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X-Ray absorption spectra at the chromium and cobalt K edges have been recorded for the metal-disordered compound [CrIII2CoIIO(MeCO2)6(py)3]·py (py = pyridine) and the associated extended X-ray absorption fine structure (EXAFS) analysed using curved-wave theory with the inclusion of multiple scattering for the co-ordinated pyridine and acetate groups; a corresponding study for the iron K-edge EXAFS of [Fe3O(MeCO2)6(py)3]·py was accomplished as a calibrant of the interpretive procedure. The dimensions Cr–µ3-O 1.87, Co–µ3-O 1.95, Cr–O (acetate) 1.98, Co–O (acetate) 2.09, Cr–N 2.26, Co–N 2.24, Cr ⋯ Cr 3.30 and Cr ⋯ Co 3.28 A were obtained from the analysis of the EXAFS, with good agreement between the values of the Cr ⋯ Co distance obtained independently from the analyses of the chromium and cobalt EXAFS. The metal–ligand and –metal distances are in good agreement with crystallographic data on related compounds. The difference in the Cr–µ3-O and Co–µ3-O distances indicates that the central (µ3) oxygen atom is slightly displaced (ca. 0.05 A) from the centre of the metal triangle, away from the CoII and towards the point midway between the two chromium(III) atoms. Such a structural change and the increased electron density of the central oxygen combine to produce a marked increase in the CrIII–O–CrIII superexchange and a reduced ligand field at the chromium(III) sites, for the Cr2CoO system as compared with CrIII3O analogues.

The following standard enthalpies (kJ mol–1) of formation of the crystalline solids at 298.15 K have been determined using solution reaction calorimetry: ΔHt⊖[Mo2(O2CMe)4c]=–1 970.7 ± 8.4, ΔHt⊖[CrMo(O2CMe)4,c]=–2 113.9 ± 6.4, ΔHt⊖[Cr2(O2CMe)4,c]=–2 297.5 ± 6.6, ΔHt⊖[Cr2(O2CMe)4.2H2O,c]=–2 875.4 ± 6.7, and ΔHt⊖[Mo2(O2CMe)2(pd)2,c](pd = pentane-2,4-dionate)=–1 805.0 ± 8.9. The corresponding metal–metal bond enthalpy contributions in the gaseous species have been assessed as D(Mo–Mo)= 334, D(Cr–Mo)= 249, and D(Cr–Cr)= 205 kJ mol–1. The bond enthalpy contribution for the co-ordination of each water molecule in [Cr2(O2CMe)4]·2H2O has been estimated as 44 kJ mol–1.

The addition complexes [(H2C:CH)2Sn(O2C·CF3)2Ln][n= 1, L = 2,2′-bipyridyl, 1,10-phenanthroline, NN′-ethylenebis(salicylidineimine) or ethylene-1,2-diamine; n= 2, L = triphenylphosphine oxide or hexamethylphosphoramide] have been prepared and characterised. These compounds (II)–(VII) all contain octahedrally co-ordinated tin(IV) with two unidentate trifluoroacetato-groups. This geometry has been confirmed by X-ray crystallography for the 2,2′-bipyridyl addition complex (II) which crystallises in the monoclinic space group P21/c, with Z= 4 in a unit cell of dimensions a= 8.04, b= 15.39, c= 17.37 A, and β= 106.6°. The structure has been determined from three-dimensional X-ray diffraction data by Patterson and Fourier methods, and refined by least-squares to a final R of 0.070 (for 2194 diffractometer intensities). (II) Exists as discrete molecules in the crystal; some mean distances are: Sn–O 2.21(4), Sn–C 2.10(2)(two mutually transσ-bonded vinyl groups), and Sn–N 2.34 ± 0.01 A, [(H2C:CH)2Sn(O2C·CF3)2] reacts with 8-hydroxyquinoline or sodium acetylacetonate to afford [(H2C:CH)2Sn(ONC9H7)(O2C·CF3)] or [(H2C:CH)2Sn(O2C5H7)2] respectively, and with carboxylic acids to produce the corresponding [Sn(O2CR)4] derivatives.

The preparation, spectroscopic properties, and X-ray crystal structures of two triphenylphosphine–gold–thiomolybdate clusters are reported. [MoS4(AuPPh3)2] crystallises in the triclinic space group P with a= 9.596(1), b= 10.630(1), c= 19.712(2)A, α= 89.06(1), β= 80.87(1), γ= 66.99(1)°, and Z= 2. [MoOS3(AuPPh3){Au(PPh3)2}] crystallises in the monoclinic space group P21/c with a= 21.251(2), b= 10.535(1), c= 23.167(2)A, β= 104.20(1)°, and Z= 4. The structures have been solved from 5 748 and 7 476 unique observed diffractometer data and have been refined to R= 0.031 and 0.036, respectively. The 95Mo n.m.r. spectra of these and related copper and silver tetrathiomolybdate(VI) clusters consist of a single resonance, whose chemical shift depends on the mole ratio of added PPh3 to Mo in the solution.

A 4Fe-ferrodoxin analogue involving functional thiolato-groups, [Fe4S4(SC6H4NH2-4)4]2–, has been prepared and, as its [NEt4]+ salt, characterised by X-ray crystallography, u.v.-visible, 1H and 13C n.m.r. spectroscopy, and electrochemistry. The geometry of the Fe4S4 core is very similar to that previously established for other clusters of this type and possesses crystallographic S4 symmetry; the Fe4 tetrahedron is slightly compressed along the S4 axis and the Fe ⋯ Fe edges group as four shorter [2.741(1)A] and two longer [2.767(1)A] distances. The NH2 groups do not participate in any secondary bonding interactions. The complex undergoes two quasi-reversible, one-electron reductions at –1.12 and –1.74 V (vs. saturated calomel electrode).

The variation of the effective magnetic moment, m, with temperature in the range 1.9–300 K, and with applied magnetic field (1.25–20.0 kG), is reported for [NEt4]3[Fe6M2S8(SPh)6(OMe)3][M = Mo (1) or W (2)], [NBun4]3[Fe6Mo2S8(SPh)9](3), and [NEt4]3[Fe6W2S8(SEt)9](4). The e.s.r. spectra at 4.2 K for powders of (2), (3), and (4), and a frozen glass in MeCN for (2) are also reported. An interpretation of these data is presented in terms of antiferromagnetic spin coupling between two FeII ions and an FeIII ion within a Fe3MS4 sub-unit, with an additional much weaker couping between the sub-units.

Monoclinic crystals of form (1) belong to space group C2/c, with a= 31.340(10), b= 9.671(3),c= 38.027(10)A, β= 110.72(5)°; the unit cell contains eight molecules of [MoO(S2CNEt2)2]2O oxo-bridged dimers (four with approximate C2 symmetry, four with crystallographic Ci symmetry), four [Mo2O3(S2CNEt2)4]– anions, and four [H5O2]+ ions. The structure has been solved with the aid of direct methods and refined by full-matrix least-squares calculations to R 0.074 using 5 508 reflections. Triclinic crystals of compound (2) belong to space group P, with a= 8.493(3), b= 12.232(4), c= 9.775(3)A, α= 107.52(5), β= 114.40(5), and γ= 95.82(5)°; the unit cell contains one centrosymmetric [MoO(S2CNEt2)2]2O oxo-bridged dimer. The structure has been solved by the heavy-atom method and refined by full-matrix least-squares calculations to R 0.052 using 2 340 reflections. The distorted-octahedral geometries around the molybdenum atoms in both compounds are similar; the respective mean dimensions are Mo–O(terminal) 1.686(13), 1.680(14)A, and Mo–O(bridging) 1.875(14), 1.859(1)A. The Mo–S lengths range from 2.436(2) to 2.694(2)A depending upon the nature of the trans ligand. The single-crystal e.s.r. behaviour of (1) shows the presence of two distinct paramagnetic molybdenum centres in the crystal which have been assigned to the [Mo2O3(S2CNEt2)4]– ion. The principal molecular g values for these two centres are 1.976, 1.978, 1.984, and 1.978, 1.981, and 1.984.

The first example of a MoOS2+ complex, cis-[MoOS(C5H10NO)2][C5H10NO = piperidine N-oxido(1–)], prepared by reaction of the corresponding cis-MoO22+ complex with B2S3, has been crystallographically characterised. The title compound crystallises in the monoclinic space group P21/n, with a= 9.671(1), b= 22.401(2), c= 13.116(2)A, β= 95.51(1)°, and eight molecules, comprising two crystallographically distinct species, per unit cell. The structure has been determined from 4 033 unique observed diffractometer data and refined to R= 0.047. The two independent molecules are virtually identical, and possess Mo–O and Mo–S bonds of length 1.723(12) and 2.106(5)A respectively and an O–Mo–S interbond angle of 116.9(7)°. The dimensions and a selection of spectroscopic properties of the title complex are compared with those of the corresponding cis-[MoY2(C5H10NO)2](Y = O or S) complexes; cis-[MoS2(C5H10NO)2] may be prepared by treating the MoOS2+ complex with B2S3.

The gas-phase core-electron ionization energies of the complexes [M2L4][M = Cr, L = O2CMe or mhp (6-methylpyridin-2-olate); M = Mo, L = O2CH, O2CMe, or mhp] are reported. These data, together with the valence d ionization energies of these molecules, show a correlation between metal electron ionization energies and the large variations in the metal–metal separations found for the dichromium(II) species. A model in which these separations are mainly determined by the electrostatic potential at the metal centre is suggested by the experimental observations and supported by ab initio calculations on [M2L4][M = Cr or Mo; L =(CH2)2PH2] reported here. This electrostatic potential is a function of the ligand; the lower the potential the shorter is the metal–metal separation, primarily because of the shallowness of the Cr–Cr potential well. The description of the metal–metal interaction, for a given Cr–Cr separation, is found to be insensitive to the nature of the ligand.

The complexes [(PhS)CuS2MoS2]2– and [(PhS)CuS2MoS2Cu(SPh)]2– have been prepared by the reaction of KSPh on the corresponding cyano-complex and isolated as their [NPrn4]+ salts. The complex [NPrn4]2[(PhS)CuS2MoS2] crystallises in the space group P21/c with a=16.534(2), b=16.608(2), c=14,863(2)A, β=104.60(1)°, and Z= 4, while [NPrn4]2[(PhS)CuS2MoS2Cu(SPh)] crystallises in the space group C2/c with a=17.064(2), b=15.646(2), c=18.448(2)A, β= 110.47(1)° and Z= 4. The structures were solved by Patterson and Fourier techniques from 4 495 and 2 404 diffractometer data, respectively, and refined to respective R values of 0.060 and 0.051. The anion [(PhS)CuS2MoS2]2– involves an essentially tetrahedral [MoS4]2– moiety co-ordinated as a bidentate ligand to a copper(I) atom [Mo-S(terminal)= 2.162(6), Mo-S(Cu)= 2.221(9), Cu-S = 2.211(13), and Mo ⋯ Cu = 2.636(1)A], the third position of the trigonal planar co-ordination about the copper being occupied by a benzenethiolate group [Cu-S = 2.188(2)A]. The anion [(PhS)CuS2MoS2Cu(SPh)]2– has crystallographic C2 symmetry and involves the co-ordination of an essentially tetrahedral [MoS4]2– moiety to two copper(I) atoms [Mo-S = 2.206(6), Cu-S = 2.212(2), and Mo ⋯ Cu = 2.632(1)A], with each trigonal planar copper(I) atom also participating in a Cu–SPh bond of length 2.171(2)A. These structural data have been used to interpret 95Mo and 1H n.m.r., vibrational, and u.v.-visible spectroscopic data.

Raman, resonance Raman at ca. 80 K, and infrared spectra of thirteen ions of the type [MS4-(CuL)n]2–(M = Mo or W; L = CN, SPh, SC6H4Me-4, Cl, or Br; n= 1–4) are reported. Totally symmetric ν(Mo–S) and ν(W–S) stretches are easily identified as giving rise to the most intense bands in the Raman and particularly resonance Raman spectra of the complexes. The pattern (i.e. the number of bands, their wavenumbers and relative intensities) observed in the ν(Mo–S) region (415–496 cm–1) is distinctive for the number of CuI ions co-ordinated to [MoS4]2–.

The syntheses of [MoO(SR)4]–(R = C6H5, C10H7, 4-CH3C6H4, 4-ClC6H4, 3-ClC6H4, 4-FC6H4, 2,6-Cl2C6H3, or C6F5) complexes by several different routes are deported. The u.v.-visible, i.r., Raman, e.s.r. spectroscopic and redox properties of these complexes are detailed. The redox potentials of the MoVI–MoV and MoV–MoIV couples correlate with the Hammett function (σ) of the aryl substituent(s); EP(MoVI–MoV)= 0.53σ+ 0.53 V (vs. saturated calomel electrode, s.c.e.) and Ep(MoV-MOIV)= 0.52σ– 0.71 V (vs. s.c.e.). The effect of the electron-withdrawing substituent(s) is also clearly seen in the u.v.–visible spectra, less so in the vibrational spectra, and is absent in the e.s.r. spectra. The synthesis of [MoO(SC6F5)4]2– has been demonstrated and this provides the first spectroscopic comparison of [MoO(SR)4]n–(n= 1 or 2) complexes.

The molecular structure and interatomic dimensions of [Mo2(O2CCF3)4] in the gas phase at 150 °C have been determined by electron-diffraction measurements and shown to be similar to those obtained by X-ray diffraction methods; dimensions obtained include Mo–Mo 2.105(9), Mo–O 2.102(6)A, and Mo–Mo–O 91.9(3)°.

57 Fe Mossbauer parameters of several [Fe6M2S8(SR)9]3–(where M = Mo or W) complexes and [Fe6W2S8(OMe)3(SPh)6]3– have been obtained at 4.2, 77, 195, and 293 K. These are seen to be very similar to those obtained for [Fe4S4(SR)4]2– complexes and, on the basis of the isomer shift, each iron atom is considered to have a net oxidation state of ca. 2.5, thus implying that each molybdenum or tungsten atom has an oxidation state between 3 and 4. Electrochemical reduction of the trianions to the 4–, 5–, 6– and, in some instances, 7– species has been monitored. The first of these reductions occurs at a potential virtually identical to that of the corresponding [Fe4S4(SR)4]2– complex, the second reduction occurs at a potential ca. 200 mV more negative. These two reductions generally appear to be reversible for most of the complexes in acetonitrile solution. However, rapid scan, staircase cyclic voltammetry showed that only [Fe6M2S8(SEt)9]3–(M = Mo or W) in acetonitrile solution approach good electrochemical reversibility.

The reaction between [Mo2(O2CCF3)4] and 2,2′-bipyridyl (1: 2) has been investigated in several solvents, and four different adducts, two 1:1 and two 1:2 compounds, have been isolated and characterised. The reaction between [Mo2(O2CCF3)4], 2,2′-bipyridyl, and [OEt3][BF4](1:2:2) has been shown to produce [Mo2(O2CCF3)2(bipy)2]-[BF4]2. Structures have been assigned to these compounds on the basis of spectroscopic data and the nature of the solution species, from which the various adducts originate, is discussed.

The standard enthalpies of formation at 298.15 K of the crystalline solids [Mo(pd)3], ΔHf⊖=–1323.9 ± 5.9 kJ mol–1, and [MoO2(pd)2](pd = pentane-2,4-dionate), ΔHf⊖=–1 365.5 ± 4.5 kJ mol–1, have been determined using solution reaction calorimetry. The mean Mo–O bond-enthalpy contributions involving the pentane-2,4-dionate groups have been assessed in the two compounds as 192.0 and 187.3 kJ mol–1, respectively.

The title complex, [Cu2Cl(Him)4]Cl, has been prepared from copper(I) chloride and imidazole (Him) in acetonitrile solution. It crystallises in the trigonal space group P3221, with a= 8.546(1), c= 21.370(3)A, and Z= 3. The trigonally co-ordinated Cu atoms are joined by a Cl bridge [Cu–Cl 2.564(2)A, mean Cu–N 1.904(5)A, Cu–Cl–Cu 95.5(1)°, Cu ⋯ Cu 3.797(3)A]. The Cl– anion lies between the two hinged Cu co-ordination planes, with Cu ⋯ Cl 3.134(2)A. [Cu2Cl(Him)4]Cl reacts readily and irreversibly with dioxygen in solution; in the solid state, only surface reaction occurs, although the molecular structure is similar to the active centre proposed for hemocyanin proteins.

Anaerobic reactions, in methanolic solution, between iron(III) or iron(II) chloride, elemental sulphur, and the lithium or sodium salt of a thiol iead to the formation of the corresponding [Fe4S4(SR)4]2– complex in good yield (R = CH2CH2OH, But, or Ph).