Electron-transfer reactions with unusual activation parameters. Treatment of reactions accompanied by large entropy decreases

Abstract

The activation enthalpies for the oxidation of Fe(H_2O)_6^(2+) by poly(pyridine)iron(III) and -ruthenium(III) complexes (reactions 1 and 2, for example) are small or slightly negative. Fe(H_2O)_6^(2+-) + Fe(bipy)_3^(3+) → Fe(H_2O)_6^(3+) + Fe(bipy)_3^(2+) (1) Fe(H_2O)_6^(2+) + Ru(bipy)_3^(3+) → Fe(H_2O)_6^(3+) + Ru(bipy)_3^(2+) (2) This has led to the suggestion that these reactions occur by a "non-Marcus" path which includes at least one distinctive feature not considered in a current outer-sphere electron-transfer model. We wish to point out that small or even negative activation enthalpies are in themselves not necessarily inconsistent with this model and indeed are predicted by it for the systems under consideration.

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