KINETICS AND MECHANISM OF THE OXIDATION OF L-ASCORBIC-ACID BY CIS-DIAQUA COBALT(III) AMMINE COMPLEXES

The kinetics of oxidation of L-ascorbic acid by cis-diaquacobalt(III) complexes, [CoL(4)(H2O)(2)](3+) (L(4) = (NH3)(4), (en)(2) or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine] was studied as a fuction of pH, L-ascorbic acid concentration, temperature, ionic stre ngth and methanol content of the solvent using stopped-flow and conven tional spectrophotometry. The results indicated that only the ascorbat e monoanion, HA(-), is involved in the redox process with the cobalt(I II) species. The rate constants for the [Co(tren)(H2O)(2)](3+) and [Co (tren)(H2O)(OH)](2+) species (k(2) and k(5)) are 0.26 +/- 0.09 and 1.2 5 +/- 0.03 dm(3) mol(-1) s(-1) respectively at 30 degrees C, and the c orresponding activation parameters are Delta H-2(double dagger) = 124 +/- 9 kJ mol(-1), Delta S-2(double dagger) = 137 +/- 30 JK(-1) mol(-1) and Delta H-5(double dagger) = 82 +/- 2 kJ mol(-1), Delta S-5(double dagger) = 26 +/- 6 JK(-1) mol(-1) The variations in the rate constants and thermodynamic parameters for the series of complexes is discussed . The Marcus cross-relationship for electron transfer has been applied to the redox process to confirm the outer-sphere mechanism and to est imate the self-exchange rate constant for the [CoL(4)(H2O)(OH)](2+/+) couple.