Asymmetric salt effects on anion/cation reactions: A comparative study of the [Fe(CN)6]4− + [Co(NH3)5pz]3+ and [Ru(NH3)5pz]2+ + [Co(C2O4)3]3− reactions

Abstract

AbstractThe kinetics of electron transfer reactions between [Fe(CN)6]4− and [Co(NH3)5pz]3+ and between [Ru(NH3)5pz]2+ and [Co(C2O4)3]3− was studied in concentrated salt solutions (Na2SO4, LiNO3, and Ca(NO3)2). An analysis of the experimental kinetic data, kobs, permits us to obtain the true (unimolecular) electron transfer rate constants corresponding to the true electron transfer process (precursor complex → successor complex), ket. The variations of both, kobs and ket, with salt concentrations are opposite for these reactions. These opposite tendencies can be rationalized by using the Marcus–Hush treatment for electron transfer reactions. The conclusion is that the negative salt effect found for the first reaction ([Fe(CN)6]4− + [Co(NH3)5pz]3+) is due to the increase of the reaction and reorganization free energies when the concentration of salt increases. In the case of the second reaction ([Ru(NH3)5pz]2+ + [Co(C2O4)3]3−), the positive salt effect observed is caused by the fact that the driving force becomes more favorable when the concentration of salt increases. Thus, it is shown that for anion/cation electron transfer reactions the kinetic salt effect depends on the charge sign of the oxidant (and the reductant). © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 81–89, 2005