KINETICS OF ELECTRODE REDUCTION OF THE TETRACHLOROCUPRATE(II) ION - AN EXAMPLE OF THE CONCERTED EC REACTION-MECHANISM

The kinetic parameters of reduction of [CuCl4](2-) ion in acetonitrile at a conventional glassy carbon electrode and at a Pt micro-electrode (10 or 20 mu m), have been determined taking into account the double-layer effect: standard rate constant k(S)(o) = 2.1 x 10(-4) cm s(-1), Gibbs energy of activation Delta G* = 26 kJ mol(-1), pre-exponential factor A approximate to 10(1) cm s(-1) and transfer coefficient alpha = 0.31. Comparison with previous data on similar reactions suggests that the fast escape of chloride ions from the inner coordination sphere, which follows the electron transfer step, retards an EC reaction. If the two steps of the reaction are practically inseparable, the barrier top broadens, resulting in a decrease in the frequency factor. A low value of the pre-exponential factor, in spite of a low activation barrier, is the main reason for the slowness of the reaction.