Modelling the hydrolysis of mixed mono-, di- and trimethyltin(IV) complexes in aqueous solutions

The formation of mixed hydrolytic species obtained by the interaction of dimethyltin(IV) {(CH3)2Sn2+ or DMT or M22+} with the monomethyltin(IV) (CH3Sn3+ or MMT or M13+) and trimethyltin(IV) {(CH3)3Sn+ or TMT or M3+} cations was studied in aqueous solutions of NaNO3, at I = 1.00 ± 0.05 mol·dm-3 and T = (298.15 ± 0.1) K, by the potentiometric technique. The formation of several mono- and polynuclear hydrolytic mixed species was observed in the two mixed systems. For the (M1) p (M2) q (OH) r mixed species we have: (p, q, r) = (1, 1, 3), (1, 1, 4), (1, 1, 5), (2, 1, 5), (2, 1, 7), (1, 3, 6) and (2, 3, 11); for (M2) p (M3) q (OH) r: (p, q, r) = (1, 1, 2), (1, 1, 3), (1, 2, 3), (1, 2, 4), (2, 1, 4), (3, 1, 4) and (3, 1, 5). The stability of these species, expressed by the following general equilibrium: pMin+ + qMjj+ + rOH- ⇄ (Mi)p (Mj)q(OH)r(np+jq-r) βpqrOH can be modelled by the empirical relationship that depends on the stoichiometry of the species: βpqrOH = a1p + a2q + a3r - y log 10 β p q r OH = a 1 p + a 2 q + a 3 r - y For the (M1) p (M2) q (OH) r species: a 1 = 6.98, a 2 = 2.73, a 3 = 9.93 and y = 1.14, while for the (M2) p (M3) q (OH) r species: a 1 = 5.03, a 2 = 1.00, a 3 = 8.04 and y = 1.71. By using the equilibrium constant X pqr relative to the formation reactions: p(M1}(p + q} } (OH)r + q(M2)(p + q)(OH}r⇄ (p + q) (M1}p(M2q(OH)r and p(M2}(p + q}} (OH)r + q(M3)(p + q)(OH}r⇄ (p + q) (M2}p(M3)q(OH)r we found that the formation of hetero-metal mixed species is thermodynamically favored and the extra stability can be expressed as a function of the difference in the stability of the parent homo-metal species. This leads, in turn, to a significant enhancement of hydrolysis. The general stability and extra stability behavior of mixed-metal (or organometal) species is discussed while also considering previous findings from this laboratory.