Speciation of dimethyltin(IV) – and trimethyltin(IV) –carbocysteinate and – glutamate systems in aqueous media

The formation of complex species in the dimethyltin(IV) and trimethyltin(IV)-carboxymethyl-L-cysteinate (carbocysteinate) systems in NaClaq, at different ionic strengths, and in a multicomponent Na+, K+, Ca2+, Mg2+, Cl- and SO42- medium representative of the seawater major composition, is discussed. Experimental results give evidence for the formation of the following species (L¼carbocysteinate): [(CH3)2Sn(L)]0, [(CH3)2Sn(HL)]+, [(CH3)2Sn(OH)(L)]-, [(CH3)2Sn(OH)2(L)]2- in the DMT–CCYS system, and [(CH3)3Sn(HL)]0, [(CH3)3Sn(L)]- and [(CH3)3Sn(OH)(L)]2- in the TMT-CCYS system. The ionic strength dependence of formation constants was taken into account by an extended Debye Huckel type equation and by the SIT (Specific ion Interaction Theory). Measurements were carried out also on the dimethyltin(IV)-glutamate and trimethyltin(IV)-glutamate systems in NaClaq, owing the strict similarity of glutamate and carbocysteinate. Results obtained show the formation of complex species having the same stoichiometry as those formed in the DMT- and TMT-carbocysteinate systems, with very similar stability, confirming that carbocysteinate behaves as a dicarboxylic amino acid without involving the sulfur-bridge potential binding site in metal coordination.