A kinetic study of pyrophosphate and peroxodiphosphate complexation of oxovanadium(IV) ion

The formation constant of the vanadyl pyrophosphate complex, VO(H_2P_2O_7), determined spectrophotometrically in acidic 1.0 M NaClO_4 solution, is logK = 4.4±0.1 at 25°C. The kinetics of the complexation of vanadyl ion by pyrophosphate and peroxodiphosphate were studied at pH 2.0 using stopped-flow spectrophotometry. The reactions are first-order in ligand and in vanadyl ion with rate constants k_2 = 2.3×10^4 and 7.5×10^3 M^(−1) sec^(−1) at 12°C for H_2P_2O_7^(2−) and H_2P_2O_8^(2−), respectively. The complexation reactions were followed by slower secondary processes which are attributed to polymerization. The results for the complexation reactions are consistent with an interchange mechanism for the formation of a monodentate complex, followed by rapid closure of the chelate ring. Peroxodisulfate, studied under similar conditions, showed no evidence of innersphere complexation