Abstract
Abstract Kinetics of aquation has been studied with pentamminecobalt(III) complexes containing aliphatic amino acids (γ-aminobutyric acid, ε-aminocaproic acid, α-amino-isobutyric acid, ornithine, and proline), aromatic acids (benzoate, anthranilic, and p-aminobenzoic acid), and α-substituted acetates (monochloroacetate and glycolate). Anation rate constants were obtained for glycine, β-alanine, and ε-aminocaproic acid, and equilibrium constants between [CoH2O(NH3)5]3+ and [Co(NH3)5amH]3+ (amH=amino acid), Keq were calculated. The free energy plot logkaq vs. logKeq gave a straight line with a gradient 0.6. This value has been discussed in terms of two possible mechanisms. The dependence of acid catalysed aquation rate constant, kH+ on pKa of carboxylate group of the amino acid is written as logkH+=0.5 pKa+ const., while that for α-substituted acetates log kH+=pKa+ const. Such a difference in the slope is interpreted as due to the electrostatic influence of–NH3+ group of coordinated amino acid.
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