Sulfite:cytochrome c activity in the pyranopterin molybdenum enzyme sulfite oxidase is inhibited by ferricyanide. Our model studies provide strong evidence supporting the stabilization of Mo(IV), and not a reduction of in-plane Mo-S-ip covalency contributions to the electron transfer regeneration pathway, as the primary reason for electron flow out of the Mo active site being thermodynamically uphill. Interestingly, it is found that anisotropic covalency contributions involving only the S-op orbitals of a coordinated dithiolate can stabilize Mo(IV) with respect to one-electron oxidation to Mo(V).