Electrochemical and homogeneous exchange kinetics for transition-metal aquo couples: Anomalous behavior of hexaaquoiron(III/II)

Rate data for electrochemical and homogeneous redox reactions involving Ru_aq ^3+/2+, V_aq ^3+/2+, Fe_aq ^3+/2+, Eu_aq ^3+/2+, and Cr_aq ^3+/2+ redox couples (where "aq" represents aquo ligands) have been analyzed and compared by using the rate relations due to Marcus in order to ascertain how the kinetics of outer-sphere electron exchange are dependent upon the metal redox center. The work-corrected rate constants for electrochemical exchange at mercury electrodes, k^e _ex, were found to be in uniformly good agreement with the rate constants for homogeneous self-exchange, k^h _ex, extracted from cross-reaction data involving outer-sphere coreactants, yielding the reactivity sequence Ru_aq ^3+/2+ > V_aq ^3+/2+ > Fe_aq ^3+/2+ ≳ Eu_aq ^3+/2+ ≳ Cr_aq ^3+/2+. However, the measured rate constant for Fe_aq ^3+/2+ self-exchange is not consistent with this sequence, being at least 10³-fold larger than the values of k^h _ex extracted from both the homogeneous cross-reaction and electrochemical data. The latter values of k^h _ex and also k^e _ex are in harmony with the relative inner-shell barriers for Fe_aq ^3+/2+ and Ru_aq ^3+/2+ calculated from crystallographic structural data. The above reactivity sequence is also consistent with the relative structural changes accompanying electron transfer as monitored by the corresponding reaction entropies.