Oxovanadium(IV) complexes of mercaptocarboxylic acids.

Complex formation between oxovanadium(IV) and a series of aliphatic mercaptocarboxylic acids (mercaptoacetic, 2- and 3-mercaptopropionic, 2-mercaptosuccinic, and meso-2,3-dimercaptosuccinic) was studied in aqueous solution by potentiometric and spectroscopic (EPR and electronic absorption) techniques. A comparison with the complexes of the corresponding hydroxycarboxylic acid analogues was also made. The most interesting finding is that VO2+ has a similar affinity to the rather soft thiolate-S donors as to the hard alcoholate-O donors, if these donors are in chelatable position with a hard carboxylate group. The bidentate mercaptocarboxylates form (CO2-, S-)-co-ordinated mono and bis complexes, while bis complex formation is rather hindered with the tridentate 2-mercaptosuccinic acid. Due to the high denticity of the ligand, 2,3-dimercaptosuccinic acid readily forms oligomeric species via the co-ordination of all potential donor groups.