STRUCTURAL CONSEQUENCES OF ELECTRON-TRANSFER REACTIONS .27. COMPETITION BETWEEN HOMOGENEOUS AND HETEROGENEOUS ELECTRON-TRANSFER PATHWAYS INAN ELECTROCHEMICAL SQUARE SCHEME

The reduction of an equilibrated isomeric mixture of CpCo(1,3-C8H8) (1 ,3) and CpCo(1,5-C8H8) (1,5) to their radical anions proceeds by a squ are scheme mechanism. The cyclic voltammetric peak heights for the cat hodic waves are dependent on analyte concentration. This is shown to a rise from the influence of the homogeneous cross-reaction (1,3)(-) + 1 ,5 reversible arrow 1,3 + (1,5)(-) near the electrode surface. Theoret ical voltammograms simulated by the fast quasi-explicit finite differe nce method are in agreement with the experimental voltammograms over a range of concentrations and scan rates. A lower limit for the rate of isomerization of [CpCo(1,5-C8H8)](-) to [CpCo(1,3-C8H8)](-) is 2 x 10 (5) s(-1), 2 orders of magnitude higher than previous estimates. Theor etical investigations show that the coupling of the cross-reaction wit h the rapid (1,5)(-) to (1,3)(-) isomerization favors the homogeneous redox pathway. Analogies are made to electron-transfer-catalyzed react ions. This is thought to be the first voltammetric quantitation of the solution electron-transfer pathway for square schemes with the reacta nts initially at equilibrium.