Complexes of oxovanadium(IV) with polyaminocarboxylic acids.

The stability constants for a series of oxovanadium(IV) complexes of polyaminocarboxylic acids were determined by potentiometry. The values obtained are almost equal to those of the corresponding nickel(II) complexes. The complexes formed by terdentate and quadridentate ligands contain 2 and 1 co-ordinated water molecules, respectively. These dissociate at pH ~4 in the first case, to give dimers-(VO)(2)(OH)(2)L(2),-and at about pH ~7, in the second case, to give the mononuclear hydroxo species VO(OH)L. Hydrolysis of the 1:1 aquo-complexes is preferred to the formation of 2:1 ligand: metal complexes even in the presence of a 10-fold molar excess of ligand. These results are of interest for better understanding of the behaviour of oxovanadium(IV) in biological systems.