Rates of electron exchange between large 1+ and 2+ ions in acetonitrile. Coulombic interaction

The rate constants for electron exchange between Ru(bpy)/sub 2/(acac)/sup +/ and Ru(bpy)/sub 2/(acac)/sup 2 +/ and between Ru(bpy)/sub 2/(hfac)/sup +/ and Ru(bpy)/sub 2/(hfac)/sup 2 +/ have been measured by the NMR line-broadening method, bpy representing 2,2'-bipyridine, acac the acetylacetonate ion, and hfac the hexafluoroacetylacetonate ion. The reactant and electrolyte concentrations and the temperature were varied, acetonitrile being the solvent and PF/sub 6//sup -/ the anion. The rate constants at 25/sup 0/C and zero ionic strength (..mu..) were calculated. The values were obtained by extrapolation to the infinite dilution; the Bronsted-Bjerrum equation was used with the distance of closest approach, a/sup 0/ = 3 +/- 3 A. The temperature dependences measured near ..mu.. = 0.02 M are small; the Arrhenius activation energies are 2.2 +/- 1.0 and 4.1 +/- 1.0 kcal mol/sup -1/, respectively. The rate constants determined at ..mu.. = 0 are compared with theoretical values and with values for other electron-transfer reactions. Use of small reactant-separation distances in precursor complexes gives the expected dependence of rate constants on the product of reactant charges. 21 references, 2 figures, 2 tables.