Reaction of Tp(iPr)Mo(VI)OS(OAr) with cobaltocene in toluene results in the precipitation of brown, microcrystalline oxosulfido-Mo (V) compounds, [CoCp2]-[(TMoOS)-Mo-iPr-O-V(OAr)] (Cp- = eta s-C5H5-, Tp(iPr-) = hydrotris(3-isopropylpyrazol-1-yl)borate, OAr- = phenolate or 2-Bu-s, 2-Bu-t, 3-Bu-t, 4-Bu-s, 4-Ph, 3,5-(s)Bu2, 2-CO2Me, 2-CO2Et or 2-CO2Et or 2-CO2Ph derivative thereof). The compounds are air- and water-sensitive and display nu(Mo=O) and v(Mo-S) IR absorption bands at ca. 890 and 435 cm(-1), respectively, 20-40 cm(-1) lower in energy than the corresponding bands in Tp(iPr)MoOS(OAr). They are electrochemically active and exhibit three reversible cydovoltammetric waves (E(Mo-VI/Mo-V) = -0.40 to -0.66 V, E([CoCp2](+) /CoCp2) = -0.94 V and E(CoCp2/[CoCp2](-)) = -1.88 V vs SCE). Structural characterization of [CoCp2][Tp(iPr)MoOS(OC6H4CO2Et-2)]center dot 2CH(2)Cl(2) revealed a distorted octahedral Mo(V) anion with Mo=0 and Mo-S distances of 1.761(5) and 2.215(2) angstrom, respectively, longer than corresponding distances in related Tp(iPr)MoOS(OAr) compounds. The observation of strong S(1s) -> (S(3p) + Mo(4d) S K-preedge transitions indicative of a d(1) sulfido-Mo(V) moiety and the presence of short Mo=O (ca. 1.72 angstrom) and Mo-S (ca. 2.25 angstrom) backscattering contributions in the Mo K-edge EXAFS further support the oxosulfido-Mo(V) formulation. The compounds are EPR-active, exhibiting highly anisotropic (Delta g 0.124-0.150), rhombic, frozen-glass spectra with g(1) close to the value observed for the free electron (g(e) = 2.0023). Spectroscopic studies are consistent with the presence of a highly covalent Mo-S pi* singly occupied molecular orbital. The compounds are highly reactive, with reactions localized at the terminal sulfido ligand. For example, the compounds react with cyanide and PPh3 to produce thiocyanate and SPPh3, respectively, and various (depending on solvent) oxo-Mo(V) species. Reactions with copper reagents also generally lead to desulfurization and the formation of oxo-Mo(V) or -Mo(IV) complexes.