NMR relaxation studies in solution of transition metal complexes. V. Proton exchange reactions in aqueous solutions of VO 2+-oxalic acid, -malonic acid systems

Abstract

The equilibria existing in the VO 2+ -oxalic acid and -malonic acid systems have been studied pH-metrically and the proton exchanges between the bulk water and the different paramagnetic species have been insvestigated by measuring the T 2 relaxation time of water protons at 298 K, I = 1.0 M NaClO 4. Mainly VOL and VOL 2− 2 type complexes are formed in both systems, the relative amounts of the protonated and mixed hydroxo complexes being very small up to pH 5. The first order rate constants of the proton exchange between the bulk water and the paramagnetic species are given. The surprisingly high value for the VOL 2− 2 type complexes is interpreted by a continuous intramolecular rearrangement of the ligands from two equatorial positions to axial-equatorial position and vice versa. The unexpectedly high rate constant for the protonated complex VOmalH + is interpreted by its fast acid dissociation process.