Kinetic study of acid dechelation of some vanadium(V)–aminocarboxylate complexes in aqueous solution

Abstract

Kinetic studies of the acid hydrolysis of vanadium(V) complexes of ethylenediaminetetra-acetic acid (H₄edta), ethylenediamine-N,N′-diacetic acid (H₂edda), and nitrilotriacetic acid (H₃nta) have been carried out by stopped-flow spectrophotometry in aqueous solution (I= 3 mol dm^–3, NaClO₄). The observed first-order (with respect to chelate) rate constant increases as a hyperbolic-type function with H⁺ concentration. Discussion of the differences in reactivity of the chelates indicates that the dissociation of the chelate complexes would proceed by a pre-equilibrium step leading to the formation of an intermediate for which the ketonic oxygen of one carboxylate group of the chelate is protonated. This first step is followed by rupture of a metal–oxygen bond, rupture of a metal–nitrogen bond, and then by complete cleavage of the multidentate ligand.