STUDY OF FORMATION CONSTANTS OF VANADIUM (III)-CATECHOLATE COMPLEXES

Catecholates, the oxygen donor ligands, have high affinity towards transition metal ions. They canstabilize the highly sensitive (+3) oxidation state of vanadium even in presence of oxygen. Pyrocatechol,gallic acid and gallic acid methyl ester were chosen for this study. They form highly colored stable complexes with vanadium(III) in 1:1, 1:2 and 1:3 M:L molar ratio. Stability constants of all these species were determined by potentiometric as well as spectrophotometric methods. Computer software BEST were utilized the potentiometric data to calculate overall stability constants. log K value for the attachment of first ligand did not vary substantially with the difference in the ring substituents on the catechols.However stabilty constants for second and third chelate rings are distinctly affected by substitution on the catechol rings which is attributed to intralig and hydrogen bonding. Gallic acid complex of V(III) showed highest values of stability constants, b1 is 1014, b 2 is 1017 and b 3 is l020. The b calculated from the spectrophotometric data are also comparable