The electronic structure of cis, trans-(L-N2S2)MoO(X) (where L-N2S2 = N, N'-dimethyl- N,N'-bis(2-mercaptophenyl)-ethylenediamine and X = Cl, SCH2C6H5, SC6H4-OCH3, or SC6H4CF3) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N2S2 chelating ligand, denoted S-180, coordinates to molybdenum in the equatorial plane such that the O=Mo-S-180-C-phenyl dihedral angle is similar to180degrees, resulting in a highly covalent g-bonding interaction between an S-180 p orbital and the molybdenum d(xy) orbital. This highly covalent bonding scheme is the origin of an intense low-energy S --> Mo d(xy) bonding-to-antibonding LMCT transition (E-max similar to16 000 cm(-1), epsilon similar to 4000 M-1 cm(-1)). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S180 contributes similar to22% to the HOMO, which is predominantly a pi antibonding molecular orbital between Mo d(xy) and the S-180 p orbital oriented in the same plane. The second sulfur donor of the L-N2S2 ligand is essentially nonbonding with Mo d(xy) due to an OdropMo-S-C-phenyl dihedral angle of similar to90degrees. Because the formal Mo d(xy) orbital is the electroactive or redox orbital, these Mo d(xy)-S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo-S-180 pi, bonding interaction observed in these complexes is analogous to the well-known Cu-S-Cys pi bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes. Finally, the presence of a covalent Mo-S pi interaction oriented orthogonal to the MOdropO bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and MO=S-sulfido bonding in xanthine oxidase.