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^at_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_k^ex 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.