Kinetics of acid-catalysed dissociation of lead(II) and copper(II) complexes of ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA)

Abstract

The acid-catalysed dissociation rate constants for PbEGTA 2− and CuEGTA 2− complexes (where EGTA is ethylenebis(oxyethylenenitrilo) tetraacetic acid) were measured in acetic acid-acetate buffer medium (pH: 3.0–4.8) and perchloric acid solutions ([H +] = 0.05–0.15 M), respectively, at a constant ionic strength of 0.15 (NaClO 4). The rate laws shown by the lead(II) and copper(II) complexes are of the form, Rate = { k d + k H[H +]}[complex] and Rate = { k d + k H 2[H +] 2}[complex], respectively. Enthalpy and entropy of activation for acid-independent and acid-catalysed pathways for both the complexes were obtained by the temperature-dependence studies of resolved rate constants in the 16–45°C range. The rate of dissociation of PbEGTA 2− is not enhanced by increasing the concentration of acetate ion in the buffer, and the amount of total electrolyte in the reaction mixture has no pronounced effect on the dissociation rates of their the lead(II) or copper(II) complex. Attempts to study the kinetics of stepwise ligand unwrapping in the binuclear Cu 2EGTA complex were unsuccessful due to the extremely rapid dissociation of this complex to yield mononuclear CuEGTA 2−.