Equilibrium and spectroscopic studies of diethyltin(iv) complexes formed with hydroxymono- and di-carboxylic acids and their thioanalogues

The complex formation of diethyltin() cation with glycolic (GA), lactic (LA), succinic (SA), malic (MA), tartaric (TA), mercaptoacetic (MAA), 2-mercaptopropionic (MPA), mercaptosuccinic (MSA) and dimercaptosuccinic acid (DMSA) has been investigated by potentiometric, spectrophotometric, H-1 NMR and Mossbauer spectroscopic methods. The mercaptocarboxylic acids yielded much more stable complexes than the corresponding hydroxy acids. Below pH 3, the carboxylate and the still protonated hydroxyl group of hydroxy acids are co-ordinated to the metal ion, while in the case of their thio analogues, {COO-, S-} co-ordinated species are dominant. With increasing pH, the metal promoted deprotonation of the hydroxyl group takes place in the presence of MA. In the neutral pH range, among the hydroxy acids, only MA and TA are able to suppress the formation of hydrolytic species of diethyltin(). Although metal co-ordinated water deprotonation was observed around pH 6-7, in the case of the mercaptocarboxylic acids, only ligand-containing complexes were formed in the whole pH range studied. Complexes displaying slow ligand exchange were detected in the case of MA and all mercaptocarboxylic acids, which allowed their structural characterization, by H-1 NMR spectroscopy. In agreement with the Mossbauer spectroscopic data, trigonal bipyramidal, {COO-, O-/S-, OH-} co-ordinated complexes are dominant around pH 7, with the exception of the octahedral geometry found in the dimer complex M2L2 of DMSA, having a {2COO(-), 2S(-)} donor set.