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

The oxovanadium(IV) complex formation with alpha-hydroxycarboxylic lig ands, namely glycolic, lactic, 2-hydroxyisobutyric, 2-hydroxy-2-methyl butyric, 2-ethyl-2-hydroxybutyric, mandelic and benzilic acids, has be en examined in aqueous solution by potentiometric and spectroscopic te chniques. The results show that the complexation starts at pH values a s low as 3 with the coordination of carboxylate(s) to form species whe re the alcoholic group(s) take(s) also part in the metal binding. On i ncreasing the pH, alcoholic groups deprotonate and form chelated compl exes involving (COO-, O-) donor set(s). Intermediate species with [(CO O-, OH)(COO-, O-)] coordination are also observed. The nature of the s ubstituents at the a-C atom affects the speciation equilibria. Electro n-withdrawing (e.g., aryl) substituents facilitate the deprotonation o f the alcoholic group and in this case mono- and bis-chelated (COO-, O -) complexes are largely the predominant species in solution. On the c ontrary, 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 to ward the trigonal bipyramid.