| In order to study how distance or steric bulk affects the electron transfer process, several series of structurally related redox couples were investigated. The series are based on the parent couples, Ru(NH(,3))(,5)pyridine('3+,2+), Co(1,10-phenanthroline)(,3)('3+,2+), and Cr(1,10-phenanthroline)(,3)('3+,2+). Additional members of each series were synthesized with methyl, hydroxymethyl, and propanoate substituents in the 4-position of the pyridine ring or the 4,7-positions of the phenanthroline ring. The diffusion coefficients for these complexes ranged from 4.7(.)10('-6) to 2.0(.)10('-6) cm('2)/sec. The diffusion coefficients were shown to be useful for the determination of the average radii for the complexes which ranged from 3.8-12.0 (ANGSTROM). Electrochemical rate constants for the chromium and cobalt complexes were measured using square wave voltammetry at mercury and glassy carbon electrodes, respectively. For adiabatic electron transfers, the rate constant is expected to increase with an increase in the ionic radius. The observed rate constants for the largest complexes were more than an order of magnitude smaller than rate constants for the smallest complexes. The bimolecular rate constants between all members of the cobalt and ruthenium series were measured using stopped-flow spectrophotometry. Marcus theory was used to calculate the cross reaction rate constants and these values were compared to the observed rate constants. The ratio of the calculated to observed value increased to over 90 for the reactions involving the largest complexes. Measurement of both electrochemical and bimolecular rate constants as functions of temperature allowed for activation parameters to be calculated. As the radii of the complexes increased, the free energies of activation became increasingly dominated by negative activation entropies. Taken together, these data provide strong evidence for nonadiabaticity in the electron transfer reactions of these complexes and demonstrate that the degree of nonadiabaticity can be controlled by modification of the ligand structure. |
