Electrochemical and Homogeneous Exchange Kinetics for Transition-Metal Aquo Couples: Anomalous Behavior

Rate data for electrochemical and homogeneous redox reactions involving R u , ~ + / ~ + , V,q3+/2+, Fe,q3+/2+, E u , ~ ~ + / ~ + , and Cr,2+/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, keex, were found to be in uniformly good agreement with the rate constants for homogeneous self-exchange, kha, extracted from cross-reaction data involving outer-sphere coreactants, yielding the reactivity sequence R u , ~ + / ~ + > V V2+ > Fe,?I2+ 5 Eu W2+ > Cr 3+/2+. aq aq However, the measured rate constant for Few3+/*+ self-exchange is not consistent with this sequence, being at least 10 -fold larger than the values of khcx extracted from both the homogeneous cross-reaction and electrochemical data. The latter values of khex and also ke,, are in harmony with the relative inner-shell barriers for Fe,PI2+ and Ru?FI2+ 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. "5