Interconversion of mononuclear cis-dioxo-Mo(VI) and oxo-Mo(V, IV) complexes of the hydrotris(3, 5-dimethylpyrazol-1-yl)borate ligand (L) by one-electron and two-electron reactions is described. In the coordinating solvent pyridine (py), LMo(VI)O(2)(SPh) is reduced by cobaltocene in one-electron steps to stable LMo(IV)O(SPh)(py). The compound LMo(IV)O(SPh)(py) 0.6 MeOH crystallizes in orthorhombic space group Pbca, with a = 13.790(2) Angstrom, b = 15.266(2) Angstrom, c = 27.807(5), V = 5853(3) Angstrom(3), and Z = 8. The complex exhibits a distorted octahedral structure with a facially tridentate ligand L and mutually cis oxo [Mo=O = 1.667(5) Angstrom], pyridine [Mo-N = 2.184(5) Angstrom], and thiolate [Mo-S = 2.390(3) Angstrom] ligands. This and other LMo(IV)O(SR)(py) (R = Ph, CH(2)Ph, CHMe(2)) complexes are also obtained from LMo(VI)O(2)(SR) via two-electron oxygen atom transfer reactions involving tertiary phosphines in pyridine. In dry solvents, the oxo-Mo(TV) complexes are oxidized by ferrocenium ion to the EPR-active cations [LMo(V)O(SR)(py)](+) which are hydrolyzed rapidly in wet solvents to LMo(V)O(OH)(SR). More generally, the complexes LMo(VI)O(2)X (X = Cl, Br, NCS, OPh, SPh, SCH(2)Ph, SCHMe(2)) react with PPh(3) at room temperature to yield OPPh(3) and unstable, coordinatively-unsaturated intermediates LMo(IV)OX. The latter are oxidized back to LMo(VI)O(2)X by Me(2)SO or can be trapped in a number of ways, depending on available ligands. For example, the complexes LMo(IV)OX(solvent) are detected in coordinating solvents, LMo(V)OClX in chlorinated solvents, LMo(V)O(OMe)X in MeOH, and [LMo(V)O](2)(mu-O) in dry toluene. However, in wet weakly-coordinating solvents, LMo(V)O(OH)X complexes are produced cleanly and can be oxidized quantitatively to LMo(VI)O(2)X. Consequently, LMo(VI)O(2)X complexes are catalysts for the oxidation of PPh(3) by O-2 in the presence of H2O. Oxygen isotope tracing shows that H2O rather than O-2 is the source of the oxygen atom which is transferred to PPh(3). This is the first model system which displays the full cycle proposed for oxidizing molybdoenzymes featuring [(MoO2)-O-VI](2+) resting states.