This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl4](2-,1-) and [Fe(SR)(4)](2-,1-) (DeltaE(0) approximate to 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (Delta/(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to Delta/(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to Delta/(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I-vert relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge (Z(eff)(Fe)) of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe) (3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases Z(eff)(Fe) and significantly lowers its redox potential.