Acid aquation of mixed chelate complexes.: Aquation of glycinato- and α- and β-alaninatoethylenebisbiguanidecobalt(III) complexes

Acid-catalysed aquation of three amino acid chelates of cobalt(III) has been investigated in order to study the influence of chelate size and ring pattern on the rates of reactions. The complex ion chosen is Co(EBB)(am)(2+), where EBB = ethylenebisbiguanide and am = glycinate (Gly), alpha-alaninate (alpha-Alan) and beta-Alaninate (beta-Alan). The observed pseudo-first order rate constant varies linearly with acid concentration. Only the cobalt(III) complex with beta-Alan shows an acid-independent path as revealed from a plot of k(obs) vs [H+], At 30 degrees, the reactivity order of the complexes is : beta-Alan < Gly <alpha-Alan. Activation parameters (Delta H* and Delta S*) have been determined in the usual way. Positive entropy of all the complexes indicates release of an 'am' chelate after the entry of solvent molecules in the coordination sphere.