Second-coordination sphere effects such as hydrogen bonding and steric constraints that provide for specific geometric configurations play a critical role in tuning the electronic structure of metalloenzyme active sites and thus have a significant effect on their catalytic efficiency. Crystallographic characterization of vertebrate and plant sulfite oxidase (SO) suggests that an average O-oxo-Mo-S-Cys-C dihedral angle of similar to 77 degrees exists at the active site of these enzymes. This angle is slightly more acute (similar to 72 degrees) in the bacterial sulfite dehydrogenase (SDH) from Starkeya novella. Here we report the synthesis, crystallographic, and electronic structural characterization of Tp*MoO(mba) (where Tp* = (3,5-dimethyltrispyrazol-1-yl)borate; mba = 2-mercaptobenzyl alcohol), the first oxomolybdenum monothiolate to possess an O-ax-Mo-S-thiolate-C dihedral angle of similar to 90 degrees. Sulfur X-ray absorption spectroscopy clearly shows that O-ax-Mo-S-thiolate-C dihedral angles near 90 degrees effectively eliminate covalency contributions to the Mo(xy) redox orbital from the thiolate sulfur. Sulfur K-pre-edge X-ray absorption spectroscopy intensity ratios for the spin-allowed S(1s) -> S-v(p) + Mo(xy) and S(1s) -> S-v(p) + Mo(xz,yz) transitions have been calibrated by a direct comparison of theory with experiment to yield thiolate S-v(p) orbital contributions, c(i)(2), to the Mo(xy) redox orbital and the Mo(xz,yz) orbital set. Furthermore, these intensity ratios are related to a second coordination sphere structural parameter, the O-oxo-Mo-S-thiolate-C dihedral angle. The relationship between Mo-S-thiolate and Mo-S-dithiolene covalency in oxomolydenum systems is discussed, particularly with respect to electron-transfer regeneration in SO.