Metal-exchange reactions between cobalt(II) complexes of polyaminocarboxylic acid ligands and copper(II) ions

Abstract

Reactions between copper(II) and polyaminocarboxylato-complexes of cobalt(II) have been studied by a stopped-flow spectrophotometric technique at an ionic strength I= 0.10 mol dm^–3 and 25 °C. Competitive associative and dissociative steps are observed with different dependences on the entering metal and on acidity. The rates of the reactions shift from first to zero order in [Cu²⁺] in the case of ethylenediaminetetra-acetate and ethylenebis(oxyethylenenitrilo)-N,N,N′N′-tetra-acetate, while only first-order behaviour is observed for ethylenedi-imino-N,N′-diacetate and N-(2-hydroxyethyl)-ethylenedinitrilo-N,N′,N′-triacetate. For all the reactions evidence is given for a stepwise unwrapping mechanism followed by attack of Cu²⁺ to give a binuclear intermediate. Experimental reactivities are compared with those computed according to the substitution lability of the exchanged metal ions and the stability of the proposed intermediate.