Intramolecular, environmental and spin state contributions to electron transfer reactivity of transition metal complexes

The object of this research is to understand relationships between structure and electron transfer reactivity of transition metal complexes by examining electrochemical activation parameters for several MN63+/2+ (M = Fe, Ni, Co, Ru) couples that undergo varying amounts of structural change as a result of oxidation or reduction. For cases involving Fe and Co unusually large values of enthalpies (ΔH‡ exp) and entropies (ΔS‡exp) of activation as well as values of electrode half-reaction entropies (ΔS Orc) are determined from measurements of the temperature dependence of electrochemical rate constants (ks,h) and formal electrode potentials (EO′). This work examines the possibility that these anomolous parameters reflect the influence of either coupled inner- and outer-shell reorganizations or spin multiplicity changes that accompany electron transfer. Electrode half-reaction entropies, ΔSO rc are measured for four M(tacn)23+/2+ (M = Fe, Ni, Co, Ru; tacn = 1,4,7-triazacyclononane) redox couples as a function of solvent and electrolyte type and concentration. These couples undergo differing amounts of inner-shell reorganization as a result of electron transfer and thereby serve to probe the possibility of coupling between inner and outer-shell reorganizations. Values of ΔSOrc are observed to be metal dependent and are shown to arise primarily from vibrational and electronic contributions to intramolecular entropy. Entropy measurements are referenced to the Ru3+/2+ couple, which is characterized by small inner-shell reorganization and should be controlled mostly by outer-shell effects. Values of Δ(ΔSOrc)M-Ru obtained from data in seven solvents are roughly solvent independent indicating that, if inner- and outer-shell reorganizations are coupled during electron transfer, this fact is not reflected in the solvent dependence of ΔS Orc