Steric and Electronic Effects in Electron Transfer Reactions

Abstract

THERE is at present no satisfactory theory to account for the rates of electron transfer from one metal ion to another via a bridging group. Two attempts have been made, both based on the assumption that the probability of electron transfer during the lifetime of the activated complex is small. The first relationship took the form¹: where k_bi is the observed overall rate constant for the reaction, k_f is the rate constant for the formation of the activated complex, C₁ is a constant, and P_rs the mobile bond order² between the atoms r and s associated with the metal ions. τ is the mean lifetime of the activated complex. This conjugation theory of electron transfer was extended by including the effect of charged reactants on the rate of formation and the mean lifetime of the activated complex³, giving at zero ionic strength the relationship: where Z_A and Z_B are the charges on the ions A and B, C₀ is a constant, D is the dielectric constant of the medium, k is the Boltzmann constant, T is the temperature, and r_AB the distance between the metal centres. A comparison has been made³ with results obtained experimentally in the chromium(II) reduction of various pentammine-cobalt(III) complexes. Agreement is better with the second theory than with the first, but not good. The suggestion was made that improvement of the M.O. calculations and the electrostatic treatment might produce substantial numerical agreement. Neither theory takes into account the steric effects of substituent groups (that is, groups present as part of the ligand but not participating directly in the electron transfer path). Such groups are important: the rate constants for the reduction of (NH₃)₅CoO₂C–R²⁺ fall off from 0.32 M⁻¹ sec⁻¹ to 0.04 M⁻¹ sec⁻¹ as R changes from CH₃ to cycloC₆H₁₁, even though the path length for the electron transfer remains unchanged⁴.