DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2331–2334 2331 Structural, spectroscopic and redox studies of mer-[RuX3L3] (L = PMe2Ph or AsMe2Ph, X = Cl or Br). Crystal structures of mer-[RuX3(AsMe2Ph)3] (X = Cl or Br) and [Ru2X5(EMe2Ph)4] (X = Br, E = P or As; X = I, E = As) Nicholas J. Holmes, Anthony R. J. Genge, William Levason * and Michael Webster Department of Chemistry, University of Southampton, Southampton, UK SO17 1BJ The complexes mer-[RuX3L3] (X = Cl or Br, L = PMe2Ph or AsMe2Ph) have been re-examined and assignments for their UV/VIS spectra proposed.Attempts to prepare analogues with X = I or L = SbR3 have been unsuccessful. Cyclic voltammetry revealed only irreversible oxidation and reduction processes and chemical oxidation with halogens resulted in decomposition to [RuX6]22, in contrast to the chemistry of related osmium compounds. The crystal structures of mer-[RuX3(AsMe2Ph)3] (X = Cl or Br) have been determined and confirm the geometrical isomer formed.Crystal structures were also determined for [Ru2X5L4] (X = Br, L = PMe2Ph or AsMe2Ph; X = I, L = AsMe2Ph) obtained by decomposition of mer-[RuX3L3] in solution, which are the first structurally characterised examples of Ru2 51 dimers of this type with bromide or iodide co-ligands. The structures are consistent with a formal Ru]Ru bond order of ��� . In previous studies we have described the effects of systematic variation of both neutral and halide ligands upon the stability, spectroscopic properties and redox chemistry of several series of osmium complexes including trans-[OsX4L2]0/2 (ref. 1), mer- [OsX3L3]0/1 (ref. 2) and trans-[OsX2L4]0/1/21 (ref. 3) (L = PR3, AsR3 or SbR3; X = Cl or Br, sometimes I). Limited information is available for ruthenium complexes with monodentate ligands, in part because of the much greater reactivity and their tendency to rearrange into halide-bridged dimers,4,5 Here we report studies of representative mer-[RuX3L3] complexes and the structures of some mixed-valence RuII]RuIII dimers formed by their decomposition.Results and Discussion Synthesis and properties The complexes mer-[RuCl3L3] (L = PMe2Ph or AsMe2Ph) were made by reaction of RuCl3?xH2O with the ligand in ethanol– concentrated HCl and converted into the bromides by metathesis with LiBr.6 Only one example of a mer-[RuI3L3] (L = AsMePh2) is mentioned in the literature,7 made by reaction of AsMePh2 with K2[RuCl5(H2O)] and KI in ethanol and with little characterisation.In our hands repeated attempts to make mer-[RuI3L3] (L = PMe2Ph or AsMe2Ph) by metathesis of the chloro complexes with LiI in a variety of solvents failed, the major products being ruthenium(II) complexes of type trans- [RuI2L4], along with smaller amounts of other uncharacterised ruthenium species. In situ monitoring of the UV/VIS spectra of these reactions showed the rapid development of species with intense absorptions at ca. 10 000 cm21, which are probably I(p)Æ RuIII(t2g) charge-transfer (CT) bands,8 but the spectra rapidly decay into ones characteristic of RuII. The reaction of [Ru(dmf)6]31 (dmf = dimethylformamide)9 with L and LiI in ethanol also gave trans-[RuI2L4] 10 along with some [Ru2I5L4] (see below). It seems possible that mer-[RuI3L3] may form transiently, but decompose too rapidly to be isolated. In a similar vein, reaction of RuX3?xH2O (X = Cl or Br) with SbMe2Ph under a variety of conditions gave trans-[RuX2(SbMe2Ph)4] 10 as major products and no examples of mer-[RuX3(SbR3)3] are known.† † We have shown elsewhere 11 that whilst trans-[RuCl2(SbPh3)4]BF4 can be made, there is no good evidence for mer-[RuCl3(SbPh3)3], although osmium(III) analogues are well characterised.2 The IR and ESR spectra of the mer-[RuX3L3] complexes (Experimental section) are in agreement with literature data.12 Electrospray mass spectrometry (MeCN solution) gave features with the appropriate isotope patterns for [RuX2L3]1, [RuX2L2]1 and [RuXL3]1 ions.The rich UV/VIS spectra of these complexes are listed in Table 1 and examples are shown in Fig. 1. The assignments of the major features in terms of ligand-tometal CT transitions in C2v symmetry follows from those of the analogous mer-[OsX3L3].2 For the osmium complexes corresponding bands are found ca. 3000–4000 cm21 to high energy compared with the ruthenium complexes, reflecting the greater ease of reduction of the latter. For osmium a small number of fac-[OsCl3L3] (L = PMe2Ph, PEt2Ph or AsMe2Ph) are known,2,6,13 but no ruthenium analogues have been characterised.Our attempts to convert mer-[RuCl3L3] into the fac isomer, by sequential treatment with NaBH4 and HCl (as used for the osmium complexes),13 failed and mer-[RuCl3L3] in toluene were not isomerised by photolysis (254 nm, 96 h). Crystal structures of mer-[RuX3(AsMe2Ph)3] (X = Cl or Br) The two compounds are isomorphous and are shown by the X-ray study to be the mer geometrical isomer with angles at the Ru atom within 98 of the idealised octahedral values (see Fig. 2 and Table 2). Despite the fact that mer-[RuX3L3] compounds Fig. 1 The UV/VIS spectra of mer-[RuX3(AsMe2Ph)3] [X = Cl (—) or Br (. . .)] in CH2Cl22332 J. Chem. Soc., Dalton Trans., 1997, Pages 2331–2334 Table 1 The UV/VIS data for the mer-[RuX3L3] complexes a Complex s(L) t2g(Ru) s(L) 1 s(X) t2g(Ru) p(X) t2g(Ru) Others b [RuCl3(PMe2Ph)3] 15.6 (240) 19.7 (1 460) 22.9 (990) 35.8 (21 160), 38.8 (16 250) [RuBr3(PMe2Ph)3] 14.5 (780) 16.5 (sh) (900) 18.1 (1000), 22.5 (840) 34.5 (12 500), 38.2 (15 000) [RuCl3(AsMe2Ph)3] 12.6 (1290), 15.3 (sh) 20.0 (1500) 23.2 (1140) 34.0 (14 770), 38.2 (19 900) [RuBr3(AsMe2Ph)3] 12.5 (1360), 14.1 (sh) 16.8 (1230) 18.8 (1160), 22.3 (970) 32.6 (13 300), 36.5 (17 450) a Emax/103 cm21 (emol/dm3 mol21 cm21) in CH2Cl2 solution.b p Æ p* of aryl rings will occur in this region.have long been known and are useful precursors to other ruthenium complexes, none has been structurally characterised, although with nitrogen donors there are a few examples of octahedral ruthenium(III) chloro complexes. There are also a few examples 14–16 of the anionic ruthenium(III) species trans- [RuCl4(PR3)2]2 (R = Et, Bu or Ph). The Ru]Cl distances in the present compound [2.339(5)–2.387(5) Å] appear typical and may be compared 14 with 2.367(4) and 2.361(4) Å found in Fig. 2 The structure of mer-[RuBr3(AsMe2Ph)3] showing the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. The corresponding chloro compound has essentially the same structure Table 2 Selected bond lengths (Å) and angles (8) for mer-[RuX3- (AsMe2Ph)3] (X = Cl or Br) X = Cl X = Br Ru]X(1) 2.339(5) 2.461(2) Ru]X(2) 2.387(5) 2.513(2) Ru]X(3) 2.345(5) 2.476(2) Ru]As(1) 2.456(2) 2.467(2) Ru]As(2) 2.473(2) 2.482(2) Ru]As(3) 2.495(2) 2.509(2) As]C 1.90(2)–1.97(2) 1.91(1)–1.96(1) C]C 1.36(3)–1.40(2) 1.35(2)–1.42(2) X(1)]Ru]X(2) 92.2(2) 91.7(1) X(1)]Ru]X(3) 173.2(2) 173.0(1) X(2)]Ru]X(3) 93.8(2) 93.7(1) As(1)]Ru]As(2) 95.9(1) 96.8(1) As(1)]Ru]As(3) 168.4(1) 168.6(1) As(2)]Ru]As(3) 93.0(1) 92.2(1) X(1)]Ru]As(1) 89.3(1) 88.4(1) X(1)]Ru]As(2) 84.6(1) 85.1(1) X(1)]Ru]As(3) 98.9(1) 99.4(1) X(2)]Ru]As(1) 82.5(1) 81.8(1) X(2)]Ru]As(2) 176.5(1) 176.5(1) X(2)]Ru]As(3) 88.9(1) 89.6(1) X(3)]Ru]As(1) 88.1(1) 88.0(1) X(3)]Ru]As(2) 89.3(1) 89.4(1) X(3)]Ru]As(3) 84.6(1) 85.0(1) trans-[RuCl4(PEt3)2]2.Structurally determined Ru]As bonds are rare and the present values [2.456(2)–2.509(2) Å] can be compared 17 with those in trans-[RuBr2{C6F4(AsMe2)2-o}2]1 [2.457(1), 2.460(1) Å] and this cation provides a comparator Ru]Br distance [2.455(1) Å]. Redox chemistry An initial aim of this study was to probe the redox chemistry of mer-[RuX3L3] type complexes. As background it is useful to recall that the osmium(III) analogues mer-[OsX3L3] undergo irreversible one-electron reductions to osmium(II) species which readily undergo halide substitution and/or dimerisation depending upon the conditions.18 In contrast electrochemically reversible one-electron oxidation produces mer-[OsX3L3]1, which can be isolated as BF4 2 salts by HNO3–HBFtment of mer-[OsX3L3].2 Cyclic voltammetric studies of mer-[RuX3L3] in CH2Cl2 containing 0.2 mol dm23 [NBun 4][BF4] at scan rates of 0.02–0.2 V s21 showed completely irreversible reduction and oxidation processes at ca. 10.1 and 11.4 V (versus ferrocene– ferrocenium at 10.58 V) showing that neither [RuX3L3]2/1 are stable on this time-scale. Attempted chemical oxidation was also unsuccessful. Addition of the appropriate halogen in CCl4 to CH2Cl2 solutions of mer-[RuX3L3] produced immediate colour changes, but the UV/VIS spectra identified the ruthenium product as the corresponding [RuX6]22,19 whilst the solid complexes decolourised rapidly when added to concentrated HNO3–HBF4 at 0 8C.Crystal structures of [Ru2X5(EMe2Ph)4] (X = Br, E = P or As; X = I, E = As) During the attempted crystallisation of several of the mer species described above, there were often crystals formed with the same colour but two distinct morphologies. Typically a few large block crystals formed in the presence of many smaller rhombic shaped ones. Hand selection and X-ray examination of these smaller crystals established that the structures were dinuclear. In the iodo case below the dinuclear product was obtained from the solution decomposition of trans- [RuI2(AsMe2Ph)4] during crystal growth.We now report the structure of the following three species: [Ru2Br5(PMe2Ph)4], [Ru2Br5(AsMe2Ph)4] and [Ru2I5(AsMe2Ph)4]. The chloro species [Ru2Cl5(PMe2Ph)4] also formed in this way and was identified by comparison with the unit-cell dimensions previously reported.15 All three compounds are of the RuII]RuIII mixedvalence type and are isomorphous.The [Ru2Br5(PMe2Ph)4] compound is shown in Fig. 3 and selected bond lengths and angles in Table 3. It was refined in the space group C2/c where the molecule has C2 crystallographic symmetry and is isomorphous with the chloro compound.15 The Ru]Ru distance [3.083(2) Å] is longer and the Ru]Br]Ru angles are more acute [73.24(6), 74.01(5)8] than the chloro derivative [2.9941(4) Å, 74.41(4) and 75.41(3)8 respectively]. As expected the terminal Ru]Br is shorter than the bridging distances and the bridging bromine not on the two-fold axis is bonded unsymmetrically to the Ru atoms (0.12 Å difference). The compounds [Ru2Br5(AsMe2Ph)4] and [Ru2I5(AsMe2Ph)4] were again refined in the space group C2/c and key structural parameters are shown in Table 3 (see also Fig. 3). As commented on recently 20 for RuII]RuII species [Ru2X3L6]1, theJ. Chem. Soc., Dalton Trans., 1997, Pages 2331–2334 2333 replacement of P by As results in a shorter Ru]Ru and the same trend is observed for these mixed-valence compounds.The Ru]X]Ru angles for X = I are about the same (728) as for X = Br and this together with the longer Ru]I bonds results in an increased Ru]Ru distance [3.197(5) Å]. The [Ru2Cl5(PMe2Ph)4] complex has been studied in some detail by Cotton and Torralba,15 and the compounds reported here are the first examples of bromide and iodide analogues. Although obtained serendipitously and at present in too small yield for detailed spectroscopic study, the structures strongly suggest that they can be regarded as having a metal oxidation state of 2.5, with a delocalised electron and a formal Ru]Ru bond order of ��� .As would be expected, the Ru]Ru distance varies with the identity of the bridging halide from 2.99 (X = Cl) 15 to 3.20 Å (I), but even in the latter the distance is shorter than in the unsymmetrical dimer [(Bu3P)3RuCl3- RuCl2(PBu3)] (3.28 Å)15 which is considered as valence-trapped RuII]RuIII with no metal–metal bond.Experimental Physical measurements were made as described previously.2 Electrospray mass spectra were obtained using a Hewlett- Packard series 1050 mass spectrometer operating in positive electrospray mode using solutions in MeCN and ESR spectra from powdered solids at 150 K on a Bruker ECS 106 spectrometer. Preparations mer-[RuCl3(PMe2Ph)3]. The compound RuCl3?xH2O (0.63 g, 2.4 mmol) was dissolved in ethanol (30 cm3) along with concen- Fig. 3 The structure of [Ru2Br5(PMe2Ph)4] showing the atom labelling scheme.Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. The two other compounds [Ru2Br5(AsMe2Ph)4] and [Ru2I5(AsMe2Ph)4] have essentially the same structure Table 3 Selected bond lengths (Å) and angles (8) for [Ru2X5(EMe2Ph)4] (X = Br or I, E = P or As)* X = Br, E = P X = Br, E = As X = I, E = As Ru]Ru9 3.083(2) 2.941(2) 3.197(5) Ru]X(br 1) 2.585(2) 2.528(2) 2.712(4) Ru]X(br 2) 2.501(1) 2.473(2) 2.677(3) Ru9]X(br 2) 2.619(2) 2.546(2) 2.738(4) Ru]X(t) 2.486(2) 2.431(2) 2.714(3) Ru]E(1) 2.311(3) 2.405(1) 2.435(4) Ru]E(2) 2.310(3) 2.410(2) 2.430(5) E]C 1.81(1)–1.83(1) 1.92(1)–1.96(1) 1.95(3)–2.00(3) Ru]X(br 1)]Ru9 73.2(1) 71.1(1) 72.2(1) Ru]X(br 2)]Ru9 74.0(1) 71.7(1) 72.3(1) X(br 2)]Ru]X(t) 178.0(1) 178.1(1) 176.9(1) * t = terminal, br = bridge (br 1 on two-fold axis).Symmetry labels: (9) 1 2 x, y, ��� 2 z [1 2 x, y, 3– 2 2 z (X/E = Br/As only)].trated HCl (1 cm3). To this PMe2Ph (0.98 g, 7.1 mmol) was added and the mixture heated to reflux under nitrogen for ca. 5 min and then cooled. A brown solid separated from the solution and was filtered off, washed with diethyl ether (2 × 15 cm3) and dried in vacuo (0.72 g, 48% based on RuCl3?xH2O) (Found: C, 46.1; H, 5.2. Calc. for C24H33Cl3P3Ru: C, 46.3; H, 5.4%). n(Ru]Cl)/cm21 (Nujol mull) 327, 300 and 270. Electrospray mass spectrum: m/z = 586, 550 (calc.for C24H33 35Cl2P3 101Ru 585, C24H33 35ClP3 101Ru 550). mer-[RuBr3(PMe2Ph)3]. The complex mer-[RuCl3(PMe2Ph)3] (0.53 g, 0.85 mmol) was suspended in ethanol (30 cm3). To this LiBr (1.77 g, 20 mmol) was added and the mixture heated to reflux under nitrogen for ca. 10 min and then cooled. A deep purple solid separated from a similar coloured solution and was filtered off, washed with water (2 × 10 cm3) and dried in vacuo (0.38 g, 59%) (Found: C, 37.9; H, 4.1. Calc. for C24H33Br3P3Ru: C, 38.2; H, 4.4%).n(Ru]Br)/cm21 (Nujol mull) 242 and 225. Electrospray mass spectrum: m/z = 675, 595 and 537 (calc. for C24H33 79Br2P3 101Ru 673, C24H33 79BrP3 101Ru 594, C16H22 79Br2P2- 101Ru 535). mer-[RuCl3(AsMe2Ph)3]. The compound RuCl3?xH2O (1.28 g, 4.90 mmol) was dissolved in ethanol (25 cm3) along with concentrated HCl (2.5 cm3). To this AsMe2Ph (3.33 g, 18.3 mmol) was added and the mixture heated to reflux under nitrogen for ca. 1 h and then cooled. A dark green solid separated which was filtered off, washed with diethyl ether (2 × 15 cm3) and dried in vacuo (3.07 g, 83%) (Found: C, 38.3; H, 4.1.Calc. for C24H33As3Cl3Ru: C, 38.3; H, 3.8%). n(Ru]Cl)/cm21 (Nujol mull) 323, 310 and 270. ESR (powdered solid 150 K): g = 2.27, 2.05 and 1.92. Electrospray mass spectrum: m/z = 718, 682 and 536 (calc. for C24H33As3 35Cl2 101Ru 717, C24H33As3 35Cl101Ru 682, C16H22As2 35Cl2 101Ru 535). mer-[RuBr3(AsMe2Ph)3]. The complex mer-[RuCl3- (AsMe2Ph)3] (1.0 g, 1.33 mmol) was suspended in ethanol (30 cm3).To this LiBr (2.30 g, 26.4 mmol) was added and the mixture heated to reflux under nitrogen for ca. 10 min and then cooled. A black solid separated and was filtered off, washed with water (2 × 10 cm3) and dried in vacuo (0.8 g, 68%) (Found: C, 33.0; H, 3.6. Calc. for C24H33As3Br3Ru: C, 32.5; H, 3.8%). n(Ru]Br)/cm21 252, 226 and 197. ESR (powdered solid 150 K): g = 2.27, 2.06 and 1.91. Electrospray mass spectrum: m/z = 807, 727 and 625 (calc.for C24H33As3 79Br2 101Ru 805, C24H33As3- 79Br101Ru 726, C16H22As2 79Br2 101Ru 625). Crystallography Details of the crystallographic studies are presented in Table 4. Data were collected on a Rigaku AFC7S diffractometer equipped with Mo-Ka radiation (l = 0.710 69 Å) and a graphite monochromator. Selected crystals were mounted on glass fibres following oil immersion and held at 150 K using an Oxford Cryosystems low-temperature device. The Lorentz-polarisation corrections and any correction for the small amount of decay were applied during data reduction.Crystal solution was by means of SHELXS 8621 and full-matrix least-squares refinement on F was carried out with the TEXSAN he space group of the binuclear systems was either Cc or C2/c with the N(z) test favouring the centrosymmetric space group and the analysis was successfully carried out in this space group. Some of the thermal ellipsoids of the carbon atoms were suggestive of disorder although individual atom sites could not be recognised.This problem could be associated with the empirical absorption corrections used and the rather large m values, genuine disorder, or the possibility of the lower-symmetry space group as difficulties over the choice of Cc versus C2/c are well known.23 Hydrogen atoms were usually included in the model at calculated positions [d(C]H) = 0.95 Å]. Other details for individual structures are as follows.2334 J.Chem. Soc., Dalton Trans., 1997, Pages 2331–2334 Table 4 Crystallographic details * mer-[RuCl3(AsMe2- Ph)3] mer-[RuBr3(AsMe2- Ph)3] [Ru2Br5(PMe2- Ph)4] [Ru2Br5(AsMe2- Ph)4] [Ru2I5(AsMe2Ph)4] Formula C24H33As3Cl3Ru C24H33As3Br3Ru C32H44Br5P4Ru2 C32H44As4Br5Ru2 C32H44As4I5Ru2 Mr 753.72 887.07 1154.25 1330.05 1565.05 Space group P21/c (no. 14) P21/c (no. 14) C2/c (no. 15) C2/c (no. 15) C2/c (no. 15) a/Å 16.070(3) 16.112(7) 16.268(9) 15.978(6) 16.214(5) b/Å 10.358(4) 10.380(2) 11.200(41) 11.508(4) 12.228(4) c/Å 18.120(4) 18.206(6) 21.479(7) 21.421(7) 22.592(8) b/8 113.68(2) 112.32(3) 102.66(3) 101.28(3) 113.32(2) U/Å3 2762(1) 2816(2) 3818(13) 3862(2) 4113(2) 2q Range for cell/8 19.0–21.0 19.0–21.0 26.6–38.2 18.8–22.2 19.0–22.8 Dc/g cm23 1.812 2.091 2.008 2.287 2.526 F(000) 1484 1700 2236 2524 2884 Crystal size/mm 0.30 × 0.20 × 0.10 0.80 × 0.60 × 0.40 0.3 × 0.4 × 0.2 0.10 × 0.25 × 0.20 0.40 × 0.30 × 0.03 Total no.observations 5368 5465 3702 3731 3957 No.unique observations (Rint) 5175 (0.079) 5268 (0.060) 3566 (0.045) 3592 (0.031) 3810 (0.21) Absorption correction y Scan y Scan y Scan DIFABS24 DIFABS Maximum, minimum transmission 0.79, 1.00 0.463, 1.000 0.734, 1.000 0.615, 1.000 0.634, 1.000 No. data in refinement 2645 [I > 4s(I)] 3577 [I > 3s(I)] 2083 [I > 3s(I)] 2203 [I > 4s(I)] 1750 [I > 3s(I)] No. parameters 240 270 195 195 115 m/cm21 44.37 82.3 62.30 93.95 77.25 hkl Ranges 0–19, 0–12, 221 to 19 0–19, 0–12, 221 to 20 0–19, 0–13, 225 to 24 0–18, 0–13, 225 to 24 0–19, 0–14, 226 to 24 S 2.66 3.37 1.75 1.94 2.49 Maximum shift/e.s.d. 0.05 0.07 0.03 0.01 0.00 Residual electron density/e Å23 2.11 to 22.53 2.01 to 21.82 1.19 to 21.41 0.91 to 21.47 1.54 to 21.47 R 0.061 0.052 0.043 0.044 0.052 R9 0.081 0.057 0.053 0.055 0.071 * In common: monoclinic; T = 150 K; Z = 4; scan mode w–2q; w21 = s2(Fo); maximum 2q = 508; R = S |Fo| 2 |Fc| /S|Fo|; R9 = [Sw(Fo 2 Fc)2/ SwFo 2]� �� . mer-[RuCl3(AsMe2Ph)3].Dark brown crystals were obtained by liquid diffusion of EtOH into a CH2Cl2 solution of the target material. Eight C atoms were treated as isotropic since anisotropic thermal parameters resulted in non-positive definite ellipsoids indicative of possible disorder problems. mer-[RuBr3(AsMe2Ph)3]. Dark brown crystals were obtained as above. Two C atoms were treated as isotropic (see comments above). [Ru2Br5(PMe2Ph)4]. Dark brown crystals were obtained by liquid diffusion of EtOH into a CH2Cl2 solution of mer- [RuBr3(PMe2Ph)3]. All C atoms were treated as anisotropic.[Ru2Br5(AsMe2Ph)4]. Dark brown crystals were obtained by liquid diffusion of EtOH into a CH2Cl2 solution of mer- [RuBr3(AsMe2Ph)3]. All C atoms were treated as anisotropic. [Ru2I5(AsMe2Ph)4]. Dark brown crystals were obtained by liquid diffusion of EtOH into a CH2Cl2 solution of [RuI2(As- Me2Ph)4]. Crystal decay was observed (8.5%). The carbon atoms were retained with isotropic thermal parameters as anisotropic ones gave no improved fit to the data and a few non-positive definite ellipsoids. No H atoms were included in the model.Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 186/529.Acknowledgements We thank the EPSRC for support and funds to purchase the X-ray diffractometer and for access to the Chemical Database Service at Daresbury. References 1 R. A. Cipriano, W. Levason, R. A. S. Mould, D. Pletcher and M. Webster, J. Chem. Soc., Dalton Trans., 1990, 339. 2 R. A. Cipriano, W. Levason, R. A. S. Mould, D. Pletcher and M. Webster, J. Chem., Dalton Trans., 1990, 2609. 3 N. R. Champness, W. Levason, R. A. S. Mould, D. Pletcher and M.Webster, J. Chem. Soc., Dalton Trans., 1991, 2777; N. R. Champness, C. S. Frampton, W. Levason and S. R. Preece, Inorg. Chim. Acta, 1995, 233, 43. 4 P. W. Armit, A. S. 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Champness, W. Levason, D. Pletcher and M. Webster, J. Chem. Soc., Dalton Trans., 1992, 3243. 18 V. T. Coombe, G. A. Heath, T. A. Stephenson, J. D. Whitelock and L. J. Yellowlees, J. Chem. Soc., Dalton Trans., 1985, 947. 19 J. C. Collingwood, P. N. Schatz and P. J. McCarthy, Mol. Phys., 1975, 30, 469. 20 G. A. Heath, D. C. R. Hockless and B. D. Yeomans, Acta Crystallogr., Sect. C, 1996, 52, 854. 21 G. M. Sheldrick, SHELXS 86, Program for crystal structure solution, Acta Crystallogr., Sect. A, 1990, 46, 467. 22 TEXSAN, Single crystal structure analysis software, version 1.7-1, Molecular Structure Corporation, The Woodlands, TX, 1995. 23 W. H. Bauer and D. Kassner, Acta Crystallogr., Sect. B, 1992, 48, 356. 24 N. Walker and D. Stuart, DIFABS, Acta Crystallogr., Sect. A, 1983, 39, 158. Received 24th February 1997; Paper 7/