Iron(III) porphyrin complexes bearing covalently attached imidazole and thiolate axial ligands are investigated using resonance Raman, electron paramagnetic resonance, and cyclic voltammetry. The thiolate ligand stabilizes a low-spin ground state in solvent-bound six-coordinate species, weakens the Fe-N-pyr bonds, and shifts the Fe-III/II potential more negative by similar to 500 mV relative to an imidazole-bound species. Density functional theory calculations reproduce the experimental observation and indicate that the covalent charge donation from thiolate to iron reduces the Z(eff) on the iron. This increases the Fe-3d orbital energies, which changes the bonding interaction present in these complexes significantly. In particular, the increase of the Fe-3d energies activates an iron-to-porphyrin pi*-back-bonding interaction not present in the imidazole-bound complex.