LIGAND-EXCHANGE KINETICS AND SOLUTION EQUILIBRIA OF CADMIUM ZINC AND LEAD NITRILOTRIACETATE COMPLEXES

Proton nuclear magnetic resonance (nmr) techniques have been used to study the aqueous solution chemistry of the nitrilotriacetate (NTA) complexes of cadmium, zinc, and lead. Complexes having NTA-to-metal ratios of 1 and 2 are present, depending upon the solution conditions, and their formation constants were evaluated from chemical shift data. The kinetics of ligand exchange between the free and the complexed forms were measured using nmr line-broadening techniques. Exchange was found to occur by both first-order dissociation of the metal-ligand complexes and by second-order processes involving free ligand and metal-ligand complexes. The rate constants for each of the reactions resulting in ligand exchange were evaluated from the dependence of the exchange rate upon ligand concentration and pH. It was observed that, when the free NTA is in the monoprotonated form, the rate of ligand exchange is significantly less than the rate when the NTA is in the totally ionized form. From comparisons of the rate constan s in these and in other systems, mechanisms to explain the dependence of the exchange rate on the degree of protonation of the reacting ligand are discussed. It is proposed that the decrease in rate is due to the relatively slow rate of proton migration from the nitrogen atom in a partially bonded reaction intermediate. Possible structures for the NTA complexes are discussed based on the kinetic rate constants, the equilibrium formation constants, and the heteronuclear coupling observed in the nmr spectra for several of the complexes. © 1969, American Chemical Society. All rights reserved.