cis,trans- (L-N2S2)(MoO)-O-V(SR) [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine; R = CH2Ph, CH2CH3, and p-C6H4-Y (Y = CF3, Cl, Br, F, H, CH3, CH2CH3, and OCH3)] are the first structurally characterized mononuclear Mo compounds with three thiolate donors, as occurs at the Mo active site in sulfite oxidase. X-ray crystal structures of the Cis,trans-(L-N2S2)(MoO)-O-V(SR) compounds, where R = CH2Ph, CH2CH3, p-C6H4-OCH3, and p-C6H4-CF3, show a similar coordination geometry about the Mo atom with all three sulfur thiolate donors in the equatorial plane. This coordination geometry places two adjacent S p(pi) orbitals parallel to the Mo=O bond, analogous to the orientation in the ene-dithiolate ligand in sulfite oxidase; the third S p(pi) orbital lies in the equatorial plane. Charge-transfer transitions from the S p to the Mo d orbitals occur at approximately 28 000 cm(-1) (epsilon: 4400-6900 L mol(-1) cm(-1)) and 15 500 cm(-1) (E: 3200-4900 L mol(-1) cm(-1)).'lhe EPR parameters are nearly identical for all the cis,trans-(L-N2S2)(MoO)-O-V(SR) compounds (g(1) similar to 2.022, g(2) similar to 1.963, g(3) similar to 1.956, A(1) similar to 58.4 x 10(-4) cm(-1), A(2) similar to 23.7 x 10(-4) cm(-1), A(3) similar to 22.3 x 10(-4) cm(-1)) and are typical of an oxo-Mo(V) center coordinated by multiple thiolate donors. The g and A tensors are related by a 24 degrees rotation about the coincident gz and Az tensor elements, reflecting the approximate C-s coordination symmetry. These EPR parameters more closely mimic those of the low pH form of sulfite oxidase and the "very rapid" species of xanthine oxidase than previous model compounds with two or four thiolate donors. The cis,trans-(L-N2S2)(MoO)-O-V(SR) compounds undergo a quasi-reversible, one-electron reduction and an irreversible oxidation that show a linear dependence upon the Hammett parameter, sigma(p), of the Y group. The cis,trans-(L-N2S2)(MoO)-O-V(SR) compounds provide a well-defined platform for the systematic investigation of the electronic structures of the (MoOS3)-O-V centers and their implications for molybdoenzymes.