Abnormal salt effects on a cation/anion reaction. An interpretation based on the analysis of the components of the experimental rate constant

Abstract

Salt effects (NaNO3) on the kinetics of the reactions [Fe(CN)6]3– + [Ru(NH3)5(pyz)]2+ = [Fe(CN)6]4– + [Ru(NH3)5(pyz)]3+ (pyz = pyrazine) were studied through T-jump measurements. An abnormal (positive) salt effect on the forward reaction was observed and a normal (negative) effect on the reverse one. These facts imply an asymmetric behavior of anion/cation reactions depending on the charge sign of the oxidant and reductant. The results can be rationalized by using the Marcus–Hush treatment for electron-transfer reactions after decomposition of the experimental rate constants into their components.