COMPLEX-FORMING PROPERTIES OF ALPHA-HYDROXYCARBOXYLIC ACIDS WITH OXOVANADIUM(IV) ION

The oxovanadium(IV) complex formation with alpha-hydroxycarboxylic ligands, namely glycolic, lactic, 2-hydroxyisobutyric, 2-hydroxy-2-methylbutyric, 2-ethyl-2-hydroxybutyric, mandelic and benzilic acids, has been examined in aqueous solution by potentiometric and spectroscopic techniques. The results show that the complexation starts at pH values as low as 3 with the coordination of carboxylate(s) to form species where the alcoholic group(s) take(s) also part in the metal binding. On increasing the pH, alcoholic groups deprotonate and form chelated complexes involving (COO-, O-) donor set(s). Intermediate species with [(COO-, OH)(COO-, O-)] coordination are also observed. The nature of the substituents at the a-C atom affects the speciation equilibria. Electron-withdrawing (e.g., aryl) substituents facilitate the deprotonation of the alcoholic group and in this case mono- and bis-chelated (COO-, O-) complexes are largely the predominant species in solution. On the contrary, the species formed by alpha-hydroxycarboxylic acids provided with groups exhibiting electron-releasing effect are more resistant to hydrolysis. UV-VIS spectroscopy shows that in the bis-chelated (COO-, O-) complexes the ligands have a trans-arrangement and, depending on the substituents, the geometry at vanadium may exhibit a distortion toward the trigonal bipyramid.