Sequestration of alkyltin(IV) cations by complexation with amino-polycarboxylic chelating agents

The binding capacity of four amino-polycarboxylic ligands (APCs) [nitrilotriacetate (NTA), ethylenediamine-N,N,N′,N′-tetraacetate (EDTA), (S,S)-ethylenediamine-N,N′-disuccinic acid (S,S-EDDS) and diethylenetriamine-N,N,N′,N″,N″-pentaacetate (DTPA)] towards mono-, di- and tri-alkyltin(IV) cations [(CH3)Sn3 +, (CH3)2Sn2 +, (C2H5) 2Sn2 +, (CH3)3Sn+ or (C2H5)3Sn+] was studied, in aqueous solutions, by ISE-H+ potentiometry, at I = 0.1 mol L- 1 (NaCl) and at T = 298.15 K. In all the systems RxSn (4 - x)+ - APC (R = CH3 or C2H5) a strong 1:1 species is formed together with protonated, hydroxo and dinuclear complexes. The values of log K are roughly linearly dependent on the charge of both the cation and the anion, and the very simple correlation log K = (1.50 ± 0.05) zanion·zcation can account for the stability of all the complexes, but hydroxo species, formed in the experimental conditions used in this investigation. The sequestering ability of APCs in each system was evaluated, at different pH values, by using the parameter pL 0.5, i.e., the total ligand concentration necessary to bind 50% of alkyltin(IV) cation trace.