Formation of tris-chelated vanadium(IV) complexes by interaction of oxovanadium(IV) with catecholamines, 3-(3,4-dihydroxyphenyl)alanine and related ligands in aqueous solution.

Complex formation between oxovanadium(IV) and a series of o-catecholic ligands was investigated in aqueous solution by EPR and electronic absorption spectroscopy and pH-potentiometry. The reaction scheme involves, besides the normal mono- and bis-catecholato oxovanadium(IV) complexes, also species where the bare vanadium(IV) ion, lacking the oxo ligand, is chelated by three catecholate ligands. A comparative examination of the systems showed that the extent to which the ligands are able to form tris complexes with VIV, by displacing the oxo group from VO2+, may be reasonably interpreted in terms of the electrostatic repulsion between the bis complex and the attacking third ligand molecule and also the base strength of the phenolate groups of the ligands.