Interaction of alkyltin(IV) compounds with ligands of interest in the speciation of natural fluids: carboxylate and hydroxycarboxylate complexes of monomethyltin(IV) trichloride

摘要

The formation and stability of some carboxylate and hydroxycarboxylate (acetate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate, malate and citrate) complexes of monomethyltin trichlorides was studied potentiometrically at 25 deg C and at different ionic strengths in NaNO_3 aqueous solution. The following quite stable species are formed in the different systems (M = CH_3Sn~(3+)): ML(OH)~+, ML_2(OH)~0, ML(OH)_2~0 and M_2L(OH)_5~0 for acetate; MLH~+, ML~0, ML(OH)~-, and ML(OH)_2~(2-) for propanetricarboxylate; MLH_2~+, MLH~0, ML; ML(OH)~92-) and ML(OH)_2~(3-) for butanetetracarboxylate; ML(OH)~0, ML()H)_2~-and ML(OH)_3~(2-) for malate; ML~0, ML(OH); ML(OH)_2~(2-) and ML(OH)_3~(3-) for citrate. Hydroxycarboxylate complexes are significantly stronger than simple carboxylate ones and this is likely to be due to the interaction of the -OH group in citrate and malate with monomethyltin(IV), whose strength was also quantified. It was found that the stability of these complexes can be roughly expressed by the simple relationship logK= a xi is the product of the charges of reactants and logK is the equilibrium constant. For simple carboxylic ligands we have a = 1.8+-0.4 and, for hydroxycarboxylic ligands, a = 3.7+-0.9. Other useful empirical relationships are reported. Moreover, hydroxycarboxylic complexes also play a prominent role in the speciation of monomethyltin(IV) under the pH conditions of interest for natural fluids.