Speciation 98: Abstracts

Organotin compounds have been accumulated in the environment as a result of their wide spreading industrial application (PVC stabilisers, fungicides, molluscides etc.) and their formation from inorganic precursors through chemical or biological processes. The generally very toxic organotin compounds entered into the living systems may interact with a variety of biological ligands. The different composition and structure of organotin complexes, may alter significantly the biological effect of the free metal ion (e.g. some, mostly dialkyltin compounds have notable anti-tumour activity). Although a number of organotin compounds are well characterised in solid state, many questions may rise concerning their form in the living organism (they may decompose, form mixed-ligand complexes etc.). To characterise these effects and interactions first we need to understand the co-ordination chemistry of organotin cations in solution and to know their biological speciation.

In our work, we have characterised the equilibrium properties of diethyltin(IV) complexes with glycolic (1), lactic (2), succinic (3), malic (4), tartaric (5), mercaptoacetic (6), mercaptopropionic (7), mercaptosuccinic (8) and m-2,3-dimercaptosuccinic (9) acids. In case of the O-donor ligands, our results revealed the co-ordination of the OH groups in both non-deprotonated (ML) and deprotonated (MLH_-1, MLH_-2) forms. The metal promoted deprotonation of hydroxyl-groups takes place at very low pH (=3-4). In case of 1, 2 and 3 the very stable (and water-soluble) hydrolytic species [(Et)₂Sn(OH)₂] is the unique complex above pH 6. The additional donor groups in 4 and 5, however, prevent the formation of the above hydrolytic species and the ligand-containing complexes are dominant in the whole pH range.

The SH-containing ligands offer much stable metal binding sites for the (Et)₂Sn(IV)²⁺ cation then OH-acids. The formation of the {COO^-, S^-} co-ordinated species is practically complete at pH 2. In equimolar systems, this type of co-ordination is also preserved in the successively formed hydroxo-mixed complexes, with exception of 9 where both thiolate groups are co-ordinated by a chelated way. In case of ligand excess bis-complexes are also formed above pH 6 with all ligands studied. The log(K₁/K₂) values of these systems are, however, extremely high values, suggesting different co-ordination mode and/or steric hindrance of the second ligand.

Back to the list of abstracts