Kinetics of ligand exchange and dissociation reactions of the calcium(II)-EGTA complex investigated by the NMR technique

Abstract

The kinetics of the ligand exchange reactions occurring in solutions containing calcium(II) ion and ethylenebis(oxyethylenedinitrilo)-tetraacetic acid (EGTA) have been studied by computer analysis of the nuclear magnetic resonance (NMR) spectra recorded at 35°C over a wide range of ligand and metal ion concentrations, in the pH range 5.0–9.5. The acidic dissociation constants of EGTA and the formation constants of its complexes formed with the calcium(II) ion have been obtained by alkalimetric potentiometric titrations in the same experimental conditions as the kinetic analysis. The experimental results are consistent with the hypothesis that the ligand exchange may occur either by dissociation reactions (with a first order dependance on the complex concentration and a zero order dependance on the incoming ligand concentration) or by symmetrical exchange reactions (with a first order dependance on both ligand and complex concentrations). Rate constant values have been obtained for the exchange reactions of the unprotonated complex with the un-, mono- and diprotonated ligand anions (20 ± 1 M −1 sec −1, 16 ± 1 M −1 sec −1, 7.0 ± 0.5 M −1 sec −1, respectively) and for the exchange reaction of the protonated complex with the diprotonated ligand anion ((9.8 ± 0.1) × 10 3 M −1 sec −1). The rate constant of the proton assisted dissociation reaction of the unprotonated CaEGTA complex has also been evaluated ((8.0 ± 0.1) × 10 6 M −1 sec −1), whereas only an upper limit has been estimated for the not catalyzed dissociation reaction of the same species. A discussion is given in which the experimental results are analyzed in order to ascertain whether EGTA may utilize its either oxygen atoms in binding to the metal ion.