The complexes LMo(VI)O(2)X [L = hydrotris(3,5-dimethylpyrazol-1-yl)bor
ate; X = Cl, Pr, NCS, OPh, SPh, SCH(2)Ph] are converted to air-stable
complexes LMo(V)O(OSiMe(3))X by one-electron coupled electron-electrop
hile transfer (CEET) reactions involving cobaltocene and the electroph
ilic reagent Me(3)SiCl. These complexes may also be obtained from LMo(
V)O(OH)X by reaction with Me(3)SiCl in the presence of base. LMo(V)O(O
SiMe(3))(SCH(2)Ph) crystallises in space group P2(1)/n, with a = 8.526
(1) Angstrom, b = 23.141 (3) Angstrom, c = 16.499 (2) Angstrom, beta
= 103.75 (12)degrees and Z = 4. The complex exhibits a distorted octah
edral structure with a facially tridentate L ligand and mutually cis o
xo [Mo = O = 1.675 (4) Angstrom], silyloxo [Mo-O = 1.932 (4) A] and th
iolato [Mo-S = 2.398 (2) Angstrom] ligands. The detailed redox propert
ies of LMo(V)O(OR)X (R = SiMe(3), alkyl, aryl) differ from those of LM
o(V)O(OH)X. Centres [(MoO)-O-V(OR)] are candidates for the stable ''in
hibited'' forms of certain molybdenum enzymes formed under conditions
which apparently disfavour the catalytically active [(MoO)-O-V(OH)] ce
ntres. In the coordinating solvent pyridine (py), both LMo(VI)O(2)(SPh
) and LMo(V)O(OSiMe(3))(SPh) are reduced in one-electron steps to stab
le LMo(IV)O(py)(SPh). LMo(IV)O(py)(SR) complexes are also obtained fro
m LMo(VI)O(2)(SR) (R = Ph, CH(2)Ph, CHMe(2)) via a two-electron oxygen
atom transfer reaction with tertiary phosphines in pyridine. Conseque
ntly, the Mo(IV) product is accessible via a concerted two-electron st
ep or via two one-electron steps.