Ionic Association Effects upon Optical Electron Transfer Energetics: Studies in Water with (CN)₅Fe^II-BPE-Fe^III(CN)₅⁵⁻

The energetics of optical electron transfer (ET) in (NC)₅Fe^III-bis(pyridyl)ethylene-Fe^II(CN)₅⁵-(1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added “inert” electrolyte. With small amounts of added NaCl, CaCl₂, or LaCl₃, the optical ET (or metal-to-metal charge transfer) energy (E^MMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl₃) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)₅Fe^III—BPE—Fe^II(CN)₅^−5·Mⁿ⁺ absorb at higher energies than symmetrical species (i.e., 1 or Mⁿ⁺⋅1⋅Mⁿ⁺), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {—Fe^II(CN)₅}³⁻ portion of 1 and (b) calculations of E^MMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH₃)₅-Ru^III—4,4′-bpy-Ru^II(NH₃)₅⁵⁺ with added Na₂SO₄. This behavior is interpreted in terms of stepwise association of SO₄²⁻ with the {(NH₃)₅Ru^III—}³⁺ and {—Ru^II(NH₃)₅}²⁺ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of E^MMCT for (NH₃)₅Ru^III—dithiaspirane—Ru^II(NH₃)₅⁵⁺ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.