A comparison of the rates of electron exchange reactions of ammine complexes of ruthenium(II) and -(III) with the predictions of adiabatic, outer-sphere electron transfer models

Abstract

The rate constants for the Ru(NH/sub 3/)/sub 6//sup 3+/2+/, Ru(NH/sub 3/)/sub 4/(bpy)/sup 3+2+/, and Ru(NH/sub 3/)/sub 2/(bpy)/sub 2//sup 3+/2+/ electron exchange reactions have been measured by a technique involving subtle modifications of the ligands. The rate constants (M/sup -1/s/sup -1/, medium) at 25/sup 0/C are 3.2 x 10/sup 3/, 0.1 M CF/sub 3/SO/sub 3/H, for the Ru(NH/sub 3/)/sub 6//sup 3+/2+/ exchange; 7.7 x 10/sup 5/, 0.1 M CF/sub 3/SO/sub 3/H, and 2.2 x 10/sup 6/, 0.1 M HClO/sub 4/, for the Ru(NH/sub 3/)/sub 4/(bpy)/sup 3+/2+/ exchange; 8.4 x 10/sup 7/, 0.1 M HClO/sub 4/, for the Ru(NH/sub 3/)/sub 2/(bpy)/sup 3+/2+/ exchange. The rate constants and activation parameters for the Ru(NH/sub 3/)/sub 4/(bpy)/sup 3+/2+/ exchange reaction were determined as a function of ionic strength (k = 4.1 x 10/sup 4/ M/sup -1/s/sup -1/, ..delta..H is not equal to 4.0 kcal mol/sup -1/, and ..delta..S is not equal to -24 cal deg/sup -1/ mol/sup -1/ at zero ionic strength and 25/sup 0/C). The rate constants determined in this work together with those for the analogous Ru(NH/sub 3/)/sub 5/py/sup 3+/2+/ and Ru(bpy)/sub 3//sup 3+/2+/ exchange reactions are compared with the predictions of theoretical models. Good linearity was found for a plot of logmore » k/sub ex/ vs. 1/anti r, where anti r is the mean separation of the ruthenium centers in the activated complex. The Marcus model derived on the basis of a reactive collision formulation gives better agreement with the observed rate constants and activation parameters at zero ionic strength than the conventional ion-pair preequilibrium model. Contrary to prediction, the increase in exchange rate with increasing ionic strength is reflected primarily in a decreased enthalpy of activation. 6 figures, 5 tables.« less