A Kinetic Study of the Reactions of Several Cobalt(III) Complexes with Ascorbic Acid

The kinetics of the reactions of [Co(phen)3]3+, [Co(bpy)3]3+, and [Co(en)(phen)2]3+ (phen=1,10-phenanthroline, bpy=2,2′-bipyridine, and en=ethylenediamine) with ascorbic acid (H2A) have been studied over the temperature range from 17.0 to 35.0 °C, at an ionic strength of 0.10 mol dm−3 (LiClO4), and over the pH range from 3.60 to 5.04 (acetate buffer). The [Co(phen)3]3+ and [Co(bpy)3]3+ ions were reduced by both HA− (k1 path) and A2− (k2 path) ions: the second-order rate constants at 25.0 °C are (0.40±0.07) dm3 mol−1 s−1 and (5.8±1.1)×106 dm3 mol−1s−1 for the [Co(phen)3]3+ ion, and (0.13±0.02) dm3 mol−1 s−1 and (2.1±0.5) ×106 dm3 mol−1 s−1 for the [Co(bpy)3]3+ ion. The [Co(en)(phen)2]3+ ion was reduced only by the A2− ion; k2=(1.0±0.2) ×106 dm3 mol−1 s−1 (25.0 °C). The [Co(phen)3]3+, [Co(bpy)3]3+, and [Co(en)(phen)2]3+ ions were not reduced by H2A. The activation parameters (ΔH and ΔS) for the ascorbate reduction of the [Co(phen)3]3+ ion were larger than those of the [Co(bpy)3]3+ ion. This is in contrast to the results of the self-exchange reactions of the [Co(phen)3]3+⁄2+ and [Co(bpy)3]3+⁄2+ couples. The mechanisms will be discussed in terms of the activation parameters and the Marcus cross relationships.