Formation, stability and empirical relationships for the binding of Sn2+ by O-, N- and S-donor ligands

This paper reports the results of a study on the interactions of tin(II) with different low molecular weight (poly)carboxylates (acetate (ac), malonate (mal), succinate (suc), adipate (adip), 1,2,3,-propanetricarboxylate (tca), 1,2,3,4-butanetetracarboxylate (btc), mellitate (mlt)), polyamines (putrescine (ptr), cadaverine (cdv), spermidine (spd), spermine (sper)), aminoacids (glycinate (gly), aspartate (asp), glutamate (glu)), and S-donor ligands (thiolactate (tla), thiomalate (tma), penicilamine (pen)), at T = 298.15 K and I = 0.15 mol L-1 in NaNO3(aq). Depending on the ligand, different MpLqHr species are formed (including the mixed ML(OH)), and the corresponding stability constants are reported. In the case of the Sn2+-ac, Sn2+-adip, Sn2+-mlt and Sn2+-asp systems, measurements at different ionic strengths (0.15  I / mol L-1  1.0) in NaNO3(aq) were also carried out, while experiments at different temperatures (298.15  T / K  313.15) have been performed only in the case of the Sn2+-adip system, in order to evaluate and model the dependence of the stability of tin(II) / ligand species on these parameters. The sequestering ability of the investigated ligands toward tin(II) has been also evaluated by calculating various pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of cation present in trace), a semiempirical parameter already proposed for the quantification of this ability in different conditions. Some empirical relationships are also presented, for a rough but immediate estimation of the stability of various tin(II) / ligand species as a function of the nature and number of the functional groups of different ligands.