Driving force effects in proton coupled electron transfer

The rates of reduction of the nickel(III) oxime–imine complexes, [NiIII(Me2L)]+ and [NiIII(Me2LH)]2+ by the corresponding iron(II) species are determined. Comparisons with rates calculated with the use of Marcus theory reveal that reactions of [FeII(Me2LH)]+, where the electron transfer product is the highly acidic [FeIII(Me2LH)]2+, show rate enhancements of several orders of magnitude. These are ascribed to a strong coupling of the electron transfer with the concomitant, thermodynamically favorable proton transfer. A free energy correlation supports this conclusion.