Reaction pattern of amino acid chelates of cobalt(III) in acidic aqueous solution

Abstract

The reactions of amino acid chelates of cobalt(III) have been investigated in order to study the influence of chelate size, ring pattern and nature of bonding on the rates of reactions. The complex ion chosen is Co(BigH)₂(am)²⁺, where BigH = biguanide and am = glycinate (Gly), <em>α</em>-alaninate (<em>α</em>-Alan), Palaninate (<em>β</em>-Alan) and cysteinate (Cyst) ions. In the case of the first three complexes kohs varies linearly with [acid]. However, for the cysteinato complex, reciprocal plot (1/<em>k</em>_ob_s versus 1/[H⁺]) is linear. The reactivity order of the complexes is: <em>β</em>-Alan < Gly < <em>α</em>-Alan < Cyst. A plot of -log <em>k </em>versus <em>pK </em>of amino acids indicates that for the Gly, <em>α</em>-Alan and Cyst complexes, basicity of the chelate ring controls the mechanism, while for the <em>β</em>-Alan complex, the mechanism is not well understood.