COMPARISON OF THE RATES OF ELECTRON EXCHANGE-REACTIONS OF AMMINE COMPLEXES OF RUTHENIUM(II) AND RUTHENIUM(III) WITH THE PREDICTIONS OF ADIABATIC, OUTER-SPHERE ELECTRON-TRANSFER MODELS

The rate constants for the Ru(NH3)33+/2+, Ru(NH3)4(bpy)3+/2+, and Ru(NH3)2(bpy)23+/2+electron exchange reactions have been measured by a technique involving subtle modifications of the ligands. The rate constants (M-1s-1, medium) at 25 °C are 3.2 X 103, 0.1 M CF3SO3H, for the Ru(NH3)63+/2+exchange; 7.7 X 105, 0.1 M CF3SO3H, and 2.2 X 106, 0.1 M HC104, for the Ru(NH3)4(bpy)3+/2+exchange; 8.4 X 107, 0.1 M HC1O4, for the Ru(NH3)2(bpy)23+/2+exchange. The rate constants and activation parameters for the Ru(NH3)4(bpy)3+/2+exchange reaction were determined as a function of ionic strength (k = 4.1 X 104M-1s-1, ΔH≠= 4.0 kcal mol-1, and ΔS≠= -24 cal deg-1mol-1at zero ionic strength and 25 °C). The rate constants determined in this work together with those for the analogous Ru(NH3)5py3+/2+and Ru(bpy)33+/2+exchange reactions are compared with the predictions of theoretical models. Good linearity was found for a plot of log kexvs. 1/r, where 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. © 1979, American Chemical Society. All rights reserved.