NMR relaxation studies in solutions of transition metal complexes. VI. Equilibria and proton exchange processes in aqueous solutions of VO 2+-glycine system

Abstract

The equilibria in aqueous solution of the VO 2+-glycine system has been studied by pH-metry in very high ligand excess, to avoid the hydrolysis of vanadyl ions. The complexes VOG +, VOG 2, VOGH 2+, VOG 2− H + and VOG 2H − −1 = VOG 2(OH) − are dominating in the system; their formation constants are given. The formation of complexes of VOGH −1 = VOG(OH), VOGH − −2 = VOG(OH) − 2 and (VO) 2G 2H −2 = (VO) 2G 2(OH) 2 have also been detected. It is stated that the VO 2G 2(OH) 2 is not the only binuclear (polynuclear) complex in the system; the composition of the other polynuclear complexes, however, cannot be stated unambiguously because of their very low concentrations. The rate constants of the proton exchange between the bulk water and the different complexes have been determined by measuring the T 2 relaxation time of the water protons. In contrast to the oxalate, and some other vanadyl complexes, the rate constant decreases by the decrease of the number of water molecules remaining in the first coordination sphere of the vanadyl ion. The exceedingly high proton exchange rate constant for the VOG 2(OH) − mixed hydroxo complex is interpreted as being due to the direct proton exchange between the bulk water and the coordinated OH group.