Kinetics of substitution of aquo ligands fromcis-diaquo-bis-(biguanide) cobalt(III) and chromium(III) ions by aspartic acid in ethanol—water mixtures

Summary

The kinetics of substitution of aqua ligands fromcis-diaqua-bis(biguanide)cobalt(III) and chromium(III) ions by aspartic acid in EtOH−H₂O media have been studied spectrophotometrically in the 30 to 45°C range. We propose the following rate law for the anation

$$\begin{gathered} d[M(BigH)_2 (Asp)^{1 + } ]/dt \hfill \\ = \frac{{k_a K_E [M(BigH)_2 (H_2 O_2^{3 + } )]_{total} [AspH^{1 - } ]}}{{1 + K_E [AspH^{1 - } ]}} \hfill \\ [M = Co^{111} and Cr^{111} ] \hfill \\ \end{gathered} $$

in the 3.8 to 5.5pH range, where K_E is the ion-pair equilibrium constant and k_a the rate constant for interchange of outer-sphere complex to inner-sphere complex. Activation parameters. ΔH^≠ and ΔS^≠, were obtained from Eyring plots. The results are consistent with a mechanism involving outer-sphere association of reactants followed by associative interchange of this outer-sphere complex into the product.