Dioxovanadium(V) complexes of some sulphur-containing ligands

The stability constants of the complexes of dioxovanadium(V) ion with some sulphur containing amino acids (L-cysteine, N-acetyl-L-cysteine, and DL- penicillamine) were determined in acidic solution at 25 degrees C and 0.1 mol dm(-3) ionic media with high ligand-to-metal ratios, using a combination of potentiometric and spectrophotometric techniques. Sodium perchlorate solution was used to maintain the ionic strength. The compositions of the formed complexes were determined and it was shown that dioxovanadium(V), VO2+, forms two mononuclear 1:1 and 1:2 (metal-to-ligand ratios) species of the types VO2H2L+ and VO2(H2L)(2)(+) with the ligands, where L represents the fully dissociated form of each ligand. The logarithms of the cumulative stability constants, beta(xyz), of the complexes, [(oxometal ion)(x)(H)(y)(ligand)(z)], are log beta(121) and log beta(142) as: 4.82, 6.26 (for cysteine), 5.14, 6.56 (for N-acetylcysteine), and 5.56, 8.06 (for penicillamine).