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

Joseph T Hupp, Michael J. Weaver

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57 Citations (Scopus)


Rate data for electrochemical and homogeneous redox reactions involving Ruaq 3+/2+, Vaq 3+/2+, Feaq 3+/2+, Euaq 3+/2+, and Craq 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, ke ex, were found to be in uniformly good agreement with the rate constants for homogeneous self-exchange, kh ex, extracted from cross-reaction data involving outer-sphere coreactants, yielding the reactivity sequence Ruaq 3+/2+ > Vaq 3+/2+ > Feaq 3+/2+ ≳ Euaq 3+/2+ ≳ Craq 3+/2+. However, the measured rate constant for Feaq 3+/2+ self-exchange is not consistent with this sequence, being at least 103-fold larger than the values of kh ex extracted from both the homogeneous cross-reaction and electrochemical data. The latter values of kh ex and also ke ex are in harmony with the relative inner-shell barriers for Feaq 3+/2+ and Ruaq 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.

Original languageEnglish
Pages (from-to)2557-2564
Number of pages8
JournalInorganic Chemistry
Issue number18
Publication statusPublished - 1983

ASJC Scopus subject areas

  • Inorganic Chemistry

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