years 1985,6a 19866b and 1987–19896c, and for cobalt for 1994.7 Following its predecessor last year, dedicated to mononuclear iron complexes, a second review of structures of iron co-ordination compounds, this time covering over 400 dimeric and oligomeric complexes, has appeared.8 Electron-transfer reactions of nickel-(III) and -(IV) complexes have been examined.9 Abiological Fe–S clusters have been reviewed10 as have [1: 3] site-di§erentiated and sulfide-bridged cubane clusters. 11 A review has appeared covering diiron(II, II) complexes, with a bridging carboxylate, that react with dioxygen forming l-peroxo species.12 Bond valence sum calculations have been reported on 227 compounds with CoOn polyhedra (n\3–8) and used to recognise the oxidation state of cobalt.13 In addition to wastewater treatment, iron compounds can be used instead of toxic heavy metals in the dye and textile industries.14 MCD studies have been reported for 24 non-haem iron(II) complexes. 15 It is stated that variable-temperature variable-field MCD (VTVH MCD) studies can generally indicate the co-ordination number and geometry of an unknown iron(II) centre.More use is being made of EXAFS and other techniques.16 [Fe(tpp)(NO)] and [MbII(NO)]Miron(II) nitrosylmyoglobinN have closely related structures with Fe–N–O ca. 155°; on the other hand, [MbIII(NO)] has Fe–N–O ca. 180° again consonant with X-ray data for models like [Fe(oep)(NO)]`.Sulfur K-edge XAS has been used to investigate covalency in rubredoxins and simple models like [NEt 4 ] [FeMC 6 H 4 (CH 2 S) 2 -oN2 ] and Na[AsPh 4 ][FeMC 6 H 4 (CH 2 S) 2 -oN2 ].17 Keggin-type ironsubstituted polyoxometallates [c-SiW 10MFe(H 2 O)N2 O 38 ]6~ are very e¶cient homogeneous catalysts for the oxidation of alkanes with H 2 O 2 .18 2 Simple binary and co-ordination compounds of hydrogen, oxygen, nitrogen and halogen donors Nanometer sized Fe 3 O 4 particles have been synthesised by hydrothermal reactions of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 165[Fe(OCH 2 CH 2 OMe) 2 ].19 An EXAFS study of b-NiO(OH) indicates that the Ni(III) sites experience a Jahn–Teller distortion, with four Ni–O bonds at 1.87Å and two at 2.03Å; in the g-phase, containing Ni(III) and Ni(IV), this is absent.20 Oxygen donors Iron is seven-co-ordinate in Ca[Fe(Hdtpa)][ClO 4 ]·7H 2 O; the protonated carboxylate group does not co-ordinate.21 The structure of Na[Fe(cdta)][ClO 4 ] has been determined in the solid state and solution.22 FeCl 3 reacts with pyridine-2,6–dicarboxylic acid forming [Fe(dipic)(H 2 O) 3 ][Fe(dipic) 2 ] in which both cation and anion contain octahedral iron(III).23 Syntheses and structures are reported for maricite (NaFePO 4 ) and sodium iron hydroxyphosphate, [Na 3 Fe(PO 4 ) 2 ·(Na 2~2xH 2xO)], two compounds implicated in boiler corrosion.24 [H 2 pip] 2 [Fe 6 (HPO 4 ) 2 (PO 4 ) 6 (H 2 O) 2 ]·H 2 O contains TBPY FeO 5 and octahedral FeO 6 polyhedra in a 3–D framework structure that contains channels which accommodate the cations.25 Quaternary ammonium ions act as templates26 for the framework in [H 3 N(CH 2 ) 4 NH 3 ] 2 - [Fe 8 (HPO 4 ) 12 (PO 4 ) 2 (H 2 O) 6 ].Two series of the familiar trinuclear carboxylates, this time with long alkyl chains, [Fe 3 O(O 2 CCnH 2n`1 ) 6 L 3 ]NO 3 (L\H 2 O, py; n\11, 13, 15, 17) have been synthesised.27 The salt [Fe 3 O(O 2 CC 13 H 27 ) 6 (py) 3 ]NO 3 has a singlelayer structure in which the interlamellar orientation of alkyl chains is alternate to those in adjacent layers.The electrochemical reduction of a range of carboxylates [Fe 3 O(O 2 CMe) 6 L 3 ]X (L\pyridine or substituted pyridine; X\NO 3 or ClO 4 ) and [Fe 3 O(O 2 CR) 6 (py) 3 ]X (R\Bu5, Ph, CH 2 Cl, CCl 3 , CH 2 CN, 4-NO 2 C 6 H 4 ; X\NO 3 or ClO 4 ) has been studied and the structure of [Fe 3 O(O 2 CPh) 6 (py) 3 ]NO 3 reported.28 The 6–methyl-2-pyridone complex, all trans-[CoCl 2 (Hmhp) 2 (H 2 O) 2 ]· 2Hmhp, is a 1–Dhydrogen-bonded polymer in which [CoCl 2 (Hmhp) 2 (H 2 O) 2 ] molecules alternate with the unco-ordinated ligand.29 The complex (CoCl 2 ) 2 ·(12–crown-4)·H 2 O has the structure [(12–crown-4)Co(H 2 O)(l-Cl)CoCl 3 ], featuring both tetrahedral and octahedral cobalt(II).30 The structure of hydrated nickel(II) perchlorate, [Ni(H 2 O) 6 ] [ClO 4 ] 2 ·2H 2 Ohas been determined.31 The complex NiCl 2 (dmf) 3 was shown by X-ray di§raction32 to be [Ni(dmf) 6 ][NiCl 4 ] whilst the structure of [Ni(dmf) 6 ][BF 4 ] 2 also shows octahedral co-ordination of nickel.33 Halide complexes Orange CsNiCl 3 has face-sharing NiCl 6 octahedra whilst turquoise Cs 3 NiCl 5 contains isolated [NiCl 4 ]2~ tetrahedra.34 [NHMe 3 ][MCl 3 (H 2 O) 2 ] (M\Mn, Ni, Co) are isomorphous, containing infinite chains of chloride edge-sharing trans- [MCl 4 (H 2 O) 2 ] octahedra.35 O 2 ` salts dissolve in anhydrous HF a§ording O 2 F, which oxidises NiF 2 to [NiF 6 ]2~ salts.36 Nitrogen donors Trigonal monopyramidal is a much less common geometry for four-co-ordination than either tetrahedral or square planar.Using the tripodal ligand tris[(N-tert- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 166butylcarbamoyl)methyl]aminato [L]3~, a series of anionic species [MII([L]3~)]~ have been synthesised (1; M\Fe, Co, Ni, Zn) and the structures of the cobalt, nickel and zinc complexes determined.They are nearly identical, suggesting that the ligand is e§ective in enforcing the geometry.37 The nickel and iron complexes can be oxidised to EPR-active M(III) species. A number of iron(II) tris(2-pyridylmethyl)amine complexes have been synthesised. [Fe(tpa)(NCMe) 2 ][SO 3 CF 3 ) 2 is low-spin whilst [Fe(tpa)(SO 3 CF 3 ) 2 ] is high-spin. Reaction in the presence of [BPh 4 ]~ a§ords [Fe(tpa)(MeOH) 2 ][BPh 4 ] 2 whilst in the presence of excess ligand six-co-ordinate [Fe(tpa) 2 ][SO 3 CF 3 ] 2 and eight-co-ordinate [Fe(tpa) 2 ][BPh 4 ] 2 could be isolated.38 The structure 2 of fac-[FeCl 3 (bpma)] has been reported.39 The structure of [Fe(phen)Cl 3 (H 2 O)], the first well characterised mono(phen) iron(III) complex, has been reported.40 The new compound [Fe(bpm)Cl 3 (H 2 O)]·H 2 O has a similar structure; [Fe 2 (bpm)Cl 6 (H 2 O) 2 ]·2H 2 O is binuclear, with bridging bipyrimidine. 41 Croconate and squarate also form dinuclear complexes [Fe 2 (bpm)(C 5 O 5 )(H 2 O) 4 ]·2H 2 O and [Fe 2 (bpm)(C 4 O 4 )(H 2 O) 6 ]·2H 2 O respectively.42 The structure of the iron(II) compound K 2 [Fe(phen)(CN) 4 ]·2.5H 2 O has been determined. 43 Iron(II) pyridinebis(imine) complexes (3; R\Me, Et, Pr*) act as very e§ective ethene polymerisation catalysts.44a Similar iron and cobalt complexes with even bulkier ligands act similarly.44b Starting with [NEt 4 ][FeTp@Cl 3 ], chloride ligands can be replaced by nitrite, resulting in the iron(III) complexes [NEt 4 ][FeTp@(ONO)xCl 3~x] (x\1–3), in which nitrite is monodentate, and the iron(II) complex [NEt 4 ]- [FeTp@(ONO)Cl] in which nitrite is chelating.45 An iron(II) methyl complex ‘supported’ by a poly(pyrazolyl)borate ligand is discussed in Section 5.The structure of the [Fe(trans-dimmac)]2` ion, one of the few low-spin iron(II) complexes of saturated amines, shows distorted octahedral geometry with rather short Fe–N distances.46 Octahedral [Fe(en) 3 ]2` ions are found in [Fe(en) 3 ][Hen][SbSe 4 ].47 A potential model for superoxide dismutase active sites, [FeL][PF 6 ] 2 [L\N(CH 2 CH 2 N––CHR) 3 ; R\1-triphenylmethyl-4-imidazolyl] has octahedrally co-ordinated iron(II); the tripodal bridging nitrogen does not co-ordinate.48 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 167[Co(pm) 2 X 2 ] (X\Cl, Br) show weak ferromagnetic interactions at low temperatures; they have chiral 3–D network structures in which the trans-CoN 4 X 2 co-ordination sphere has pyrimidine rather than halogen bridges.49 [CoL 2 (NCS) 2 ] (L\pyrazine or pyrimidine) have sheet-like structures in which the azine ligands bridge between cobalt atoms; compressed octahedral co-ordination of cobalt is made of four long Co–N (azine) and two short axial Co–N (NCS) bonds.The pyrazine complex is antiferromagnetic at low temperatures whilst the pyrimidine complex displays magnetic ordering below 8.2 K. In contrast, pyridazine forms a monomeric complex trans-[Co(pyz) 4 (NCS) 2 ].50 Three cobalt(II) complexes of 4,4@-bipyridine have been studied.51 [Co(4,4@-bipy)(SO 4 )(H 2 O) 2 ]·2H 2 O and [Co(4,4@-bipy)Cl 2 (dmso) 2 ] have chains containing linear Co–bipy–Co groupings whereas [Co(4,4@-bipy)(OAc) 2 ] has double chains with acetate bridges.The structure of fac-[Co(bipy)(py)(N 3 ) 3 ] has been determined.52 Hydroxoaquatetraminecobalt(III) complexes in microemulsions catalyse the hydrolysis of nerve agent simulants.53 The tetrahedral high-spin cobalt(II) complexes [CoXMN(SiMe 2 CH 2 PPh 2 ) 2N] (X\Cl, Br, I) undergo alkylation forming the low-spin square-planar cobalt(II) compounds [CoRMN(SiMe 2 CH 2 PPh 2 ) 2N] (R\Me, CH 2 Ph, CH 2 SiMe 3 , Cp).PhCH 2 X oxidizes [CoXMN(SiMe 2 CH 2 PPh 2 ) 2N] to the five-co-ordinate paramagnetic Co(III) compounds (a combination still rare enough to be remarked on) [CoX 2MN(SiMe 2 CH 2 PPh 2 ) 2N]. In toluene, MeI or MeBr react with [CoMeMN(SiMe 2 CH 2 PPh 2 ) 2N] to form [CoXMN(SiMe 2 CH 2 PPh 2 ) 2N], probably via the unstable [Co(Me)XMN(SiMe 2 CH 2 PPh 2 ) 2N].Structures are reported for [CoIMN(SiMe 2 CH 2 PPh 2 ) 2N], [Co(CH 2 Ph)MN(SiMe 2 CH 2 PPh 2 ) 2N] and [CoBr 2MN(SiMe 2 CH 2 PPh 2 ) 2N].54 A number of cobalt complexes of 2,2@- dipyridylamine have been studied.55 [Co(dpa) 2 (Me 2 CO) 2 ][ClO 4 ] 2 and [Co(dpa)Cl 2 ] have octahedrally and tetrahedrally co-ordinated Co(II) respectively.Under aerobic conditions, oxidation to Co(III) occurs with the formation of [Co 2 (dpa) 4 (l-O 2 )(l-OH)]- [ClO 4 ] 3 . Deprotonated dpa is a ligand in [Co(dpa[H) 2 ] whilst the protonated ligand is found in [Hdpa] 2 [CoCl 4 ]. The crystal structure of [Ni(5–Mepz) 6 ][ClO 4 ] 2 has been determined; the EPR spectra of [Ni(5–Mepz) 6 ]X 2 (X\ClO 4 , BF 4 ) show a temperature-dependent zero-field splitting.56 [Ni(NH 3 ) 6 ]C 60 ·6NH 3 , prepared in liquid ammonia, is a new recruit to the ranks of nickel hexammine complexes; it has a distorted rock salt structure.57 The structure of cis-[Ni(phen) 2 (NCS) 2 ] has been reported.58 [Ni(bipy) 2 (Cr 2 O 7 )(NCMe)] = has a helical chain structure, whilst in [Ni(en) 2 - (Cr 2 O 7 )] = the chains cross.59 In [Ni(bipy)(O 2 CMe) 2 (H 2 O) 2 ] and [Ni(dmbipy)(O 2 CMe) 2 (H 2 O) 2 ] the molecules are self-assembled into 1-D infinite zig-zag chains by double intermolecular hydrogen bonds; the chains are stacked through the aromatic rings.In [Ni(phen)(O 2 CMe) 2 (H 2 O) 2 ]·0.5H 2 O, hydrogen bonds link molecules to form a 1-D chain, associated with another chain through double Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 168hydrogen bonds.60 The bimetallic assemblies [NiIIL 2 ] 3 [FeII(CN) 6 ]X 2 (L\en, tn, X\PF 6 , ClO 4 ) have 3-D networks with Fe–CN–Ni linkages leading to a ferromagnetic interaction between neighbouring Ni and Fe.61 A number of thermochromic nickel ammines have been studied.62–66 The dependence of alkyl group size in substituted en ligands upon nitrate co-ordination in nickel ammine complexes was discussed.Syntheses are reported of cis-[Ni(dpren) 2 (NO 3 )] [NO 3 ] and cis-[Ni(dipren) 2 (H 2 O) 2 ][NO 3 ] 2 ·2H 2 O; the latter undergoes loss of four water molecules on melting, accompanied by a colour change from greenish blue to deep green.62 Syntheses are reported for [Ni(dpren) 2 (H 2 O) 2 ]X 2 (X\Cl, Br, I, CF 3 CO 2 , CF3 SO 3 ), [Ni(dpren) 2 (NCS) 2 ], [Ni(dipren)Cl 2 ] and [Ni(dipren)(CF 3 CO 2 ) 2 (H 2 O) 2 ].[Ni(dpren) 2 (H 2 O) 2 ]X 2 lose water on heating, with various colour changes; on further heating, [Ni(dpren) 2 X 2 ] (X\Cl, Br) yield monodpren complexes.63 The thermal dehydration of a number of trans- [Ni(diamine) 2 (H 2 O) 2 ]X 2 systems has been studied.64 Some compounds such as trans-[Ni(stien) 2 (H 2 O) 2 ]Cl 2 convert first to cis-[Ni(diamine) 2 X 2 ], and on further heating isomerise to the trans-isomer. Rehydration occurs on standing to give the original complex.The nickel trifluoroacetate complexes [NiL 2 (CF 3 CO 2 ) 2 ] (L\meen, eten, pren or ipren) have been synthesised; [Ni(pren) 2 (CF 3 CO 2 ) 2 ] and [Ni(ipren) 2 (CF 3 CO 2 ) 2 ] have trans NiN 4 O 2 co-ordination spheres.They exhibit irreversible blue to light violet thermochromism whilst [Ni(meen) 2 (CF 3 CO 2 ) 2 ] shows a blue-violet to blue thermochromism that is reversed in a humid atmosphere.65 The structure of the high-temperature isomer of [NiL 2 (NCS) 2 ] (L\N,N-dimethylpropane- 1,3-diamine) has been determined;66 the isomers di§er in their molecular packing.Mixed ligand chelates of N- or N,N@-methylated ethylenediamines and tropolonate or hinokitiolate ligands have either octahedral or square-planar coordination, sometimes exhibiting an equilibrium in solution.67 [Ni(bpma) 2 ][ClO 4 ] 2 exhibits cis facial co-ordination of the bpma ligands; bond lengths were compared within the series [M(bpma) 2 ]2` (M\Mn, Fe, Ni, Cu, Zn).68 Reaction of NiCl 2 and tpa[ClO 4 ] 3 in the presence of NEt 3 yields [(tpa)Ni(l-Cl) 2 Ni(tpa)][ClO 4 ] 2 which in alkaline solution forms [(tpa)Ni(l-OH) 2 Ni(tpa)][ClO 4 ] 2 .This fixes atmospheric CO 2 a§ording [(tpa)Ni(l-HCO 3 ) 2 Ni(tpa)][ClO 4 ] 2 .69 A pyrazolate-based ligand [HL\3,5-bis(R 2 NCH 2 )pyrazolyl; R 2 N\Me 2 - N(CH 2 ) 3 NMe] forms a dinuclear nickel complex [LNi 2 (l-OH)(NCMe) 2 ][ClO 4 ] 2 in which the two nickels are bridged by the pyrazolate and hydroxide; each five-coordinate nickel has one MeCN completing its co-ordination sphere.A similar compound MR 2 N\[Me 2 N(CH 2 ) 3 ] 2 NN has extra ligand side arms so that the complex [HLNi 2 (l-OH)(NCMe) 2 ][ClO 4 ] 3 has an intramolecular N· · ·H· · ·N bridge.The nickel ions are su¶ciently close to exhibit strong antiferromagnetic coupling (J\[46.7 cm~1).70 Several nickel(II) complexes of a pentadentate ligand (pyN 4 ) have been synthesised with the formulae [Ni(pyN 4 )(H 2 O)]X 2 (X\Cl, I), [Ni(pyN 4 )(OClO 3 )][ClO 4 ] and [Ni(pyN 4 )X][PF 6 ] 4 as well as binuclear [(pyN 4 )Ni(l-Cl)Ni(pyN 4 )][PF 6 ] 3 .71 The binuclear compound shows antiferromagnetic coupling, resulting in a S\0 ground state at low temperatures.[Ni(Medien)(NCS)(l-NCS)]n has a chain structure in which nickel atoms are linked by cis-thiocyanates, causing ferromagnetic coupling.72 cis-[(l- jN,NA-N 3 ) 2MNi(dl-cth)N2 ][ClO 4 ] 2 exhibits significantly stronger antiferromagnetic coupling than cis-[(l1,3 -N 3 ) 2MNi(dl-cth)N2 ][PF 6 ] 2 ·dmf.73 The azide- and oxalate- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 169bridged dimer[MNi(Medpt) 2 (l-ox)(l-N 3 )Nn][ClO 4 ]n has a bond-alternating S\1 spin chain.74 Compounds [Ni 5 (l5 -tpda) 4 X 2 ]2` (X\Cl, CN, NCS, N 3 ) and [Ni 5 (l5 - tpda) 4 (NCMe) 2 ]2` all contain a linear chain of five nickels, terminated by two axial ligands and wrapped helically by the four tpda ligands.The nickels are not magnetically equivalent, the three inner ones being in square-planar low-spin environments whilst the outer two are square-pyramidal high-spin; magnetic behaviour indicates antiferromagnetic interaction of the two high-spin nickels.75 3 Complexes of tertiary phosphines The structure of trans-[FeHCl(depe) 2 ] has been determined.76 A compound believed to be [MFeCl(depe) 2N2 (l-N 2 )][BPh 4 ] 2 has been shown to have the structure trans- [Fe(N 2 )Cl(depe) 2 ][BPh 4 ]; [Fe(N 2 )X(depe) 2 ][BPh 4 ] (X\Cl, Br) are much less stable than the corresponding hydrides and undergo dinitrogen exchange even in the solid state at room temperature.77 In the presence of TlBF 4 , trans-[FeHCl(dppe) 2 ] reacts with P–– – CBu5, forming trans-[Fe(g1-P–– – CBu5)H(dppe) 2 ][BF 4 ].The related trans- [Fe(g1-P–– – CBu5)H(dppe) 2 ][BPh 4 ] has an exceptionally short P–– –C bond. On reaction with HBF 4 this forms trans-[Fe(g1-PF––CHBu5)H(dppe) 2 ]` and trans-[Fe(g1- PF 2 CH 2 Bu5)H(dppe) 2 ]`. trans-[FeHCl(dppe) 2 ] reacts with P–– – CBu5, in the presence of TlBF 4 and NH 4 BF 4 forming trans-[Fe(PH 3 )H(dppe) 2 ][BF 4 ].78 The dihydrogen ligand in trans-[FeH(H 2 )(dppm) 2 ]` is replaced by other ligands such as MeCN, py,N 2 and ethene; evidence was obtained for trans-[Fe(H 2 ) 2 (dppm) 2 ]2`, which undergoes substitution forming trans-[Fe(NCMe) 2 (dppm) 2 ]2`.The structures of trans- [FeH(NCMe)(dppm) 2 ][BF 4 ] and trans-[Fe(NCMe) 2 (dppm) 2 ][BF 4 ] 2 were reported. 79 cis-[FeH 2 (dppe) 2 ] reacts with acids in thf forming trans-[FeH(H 2 )(dppe) 2 ]` in a reaction first order in both reactants.80 cis-[Fe(bpe5) 2 Cl 2 ] is the first cis- [Fe(PP) 2 Cl 2 ] (PP\bidentate phosphine) system.81 In solution, it exhibits temperature- dependent paramagnetism owing to dissociation of chloride. Cobalt powder reacts with [IBz 2 P(CH 2 ) 2 PBz 2 I] forming [CoMBz 2 P(CH 2 ) 2 PBz 2NI 2 ]; aerial oxidation converts it to a phosphine oxide complex.82 [Ni(dppe) 2 ] reacts with DCl via the initial formation of [NiD(dppe) 2 ]`; at low acid concentration, the nickel-containing product is [NiCl 2 (dppe)], whose structure has been determined.83 [Ni(dppe)(SC 6 H 4 X)] (X\O, CO 2 , NH) have been reported;84 square-planar co-ordination for nickel has been confirmed for X\O.[Ni(dppmeSe) 2 ]Cl 2 has a square-planar NiP 2 Se 2 chromophore.85 A square-planar geometry is also found86 in the cation of [NiMPh 2 P(CH 2 ) 4 PBuPh 2N(S 2 CNEt 2 )]- [ClO 4 ]. The ligand tapa forms the nickel complexes [NiCl 2 (tapa) 2 ] and Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 170[Ni(CN) 2 (tapa) 3 ], the latter having a TBPY structure with axial cyanides.87 4 Complexes with sulfur, selenium and tellurium donors [FeCl 2 (th) 2 ] contains infinite linear chains where iron atoms are doubly linked by halide bridges;88 a similar structure was deduced for [NiBr 2 (th) 2 ] from powder neutron-di§raction data.89 [CoMN(SiMe 3 ) 2N2 ] reacts with RSH [R\2,6–(Me 3 Si) 2 C 6 H 3 ] to form the tetrahedral thiolate [Co(SR) 2 (thf) 2 ].90 [MCo(Py 2 S) 2 Cl 2Nn] has a two-layer interwoven sheet structure whilst [MCo(Py 2 S) 2 (NCS) 2 (H 2 O) 2Nn] has a double-stranded linear infinite structure.91 A number of complexes of the ‘extended reach’ S-donor ligands ebpt and p-xbpt have been synthesised.92 [MCo(NCS) 2 (ebpt)Nn] has a chain structure with tetrahedrally co-ordinated cobalt, whereas [MCoCl 2 (ebpt)N2 ] is a dimer with 18–membered rings.Polymeric [MCo(p-xbpt)N2 ][ClO 4 ] 2 ·MeNO 2 has a sheet structure with 52–membered rings; cobalt is again tetrahedrally co-ordinated. [MNi(NO 3 ) 2 (p-xbpt)Nn] has bidentate nitrates; bridging p-xbpt ligands link the octahedrally co-ordinated nickel atoms into chains. Compounds of the ligand ebpyt include [Fe(ebpyt)Cl 2 ], [Co(ebpyt)Br 2 ] and [Ni(ebpyt) 3 ][ClO 4 ] 2 ·2MeOH·MeNO 2 .Of these, the cobalt complex has a chain structure in which ebpyt ligands link tetrahedrally co-ordinated cobalts, whilst the nickel compound has NiS 6 centres in which the metal atoms are again linked by ebpyt bridges into a 3-D polymeric array.93 Compounds with amide-substituted arylthiolate ligands, such as [NMe 4 ] 2 [Fe(SC 6 H 4 NHCOMe-o) 4 ], [NMe 4 ] 2 [FeMSC 6 - H 3 (MeCONH) 2 -2,6N4 ], [NEt 4 ] 2 [M(SC 6 H 4 NHCOBu5-o) 4 ]·2EtCN (M\Fe, Co) and [PPh 4 ] 2 [CoMSC 6 H 3 (CF 3 CONH) 2 -2,6N4 ]·2Et 2 O appear to have unexpectedly short M–S distances as well as significantly shifted Fe(III)–Fe(II) redox potentials, due to intramolecular NH· · · S hydrogen bonds.94 Iron complexes of the ligand ns 3 have attracted attention.95 The paramagnetic TBPY iron(III) anionic complex [Fe(ns 3 )Cl]~ is reduced by CO–Na forming the paramagnetic TBPY iron(II) complex anion 5.The latter reacts with FeCl 2 forming an unusual paramagnetic Fe 3 S 4 carbonyl cluster 6. Dithiocarbamates and related ligands Electrochemical oxidation of [Co(Q 2 CNR 2 ) 3 ] (Q\S, Se) in MeCN involves96 [Co(Q 2 CNR 2 ) 3 ]`, [Co 2 (Q 2 CNR 2 ) 5 ]` and [Co(Q 2 CNR 2 ) 2 (NCMe) 2 ]`.The structure of [Co(S 2 CNEt 2 ) 3 ] has been reported.97 Soft X-ray photoreduction of Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 171[Ni(S 2 CNEt 2 ) 3 ][BF 4 ] gives a Ni(II) square-planar species, whilst [PPh 4 ]- [Ni(S 2 COEt) 3 ] is photoisomerised to a square-planar Ni(II) species.98 Adducts of [Ni(S 2 COPr*) 2 ] with amine ligands, such as [Ni(S 2 COPr*) 2 (dpa)]·Me 2 SO and [Ni(S 2 COPr*) 2 (amp)], have a cis-NiN 2 S 4 co-ordination sphere.99 [NiMPh 2 P(CH 2 ) 4 PPh 2N(S 2 CNEt 2 )] contains square-planar nickel(II).100 [NBu 4 ] 2 - [Ni(S 2 C––NC 6 H 4 CN-p) 2 ] also has distorted square-planar co-ordination of nickel.101 The electronic structure of [Ni(mnt) 2 ]~ has been probed by ENDOR and ESEEM (electron stimulated echo envelope modulation).102 Metal complexes of dmit have potential as molecular metals.A number of new salts of [Ni(dmit) 2 ]~ are semiconductors, though their conductivity increases on doping with iodine. The structure of [eda]- [Ni(dmit) 2 ] shows short S · · · S contacts between cation and anion.103 The bonding between [Ni(dmit) 2 ] monomers in the solid state has been analysed via density functional calculations.104 New selenoether complexes of nickel include [NiX 2 ([16]aneSe 4 )] (X\Cl, Br, I) and [NiX 2 (MeSeCH 2 CH 2 SMe) 2 ].105 5 Complexes with r-bonded carbon donors The colourless paramagnetic r-bonded methyl (7; R\Bu5) is unusual in being a four-co-ordinate iron(II) alkyl.It is the precursor for a number of iron(II) poly(pyrazolyl) borate complexes.106 The compound formerly reported as an iron(0) species [Li(OEt 2 )] 4 [FePh 4 ] is now believed to be an iron(II) compound [Li(OEt 2 )] 4 trans- [FeH 2 Ph 4 ].107 The unstable six-co-ordinate cobalt(II) isocyanide complex [Co(CNCy) 4 (PPh 3 ) 2 ][ClO 4 ] 2 has been isolated by rapid precipitation and filtration, the usual product being the cobalt(I) species [Co(CNCy) 3 (PPh 3 ) 2 ][ClO 4 ].On attempted recrystallisation of the cobalt(II) complex, [Co(CNCy) 3 (PPh 3 ) 2 ][ClO 4 ] 2 is obtained.108 6 Complexes with porphyrins and other macrocycles A resonance Raman study of dioxygen adducts of double-sided encumbered iron(II) porphyrins shows high O–O stretching frequencies, possibly owing to a decreased Fe–O–O angle caused by the narrow cavity.Skeletal vibrations were also readily assigned.109 BF 3 ·OEt 2 reacts with [K(18-crown-6)][Fe(tpivpp)(NO 2 ) 2 ] forming the unstable molecule [Fe(tpivpp)(NO 2 )]. This disproportionates forming Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 172[Fe(tpivpp)(NO)] and [Fe(tpivpp)(NO 3 )]; the latter has six-co-ordinate iron with a symmetrical bidentate nitrate.110 [N(PPh 3 ) 2 ][Fepc(OMe) 2 ] contains octahedrally co-ordinated iron,111 with the methoxides trans. The dimeric species [N(PPh 3 ) 2 ] 2 [MN 3 Fe(pc)N2 N]I 3 ·2Et 2 O has an almost linear (177°) Fe–N–Fe skeleton.112 Iron porphyrin thiolates are studied as models for cytochrome P-450.Two compounds with single and doubleNH· · · S hydrogen bonds, [Fe(oep)MSC 6 H 3 (RCONH) 2 -2,6N] and [Fe(oep)(SC 6 H 4 RCONH-2)], have been studied crystallographically.The hydrogen bond elongates the Fe–S bond, stabilises the Fe(III) state, makes the compounds less air- and water-sensitive and also makes the redox potential more positive; together this is more significant than the e§ects of steric hindrance.113 A number of complexes have been made from tetra(cyclohexyl)porphyrin. 114 High-spin [FeCl(thcp)] reacts with cyanide forming [Fe(CN) 2 (thcp)]~ which is low-spin with the unusual (dxzdyz)4(dxy)1 ground state. Imidazole complexes such as [FeL 2 (thcp)]` (L\imidazole, 1-methylimidazole, 1,2-dimethylimidazole) are also low-spin. Iron(III) porphycenes [Fe(etiopc)R] (8, R\Ph, 3,5–F 2 H 3 ,C 6 , 3,4,5- F 3 H 2 C 6 , 2,3,5,6-F 4 HC 6 ) can be high- or low-spin depending upon the axial ligand and temperature.115 The first one-electron oxidation step involves the metal; subsequent electrons are removed from the porphycene ligand.The iron atom is nearly in the basal plane in [Fe(etiopc)(3,5-F 2 H 3 C 6 )]. The complexes [FeX(tmcp)] ( X\Cl, Br, OH) have been studied; the structure of high-spin [FeCl(tmcp)] is unusual as the porphyrin is strongly rußed and domed, and the Fe–N distances are distinctly short.116 Low-spin derivatives of iron(III) chiroporphyrin, with cyanide and imidazoles include examples with the unusual (dxzdyz)4(dxy)1 ground state.117 Complexation of FeCl 3 with 1,4,7-tris[3,4-bis(decyloxy)benzyl]-1,4,7- triazacyclononane yields a liquid crystalline product.118 Of the iron(II) and iron(III) complexes [Fe([9]aneN 2 S) 2 ][ClO 4 ] 2 , [Fe([9]aneN 2 S) 2 ][ClO 4 ] 3 and [Fe([9]- aneNS 2 ) 2 ][ClO 4 ] 2 , the last two are low-spin, but the first exhibits spin–equilibrium behaviour.119 Seven-co-ordinate iron(III) complexes of [15]aneN 5 and related ligands are catalysts for the dismutation of superoxide ion.120 [Co(NO)(oep)] has a bent Co–N–O linkage (Co–N–O 122.7°) with the Co–N(NO) vector between the pair of short Co–N (porphyrin) bonds,121 but with less tilt than in similar iron compounds.This is seen as a bonding e§ect rather than a consequence of crystal packing. Stopped-flow EXAFS has been applied to the study of the reaction between Co(II) and a water-soluble sulfonated porphyrin.122 Several cobalt(III) complexes of cyclic thioethers have been prepared, including [Co([9]aneS 3 )Cl 3 ] and Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 173[Co([20]aneS 6 )][BF 4 ] 3 , and the structure of the latter determined.123 A decaaza macrocycle ligand (L1) forms a dicobalt complex [Co 2 (NCMe) 2 L][ClO 4 ] 4 in which trigonal-prismatic co-ordination is imposed because of the interaction between the MeCN ligands. However [Co 2 (l-OH)L][ClO 4 ] 3 is octahedral.124 [Ni(O 2 tpp)X 2 ] (X\halogen) react with aryl Grignards at 203K to produce rare paramagnetic organonickel(II) porphyrins such as [Ni(O 2 tpp)(Ph)X] and [Ni(O 2 tpp)Ph 2 ].On warming they decompose to the EPR-active Ni(I) species [Ni(O 2 tpp)Ph].125 [NiL]Cl 2 (L\3,14-dimethyl-2,6,13,17-tetraazatricyclo- [14,4,01.18,07.12]docosane) adds X~ (X\NCS, N 3 ) to form the tetragonally distorted high-spin [NiLX 2 ].126 Binding of pyrrolidine or piperidine to [Ni(tpp)] has been studied spectroscopically and the structure of [Ni(tpp)(pipd) 2 ] determined; there is a relationship between the orientation of the axial ligands and macrocyclic distortions. 127 [Ni([15]aneN 4 )N 3 ][PF 6 ] has a quasi 1-D structure with a superlattice along one axis.128 A dinickel complex of a macrocyclic ligand with two cyclam ligands arranged face-to-face has been described.129 The mixed complex [Ni([9]aneN 3 )([9]- aneS 3 )][ClO 4 ] 2 undergoes electrochemical and chemical oxidation to the corresponding Ni(III) species.130 A number of nickel(III) complexes of [14]aneN 6 ligands additionally bearing pendant arms (which in this case do not co-ordinate) have been synthesised. 131 7 Schi§ base and related complexes [Fe(salen)]` and [Fe(CN) 6 ]3~ react to form [MFe(salen)N2MFe(CN) 6N]~ units which have extended 2–D structures with Fe(S\5/2)–NC–Fe(S\1/2)–CN bridging units. The assembly depends upon the counter ion and the solvent; [NEt 4 ][MFe(salen)] 2 - [Fe(CN) 6N] is metamagnetic, and in ferromagnetic [MFe(salen)(MeOH) 2NMFe- (salen)N2MFe(CN) 6N] the [Fe(salen)(MeOH) 2 ]` ions act as counter cations between the interlayers.132 The structure of an oxygen-carrying Schi§ base complex 9, whose dimerization is prevented by bulky substituents, is reported.133 A five-co-ordinate high-spin cobalt Schi§ base complex reversibly binds O 2 in a Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 174range of solvents.134 When an oxygen-binding polymeric cobalt Schi§ base complex is compounded with a conductive carbon powder, the material can release absorbed oxygen when a voltage is supplied.135 Air-oxidation of a cobalt(II) salen system leads136 to an intramolecularly bridged compound 10 that is a simple model for coenzyme B 12 .The structure of the dimeric complex [Co 2 (sal-m-phen) 2 ] shows two bis(salicylideneaminato)cobalt(II) residues linked by two m-phenylene groups; cobalt has distorted tetrahedral co-ordination.137 Cobalt and nickel complexes have been prepared with similar ligands; they all show weak antiferromagnetic interactions between the metal ions.Nickel(II) and iron(III) complexes of the unsymmetrical N 3 O Schi§ base H 2 ambprsal (formed by reaction between 2-aminobenzaldehyde, propane- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 1751,3-diamine and salicylaldehyde), planar [Ni(ambprsal)] and high-spin [FeCl(ambprsal)], have been synthesised.138 Nickel salicylaldimine complexes 11 act as catalysts for the polymerisation of ethene under mild conditions in the presence of a phosphine scavenger such as [Ni(cod) 2 ].139 The substitution of nickel for copper in [Cu(amben)]140 is catalysed by anions in the order [ClO 4 ]~\Br~\SCN~@Cl~, possibly reflecting their ability to form [Ni(dmf) 2 X 2 ].The reversal of the Irving–Williams order reflects the stability of the d8 square-planar complex [eqn.(1)]. A planar nickel complex 12 of a potential anti-inflammatory agent has been reported. 141 Nickel(III) Schi§ base complexes have been produced by electrochemical oxidation of Ni(II) complexes derived from naphthaldehyde, and identified by EPR spectroscopy. 142 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 1768 Nitrosyl complexes X-Ray absorption spectroscopy143 of the inactive form of the enzyme nitrile reductase from Rhodoccus sp.R312 confirms that NO is bound to low-spin Fe3`.The structure of [Co(NO)(oep)] and other nitrosylporphyrins have already been alluded to.16,121 9 Iron binding agents The co-ordination chemistry of siderophores, including the thermodynamics and kinetics of iron chelation and release, has been reviewed.144 Hexadentate tris(salicylate) ligands have been used as models for triprotonated iron(III)-containing enterobactin.145A new macrocyclic bis(amine, amide, hydroxamate) ligand has a high complexing power for iron(III).146 A hexadentate chelating agent containing three 3-hydroxy-2(1H)-pyridinone moieties has a very high a¶nity for iron(III).147 A watersoluble tripodal ligand with three 2,2@-dihydroxybiphenyl groups connected to a tren framework has been synthesised as a new iron(III) chelator.148 Four amonabactins are siderophores from the pathogenic organism Aeromonas hydrophila; because they are tetradentate ligands, one molecule cannot complete the co-ordination sphere of one Fe3` and so they form complexes with the stoichiometry Fe 2 L 3 at high pH and excess ligand.(At lower pH and ligand concentration, 1: 1 complexes are formed.)149 The presence or absence of oxalate a§ects the oxidation state and phase of iron oxide or hydroxide produced by hydrolysis.150 Lariat crown ethers have been used for molecular recognition of ferrioxamine B through complexation of a pendant amine function by the lariat ether.151 10 Polynuclear complexes with oxo, hydroxo and related bridges, particularly those of iron A highlight is the synthesis of the 18–iron cyclic system, [MFe(OH)(xdk)Fe 2 (OMe)(O 2 CCH 3 ) 2N6 ], the largest cyclic cluster yet characterised.152 A ferric citrate complex has the composition [Hneo] 7 [Fe 9 O(cit) 8 (H 2 O) 3 ].This has a ‘triple-decker’ structure with three parallel triangular triiron units forming a slightly distorted trigonal prism.The two outer units are connected by three bridging citrates to the central triangle; the two remaining citrates cap the two ends. The central triangle has an oxo-bridged Fe 3 O core whilst the two terminal units have a Fe 3 O 4 core.153 Resonance Raman spectra of a number of compounds with a [Fe 2 (l-O) 2 ] ‘diamond’ core, [Fe 2 (l-O) 2 L 2 ][ClO 4 ] 3 (L\tpa, 5-Me 3 tpa, 5-Me 2 tpa, 5-Metpa, 5-Et 3 tpa, 3- Me 3 tpa) have been examined.154 It was proposed that a band between 650 and 700cm~1 is a signature for an iron cluster with such a core.A synthetic model of the deoxy form of hemerythrin 13 contains five- and six-coordinate iron, thus having a vacant site for the co-ordination ofO 2 ; it forms a dioxygen adduct with an optical spectrum very similar to that of oxyhemerythrin.155 The dimer [L(H 2 O)Fe(l-O)FeL(OH)][ClO 4 ] 3 ·H 2 O ML\N,N@-dimethyl-N,N@- bis(2-pyridylmethyl)ethane-1,2-diamineN has a hydrogen-bonded [(H 2 O)Fe(l- O)Fe(OH)] 3 ` unit with an Fe–O–Fe angle of 137.5° and significant antiferromagnetic Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 177coupling between the irons.156 On dissolution in MeCN it undergoes protonation and m4:.Fe–O–Fe shifts from 438 to 600cm~1, corresponding to a change in Fe–O–Fe to 111°, possibly associated with the formation of a diamond-shaped [(H 2 O)Fe(l-O)(l- OH)Fe(OH)]3` core. [FeIIFeIII(bplm)(OAc) 2 ] has a l-phenoxobis(acetato)diiron core with antiferromagnetically coupled high-spin irons; it is suggested as a model for the reduced form of purple acid phosphatase.157 In the presence of air, a methanolic solution of Fe(ClO 4 ) 3 containing tpa forms158 a dinuclear (l-oxo)(l-formato) complex [Fe 2 (tpa) 2 (l-O)(l-O 2 CH)][ClO 4 ] 3 ; if the reaction is performed in EtOH, the product is [Fe 2 (tpa) 2 (l-O)(l-O 2 CCH 3 )][ClO 4 ] 3 .Other compounds with l-oxo bridges supported by carbonate, bicarbonate and acetate bridges include [MFe-(bispicMe 2 ([)chxn)N2 (l-O)(l-CO 3 )][ClO 4 ] 2 ·4H 2 O, [MFe(bispicMe 2 ([)chxn)N2 (l-O)(l- HCO 3 )][ClO 4 ] 3 ·H 2 O and [MFe(bispicMe 2 ([)chxn)N2 (l-O)(l-OAc)][ClO 4 ] 3 · H 2 O whose structures have been reported.159 Diiron(II) complexes [Fe 2 (l-L)(l- O 2 CR)(O 2 CR)(base) 2 ] [L\dinucleating bis(carboxylate) based on mxylylenediaminebis( Kemp’s triacid imide); base\pyridine- or imidazole-derived ligand] have been synthesised as models for carboxylate-bridged non-heme diiron enzymes.The bridging carboxylate ligand shifts between monodentate and bidentate mode. Solutions of these iron(II) compounds turn deep blue when oxygenated, due to the formation of diiron(III) peroxo species, with a stoichiometry of one O 2 per Fe 2 centre.160 A dicarboxylate ligand based on a porphyrin rather than xylylenediamine binds three iron atoms, one in the porphyrin ring and two linked by two dicarboxylate bridges as a [Fe 2 (l-O 2 CR) 2 ]2` unit.161 The compound [Fe 2 L 2 (l-OMe) 2 ]·0.5MeOH [L\1,2-bis(2@-hydroxybenzyl)ethane-1,2-diamine] contains two asymmetric methoxo bridges with Fe–O distances of 1.980 and 2.040Å.162 [Fe 2 O(bipy) 4 Cl 2 ][ClO 4 ] 2 ·0.25MeCN·0.25MeOH·0.25H 2 O has a slightly bent oxo bridge (167.0°) and shows strong antiferromagnetic coupling between the Fe(III) centres.163 On recrystallisation from MeCN, [NEt 4 ][M 2 (OAc) 5 py 2 (l-OH 2 )] (M\Fe, Co) convert to trinuclear [NEt 4 ] 2 [M 3 (OAc) 8 ].The iron compound undergoes aerial oxidation to the mixed-valence compound [NEt 4 ][Fe 3 (l3 - O)(OAc) 7 (H 2 O)].Oxygen and peroxide oxidation of pyridine solutions of [NEt 4 ]- [Fe 2 (OAc) 5 (py) 2 (l-H 2 O)] in the presence of chloride gives [Fe 4 (l3 -O) 2 (OAc) 6 (py) 4 - Cl 2 ], which has a [FeIII 4 (l3 -O) 2 ] core. All these compounds catalyse the oxidation of adamantane to adamantanols and adamantanone under Gif conditions.164 Iron(III) nitrate reacts with H 5 dhpta and excess RCH(NH 2 )CO 2 H (R\H, Me) to a§ord tetranuclear iron(III) complexes Na[Fe 4 (dhpta) 2 (l-O)(l-OH)(O 2 CCHRNH 3 ) 2 ].These contain two identical dimer units, each bridged by dhpta, the units being linked by the oxo-, hydroxo- and carboxylate bridges.165 The high O–O stretching frequency in the binuclear l-1,2 peroxide-bridged iron(III) complex [Fe 2 (O 2 )(O 2 CPh) 2 (TpP3* ) 2 ] is due to strong coupling between the Fe–Oand O–O stretching vibrations rather than to an especially strong O–O bond.166 Use of multidentate ligands is now a well-established approach to cluster synthesis.Reaction of FeCl 3 with sodium benzoate and 2,2@-bipyridyl in MeCN gives [Fe 4 O 2 (O 2 CPh) 7 (bipy) 2 ][FeCl 4 ].Linking two bipy units together in 1,2–bis(2,2@- bipyridin-6-yl)ethane (L) a§ords a tetradentate ligand; its reaction with FeCl 3 and sodium benzoate in MeCN results in a six-iron cluster [Fe 6 O 4 Cl 4 (O 2 CPh) 4 L 2 ]- [FeCl 4 ] 2 which has a [Fe 6 (l3 -O) 4 ]10` core. Carrying out the reaction in MeOH gives a di§erent cluster, [Fe 2 (l-OMe) 2 (O 2 CPh)L][FeCl 4 ], with an [Fe 2 (l-OMe) 2 ]4` Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 165–188 178core.167 [(bipy) 2 (NO 3 ) 2 Fe(l-O)Fe(NO 3 ) 2 (bipy) 2 ] contains seven-co-ordinate iron.168 The compound Fe 3 (OAsPh 3 ) 4 Cl 6 (MeCN) is in fact [(Ph 3 AsO) 4 Fe(l-O)FeCl 3 ]- [FeCl 4 ]·MeCN.169 l-Oxo-bridged binuclear iron(III) complexes of the tetradentate ligand 2,2@: 6@,2A: 6A,2@@@-quaterpyridine, [M(H 2 O)(qtpy)FeN2 (l-O)][ClO 4 ] 4 ·2H 2 O, have been synthesised.170 Use of a poly(pyrazolyl)borate ligand permits the isolation of dimeric cobalt- and nickel-(III) complexes [TpM%3M(l-O) 2 MTpM%3] (M\Co, Ni) in which the metals are in SPY five-co-ordination. These are the first dinuclear nickeland cobalt-(III) bis(l-oxo) complexes to be characterised (though the nickel compound decomposes rather rapidly at room temperature).The corresponding M(II) complexes [TpM%3M(l-OH) 2 MTpM%3] were also isolated.171 11 Other cluster complexes Structure–function correlations in high-potential proteins, which contain [Fe 4 S 4 ] clusters, have been reviewed.172 [NEt 4 ] 3 [Fe 4 S 4 (SH) 4 ][NEt 4 ]Cl has been synthesised and its crystal structure determined.173 The structure previously reported to be of [NPr 4 ] 2 [Fe 4 S 4 (SH) 4 ] has been shown to be that of [NPr 4 ] 2 [Fe 4 S 4 Cl 4 ], formed by reaction with the solvent during crystal growth.The Fe–S bond lengths for [Fe 4 S 4 (SR) 4 ]z (z\[1, [2, [3) clusters in a range of oxidation states were compared. 173 The syntheses and structures of two cycloalkylthiolate clusters [NMe 3 Bz] 2 - [Fe 4 S 4 (SC 5 H 9 ) 4 ] and [NEt 4 ] 2 [Fe 4 S 4 (SC 6 H 11 ) 4 ] have been reported,174 as has that of the dithiocarbamate175 [NEt 4 ] 2 [Fe 4 S 4 (S 2 CNEt 2 ) 4 ].Successive protonation of [Fe 4 S 4 (SPh) 4 ]2~ labilizes the thiols towards substitution, whilst in the case of [Fe 4 S 4 Cl 4 ]2~ the first protonation labilizes the complex but the second protonation inhibits substitution of the chlorides.176 The ligand N,N@-diethyl-3,7–diazanonane- 1,9–dithiol forms hexanuclear complexes containing both stair-like [Fe 6 (l3 -S) 4 (l- SR) 4 ] and nest-like [Fe 6 (l3 -S) 2 (l-S) 2 (l4 -S)(l-SR) 4 ] cores.177 Cuboidal iron–sulfur clusters with bulky phosphines [Fe 4 S 3 (NO) 4 (PR 3 ) 3 ]n` (R\Et, Pr*, Cy; n\0, 1) have been characterized.178 [Fe 4 S 4 (SEt) 2 (CNBu5) 6 ] reacts with PhCH 2 SSSCH 2 Ph to form [Fe 8 S 12 (CNBu5) 12 ].This is a cluster based on two Fe 4 (l3 -S) 3 (l3:g2,g1-S 2 ) units bridged by two l-S ions. [Fe 4 S 4 (SEt) 2 (CNBu5) 6 ] has two octahedrally co-ordinated and two tetrahedrally co-ordinated irons.179 Iron and cobalt formamidinate clusters such as [M 4 O(dphf) 4 ] have the basic beryllium acetate structure.180 A ‘witches-brew’ reaction between CoCl 2 , Habt, NaOMe and PBu/ 3 , followed by addition of excess Li 2 S, a§ords a triangular sulfur-capped tricobalt cluster [Co 3 (l3 - S)(abt) 3 (PBu/ 3 ) 3 ].181 The optically active partial cubane cluster [Co 4 (l3 -OH) 3 (l3 - O)(edma) 3 ]Cl has been synthesised and resolved.182 The mixed-valence compound [Co 6 L 2 (CH 3 CO 2 ) 2 (OMe) 6 L@]·2MeOH [L\2,6-bis(salicylideneaminomethyl)-4- methylphenol; L@\salicylazine] has a structure made of two linked defective cubane cores.183 Syntheses and structures are reported184 for the symmetrical cubane [Co 4 (l3 -O) 4 (l3 -CH 3 CO 2 ) 4 (py) 4 ]·5CHCl 3 and the partial cubane [Co 4 (l3 -O) 4 (l3 - OH) 3 (l3 -CH 3 CO 2 )(CH 3 CO 2 )(py) 6 ][PF 6 ] 2 ·2H 2 O.The mixed cluster [Co 3 Fe(mp) 4 - (HMp)(PBu 3 ) 3 ], which contains a Co 3 FeS 4 O 3 core, results from another ‘witches brew’ synthesis involving CoCl 2 , FeCl 3 , H 2 mp, NaOMe and PBu 3 .185 [NH 4 ]- [Co 8 (OAc) 8 (OMe) 16 ][PF 6 ] is the result of a self-assembly reaction in methanolic solutions of cobalt(III) acetate in the presence of NH 4 PF 6 .186 Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 165–188 179Reaction of NiCl 2 ·6H 2 O with Bu5CO 2 H and KOH forms a nonanickel cluster [Ni 9 (Bu5CO 2 H) 4 (l4 -O) 3 (l3 -OH) 3 (Bu5CO 2 ) 12 ], which contains both Ni-(II) and -(III). It is cleaved by donors such as pyridine, a§ording [Ni 2 (py) 4 (O 2 CBu5) 2 (l-O 2 CBu5) 2 (l- OH)] which gives the Ni(II) compound [Ni 2 (py) 2 (HO 2 CBu5) 2 (O 2 CBu5) 2 (l-OH 2 )] on pyrolysis.187 12 Complexes with unusual magnetic properties, including spin-crossover compounds The Kikuchi cluster variation method has been applied to modelling two step highspinflow- spin transitions.188 Iron(II) complexes Iron(II) complexes of 1,2,4-triazole, tetraazole and their derivatives that exhibit thermo- and photo-induced spin state transitions have been reviewed.189 Tris(2- pyridylmethyl)amine complexes of iron(II) illustrate how magnetic properties depend upon the anion.38 The Preisach model has been applied to a study of the hysteresis loops in [Fe(btr) 2 (NCS) 2 ]·H 2 O and [Fe(bt) 2 (NCS) 2 ].190 Three polymorphs of [Fe(dppa)(NCS) 2 ] all have octahedral co-ordination with cis-thiocyanates but with slightly di§erent crystal packing in the solid state.191 One polymorph has an S\2 ground state from 4.5 to 295 K, the second shows a gradual high-spinflow-spin transition without hysteresis and the third an abrupt high-spinflow-spin transition with hysteresis.[Fe(pybzim) 2 ][ClO 4 ] 2 ·H 2 O is diamagnetic \85K and high-spin (k%&& \4.5–5.3 kB )[160 K; it exhibits a spin–equilibrium.192 In [Fe(bzimpy) 2 ]- [ClO 4 ] 2 ·0.25H 2 O, iron is octahedrally co-ordinated by six nitrogens; it exhibits a spin-crossover with broad hysteresis.193 All of the complexes [Fe(bzimpy) 2 ][BPh 4 ] 2 · xH 2 O, (x\4, 2 and 0) have been synthesised; the tetrahydrate exhibits a spin transition centred at room temperature.194 The dihydrate and anhydrous complex are high-spin. The [Fe(bzimpy) 2 ]2` ion exhibits spin-crossover behaviour in 50% propanediol- 1,2–carbonate–MeOH.195 The structure of the spin equilibrium compound [Fe(pm-bia) 2 (NCS) 2 ] has been determined in both spin states and shows an unusually large di§erence in Fe–N bond lengths; it has a very abrupt spin state transition between the S\0 and S\2 states with a 5K hysteresis loop and exhibits LIESST at low temperatures.196 The syntheses and magnetic properties of [Fe(dpq) 2 (NCS) 2 ]· Me 2 CO and [Fe(abpt) 2 (NCX) 2 ] (X\S, Se) are reported.197 The first has an FeN 6 co-ordination sphere and is high-spin; the other two show S\5/2 and S\1/2 spin equilibria.A wide-angle X-ray scattering study of [Fe(Htrz) 2 (trz)][BF 4 ] and [Fe(Htrz) 3 ][NO 3 ] 2 revealed polymeric structures; they are low-spin at room temperature and high-spin above room temperature.198 In the low-spin state, the chain is linear; the magnetic transition causes an elongation and deformation of the chain.[Fe(hyetrz) 3 ][3-O 2 NC 6 H 4 SO 3 ] 2 ·3H 2 O is low-spin at room temperature; on heating, the three water molecules are lost and the anhydrous compound transforms via a metastable low-spin state to a high-spin form.199 A more extensive study showed that [Fe(hyetrz) 3 ]X 2 ·3H 2 O (X\Cl, Br, I, NO 3 , BF 4 , ClO 4 , PF 6 ) contain linear chains of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 180Fe(II) ions involving the neighbouring N(1) and N(2) atoms in the triazole ring. Most of these compounds, like the 3-nitrophenylsulfonate, exhibit non-classical spin-crossover behaviour when the lattice water molecules that stabilise the low-spin state are removed.However, dehydration of the iodide and perchlorate shifts the spin-state transition temperature, which increases with decreasing ionic radius. A rarity, [Fe(hyetrz) 3 ]I 2 exhibits a crossover at room temperature (291 K) with a 12K hysteresis loop.200 At room temperature, [FeM(Et 2 N) 3 terpyN2 ][PF 6 ] 2 has k%&& \1.6 kB indicating thermal population of the S\2 state; studies on Co(II) complexes support the view that electron-donating p-substituents in the terpyridine rings weaken the ligand field.201 Complexes of a ligand derived from terpy, by replacing a terminal pyridine ring with a thiazole ring, have been studied.[Fe(thzby) 2 ][PF 6 ] 2 is low-spin, as is [Fe(mthzby) 2 ]- [BF 4 ] 2 , though in the latter there is evidence of a high-spin species at high temperatures.Replacing the central pyridine ring with a thiazole, forming pythiaz, reduces the ligand field, so that [Fe(pythiaz) 2 ][BF 4 ] 2 is high-spin. The structures of all three compounds show the characteristic Fe–N bond lengths for the spin state in question. 202a A further study in which the terminal pyridine ring of terpy is replaced by a triazole system, forming the ligands 6–(1,2,4–triazol-3-yl)-2,2@-bipyridine, 6-(1-methyl- 1,2,4-triazol-3-yl)-2,2@-bipyridine and 6-(1,5-dimethyl-1,2,4-triazol-3-yl)-2,2@-bipyridine, produced complexes that are low-spin at room temperature but where there is some population of the high-spin state on increasing the temperature.202b The complexes [Fe(bpyam) 2 ]X 2 (X\BF 4 , ClO 4 , CF 3 SO 3 ) are high spin; on cooling, the perchlorate and triflate undergo a partial transition to a singlet state.203 The structure of [Fe(bpyam) 2 ][BF 4 ] 2 was determined.The spin-crossover compound bis[hydrotris( 1,2,4-triazolyl)borato]iron has been studied as pure and diluted solids and also in solution.204 Iron(III) complexes The first low-spin iron(III) semiquinonate complex has been reported.205 [Fe(LN 4 Me 2 )(dnbsq)][ClO 4 ] 2 ·2.5H 2 O is low-spin with the unpaired electron of the Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 181semiquinonate strongly antiferromagnetically coupled with the unpaired electron of the metal. Iron(III) cryptates and podands have been studied 14.The cryptates, with FeN 6 co-ordination spheres, are S\3/2 systems compared with analogous azaphenolates with FeN 4 O 2 co-ordination spheres.206 Another example of an S\3/2 iron(III) system is provided by a complex 15 with a square-pyramidal FeClN 2 S 2 co-ordination sphere.207 Six-co-ordinate iron(III) Schi§ base species of the general type [Fe(salen@)(Him) 2 ]Y [e.g.salen@\N,N@-4-chloro-o-phenylenebis(3-methoxysalicylidineimine); Y\ClO 4 , PF 6 , BPh 4 ] can be high-spin, low-spin or exhibit a S\5/2fS\1/2 spin–equilibrium, 208 as in the case of [Fe(salen@)(Him) 2 ][ClO 4 ]. Magnetic properties are reported for iron(III) complexes of tridentate (O,N,O; O,N,S; and O,N,N) and of tetradentate (O,N,N,O) azomethine ligands.209 Spin–equilibria are observed for compounds of the (O,N,S) ligand such as 16 and for [FeXL(NCS)] [X\py or Me 2 CO; L\S-substituted N1-salicylidene-N4-M5–(R)-salicylideneisothiosemicarbazoneN].210 Spin– equilibrium behaviour is also reported for dinuclear diiron(III) complexes with thiolate bridges.211 Cobalt complexes [Co(bipy) 3 ]2` ions are high-spin in solid salts or solution; when encapsulated in zeolite-Y supercages, the normally trigonally distorted cation undergoes a twist that results in a near-octahedral geometry, causing it to exhibit spin-crossover behaviour. 212 Intramolecular electron transfer between cobalt(II) semiquinonate and cobalt( III) catecholate has been investigated in the compounds [Co(N–N)(3,6-dbbq) 2 ] Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 165–188 182(N–N\tmmda, tmen, tmpda).The transition temperature for redox equilibrium is lowest for [Co(tmpda)(3,6-dbbq) 2 ], attributed to the flexibility of the six-membered metal–chelate ring. The structures of [CoII(tmmda)(3,6-dbc)(3,6-dbsq)], [CoII(tmedn)(3,6-dbc)(3,6-dbsq)] and [CoIII(tmpda)(3,6-dbsq) 2 ] were reported.213 The compound [Co(L)(3,6-dbbq) 2 ] (L\phen or 4,7-substituted phen) shows solvent dependent redox equilibria214 and is mainly quinone-centred rather than metal-centred in CH 2 Cl 2 .A cobalt complex 17 of a Schi§ base diquinone [CoIII(cat-N-bq)(Cat-Nsq)] exhibits an equilibrium in solution forming [CoII(cat-N-bq) 2 ]. This is temperature- dependent, the solutions changing colour from green-blue to brick brown on cooling together with the appearance of an EPR signal.215 References 1 A.K.Powell, Struct. Bonding, 1997, 88, 2. 2 M. J. Maroney, G. Davidson, C. B. Allan and J. Figlar, Struct. Bonding, 1998, 92, 2. 3 E. Nordlander, A. Thapper, J. King, C. 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