Oxovanadium(IV) complexes of phosphonic derivatives of iminodiacetic and nitrilotriacetic acids

The complex formation between (VO)-O-IV and phosphonic derivatives of imino diacetic (Ida) and nitrilotriacetic acids (Nta) has been studied by combine d application of pH-potentiometric and spectroscopic (EPR and electronic ab sorption) techniques. Differently from aminomonophosphonates, which are abl e to bind the metal ion rather weakly forming five-membered chelated rings, the mixed carboxylic-phosphonic and pure phosphonic derivatives of Ida and Nta form chelated systems resulting in very stable complexes mostly of the 1 : 1 type. Substitution of CO2- by PO32- increases the stability of the c omplexes due to the higher basicities of the phosphonic functions. However, the higher spatial requirement of the phosphonate groups and the greater e lectrostatic repulsion between the dinegative phosphonate arms would mostly compensate this effect. Spectroscopic evidence has been found for a consid erable distortion in the geometry of the aminophosphonate complexes. The di fferences in the observed stability trends between the corresponding copper (II) and (VO)-O-IV complexes are also discussed.