Rate data for electrochemical and homogeneous redox reactions involving Ruaq3+/2+, Vaq3+/2+, Feaq3+/2+, Euaq3+/2, and Craq3+/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, Khex, extracted from cross-reaction data involving outer-sphere coreactants, yielding the reactivity sequence Ruaq3+/2+ < Vaq3+/2+ < Feaq3+/2+ Euaq3+/2+ < Crat3+/2+. However, the measured rate constant for Feaq3+/2 self-exchange is not consistent with this sequence, being at least 103-fold larger than the values of khex extracted from both the homogeneous cross-reaction and electrochemical data. The latter values of khex and also kkex in harmony with the relative inner-shell barriers for Feaq3+/2 and Ruaq3+/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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry