Oxovanadium(IV) complexes of ligands containing phosphonic acid moieties

Complex formation between oxovanadium(rv) and phosphonate ligands was investigated by pH potentiometric and spectroscopic (EPR and electronic absorption) techniques showing that, in contrast to monophosphate, which is able to bind the metal ion forming four-membered chelated rings, methylphosphonate acts only as a monodentate ligand so that hydrolysis sets in at very low pH. Other ligands have also been considered, phosphonoformic acid, its higher homologue phosphonoacetic acid, methylenediphosphonic acid and I-hydroxy(methyl)methylenediphosphonic acid, which adopt a chelating behaviour and form much more stable complexes. Spectroscopic and potentiometric evidence for the binding of methylenediphosphonic and l-hydroxy(methyl)methylenediphosphonic acid by oxovanadium(rv) yielding trinuclear species similar to the well-characterized analogue formed by diphosphate has been obtained.