The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to MO-S-cysteine bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)(4)], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)(2)CN)(3))(4)], and [PPh4(])[MoO(P-SPhCONHC(CH2O(CH2)(2)COOCH2CH3)(3))(4)]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively, The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4](-) cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)(4)](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.