The [M0O(bdt)~]-/[MoO(bdt)~]2-(H2bdt = benzene-I ,2-dithiol) redox couple, in dimethylformamide solution, is a fully reversible process at €+ = -350 mV vs. standard calomel electrode and each of these anions has been characterised by X-ray crystallo raphy; the metal-ligand bond lengths of the former are slightly shorter than those of the latter, MoV-0 = 1.668(3) 1, MolV-0 = 1.699(6) A, mean MoV-S = 2.377(1) A, MolV-S = 2.388(2) A.

F, ) complexes b y several different routes are reported. The u.v.-visible, i.r., Raman, e.s.r. spectroscopic and redox properties of these complexes are detailed. The redox potentials of the MoV1-MoV and MoV-MoIV couples correlate with the Hammett function (0) of the aryl substituent(s); EP( MoV'-MoV) = 0.530 + 0.53 V (vs. saturated calomel electrode, s.c.e.) and E P ( MoV-MoIV) =0.520 -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(SC,F,),]*-has been demonstrated and this provides the first spectroscopic comparison of [Mo0(SR)Jn-( n = 1 or 2) complexes.

Stoicheiometric thiol exchange between [MO(SPh)4]-( M = M o or W) and HSCH2CH2SH provides a convenient synthesis for the [MO(SCH2CH2S)2]-complexes: the structures of the [PPh4]+ salts of these anions have been determined t o provide a clear structural comparison between these homologues [the two anions have very similar dimensions Mo-0 1.678(5), W-0 1.641(10), Mo-S 2.372(3), W-S 2.363(6) A]; on the basis of e.s.r. spectroscopic studies, it is concluded that [MOO(SCH~CH~S)~]-is formed when [MOO& reacts with HSCH2CH2SH and when this dithiol reacts with the oxidised molybdenum centre of xanthine oxidase.

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