Kinetics and mechanism of the anation of cis (aquo)(ammine) bis(ethylenediamine) Co(III) by oxalate

Abstract

The kinetics of the anation of cis (aquo)(ammine)bis(ethylenediamine)Co(III) by H₂C₂O₄, HC₂O₄^- and C₂O₄^2- have been investigated at 50-60°C and I = 1.0 mol dm^-3 (ClO₄^-). Association of the aquo cation with oxalic acid could not be identified. The association constants of the aquo-HC₂O₄^- and aquo-C₂O₄^2- ion-pairs are essentially temperature independent and have values 1.5 and 5.8 dm³ mol^-1 (50-60°C) respectively. The interchange reactions of the ion-pairs to yield the innersphere complexes, cis[(en)₂(NH₃)CoC₂O_4]⁺ or its acid form, are assigned a dissociative mechanism, Id, involving Co □ O bond breaking. The mechanism of the anation of the aquo cation by H₂C₂O₄ may be Id but the possibility of rate limiting addition of oxalic acid to the Co □O bond can not be completely ruled out.