Interaction of methyltin(IV) compounds with carboxylate ligands. Part 1: Formation and stability of methyltin(IV)-carboxylate complexes and their relevance in the speciation of natural waters.

Quantitative data on the stability of mono-, di- and trimethyltin(IV)-carboxylate complexes (acetate, malonate, succinate, malate, oxydiacetate, diethylenetrioxydiacetate, tricarballylate, citrate, butanetetracarboxylate and mellitate) are reported at t = 25 ◦C and I → 0 mol l−1. Several mononuclear, mixed proton, mixed hydroxo and polynuclear species are formed in these systems. As expected, the stability trend is mono- > di- > trimethyltin(IV) and mono < di < tri < tetra < hexa for the organotin moieties and carboxylate ligands investigated, respectively. Moreover, ligands containing, in addition to carboxylic,–O–and—OH groups show a significantly higher stability with respect to analogous ligands with the same number of carboxylic binding sites. The results obtained fromall the systems investigated allowed us to formulate the following empirical predictive equation for correlation between complex stability and some simple structural parameters, log β = −6.0 + 1.63ncarb + 1.4nOH + 4.58r + 3.9zcat where ncarb and nOH are the number of carboxylic and alcoholic groups in the ligand, respectively, r is the stoichiometric coefficient ofH+ (positive) orOH− (negative) and zcat is themethyltin cation charge (CH3)xSnz+ (z+ = 4 − x).Distribution diagrams for some representative systems are also reported and are discussed in the light of speciation studies in natural waters. A literature data comparison is made with carboxylate complexes of other metal ions with the same charge as the organotin cations investigated here.