In recent years, Balzani and co-workers have promoted an approach designed to disentangle nuclear and electronic factors in electron-transfer reactions of ground and of electronically excited states.1"4 On the basis of this approach, they have reported values of xn and AG,,* in eq 1 for the self-exchange reactions kn = (ká/k^)KU{kbT/h) exp(--AGXX*/RT) (1) of several redox couples. Definitions in eq 1: ku, second-order rate constant for self-exchange reaction; k¿ and k^, rate constants for formation and for dissociation, respectively, of the outer-sphere complex formed between the two oxidation states; k,, and AG,,*, electronic and nuclear factors, respectively, for internal electron transfer within the outer-sphere complex. The reported1•3 values of ku and AG,,*, when introduced in eq 1, yield values of kn that differ from the measured rate constants by several orders of magnitude.5 Since the authors failed to point out1•3 these large discrepancies,5 they did not offer any explanation. Evidently, this is a highly unsatisfactory situation since a very important and useful result of electron-transfer theory is that the rate constants for cross-reactions can be calculated, in principle, from the measured reduction potentials and self-exchange rate constants of the individual couples, and whenever discrepancies occur and are noted, some insight into the details of the mechanisms of the reactions under consideration are revealed.6•7 It is the purpose of this communication to show that Balzani's approach1-3 cannot be reconciled with the measured self-exchange parameters of the redox couples and to point out several other problems with the approach. Moreover, we present an alternate treatment that makes use of the measured properties of the individual couples and of Marcus' cross relationship modified8 to take into account nonadiabaticity.
