The first structural characterization of a mononuclear, EPR-active, oxosulfido-Mo(V) compound related to the very rapid form of xanthine oxidase (VR-XnO) is reported. The compound, [CoCp2][Tp(iPr)Mo(V)OS-(2-OC6H4CO2Et)] [Cp = cyclopentadienyl; = Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate], exhibits a distorted octahedral geometry with Mo=O and Mo=S distances of 1.761(5) and 2.215(2) angstrom, respectively, and an O=Mo=S angle of 107.33(14)degrees. Significantly, the Mo-V=S distance is much shorter than the value of 2.36 angstrom reported for oxosulfido-Mo(V) compounds (Singh, R; et al. Inorg. Chem. 1989, 28, 8) but close to the range established for VR-XnO by protein crystallography. The methyl and phenyl esters were also prepared but the latter is highly reactive and undergoes an intramolecular, radical-based cyclization/elimination reaction to form [CoCp2]-[Tp(iPr)Mo(IV)O{2-OC6H4C(O)S-kappa O,kappa S}]. This study provides the first definitive measurement of the Mo-V=S bond distance in an unambiguously characterized oxosulfido-Mo(V) compound and supports the presence of a short (ca. 2.22 angstrom) Mo=S bond in VR-XnO. It also demonstrates that the Mo-V=S moiety participates in radical-based reactions that are facilitated by the facile redox interplay of Mo and S and by substrates susceptible to radical eliminations.