Coupled electron-transfer and spin-exchange reactions

Abstract Electron-transfers accompanied by a change in metal atom spin-state occur widely in chemistry and biology. An appropriate framework for interpreting such behavior is an electrochemical scheme of squares consisting of two one-electron electrode reactions and two spin-equilibrium steps. The presence of coupled electron-transfers and spin-exchanges is reflected in the thermodynamic and kinetic parameters of such events. Oxidation–reduction potentials and electron-transfer rate constants depend on the positions of the spin-state equilibria in the two oxidation states. Electrochemical activation parameters exhibit significant contributions from the large enthalpic and entropic differences that characterize spin-exchange reactions. The entropy of an electrochemical half-reaction, Δ S ° rc , coupled to a spin-exchange reflects the increases in vibrational and electronic entropy that accompany low- to high-spin conversion. Electrochemical enthalpies and entropies of activation, Δ H ‡ and Δ S ‡ , are influenced by the temperature dependence of the accompanying spin-state transitions and are useful in mechanism diagnosis. The present review describes examples from the literature and our own laboratory regarding Fe(III/II) (d 5 /d 6 ) and Co(III/II) (d 6 /d 7 ) couples that exhibit the above behavior.

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