STUDIES ON THE SOLUBLE INTERMEDIATES IN THE HYDROLYSIS OF DIMETHYLTIN DICHLORIDE: EQUILIBRIUM CONSTANT FITTING WITH A HIGH SPEED DIGITAL COMPUTER

Abstract

The hydrolysis of (CH3)2SnCl2 has been studied in a medium 0.1 M in (Na+)Cl− by a potentiometric titration technique. The complexes formed by the hydrolysis reactions together with their formation constants have been determined by analysis of the 394 data sets using a high speed digital computer. The data can be fitted equally well over the range pH 1.5 to 9 by the four formation constants for (CH3)2SnOH+ (log *β11 = −3.245 ± 0.004), [(CH3)2SnOH]2+2 (log *β22 = −5.00 ± 0.02), (CH3)2Sn(OH)2 (log *β21 = −8.516 ± 0.004), and [(CH3)2Sn]4(OH)6+2 (log *β64 = −16.85 ± 0.02) or by the five constants for (CH3)2SnOH+ (log *β11 = −3.251 ± 0.004), [(CH3)2SnOH]2+2 (log *β22 = −5.05 ± 0.02), (CH3)2Sn(OH)2 (log *β21 = −8.535 ± 0.005), ((CH3)2Sn)2(OH)3+ (log *β32 = −9.81 ± 0.01), and ((CH3)2Sn)3(OH)4+2 (log *β43 = −11.52 ± 0.03). Thus there is little question about the existence in the solutions of the simple monohydroxo complex and its dimer or of the neutral dihydroxide. Only very low concentrations of species containing more than four tin atoms can exist in aqueous solution. In addition, the data determined previously for the hydrolysis of (CH3)2Sn(ClO4)2 in a medium 3.0 M in (Na+)ClO4− have been refined by the least squares procedure. These 762 data sets can also be fitted equally well by the set of constants log *β11 = −3.55 ± 0.01, log *β22 = −4.52 ± 0.01, log *β21 = −9.00 ± 0.01, and log *β64 = −16.14 ± 0.03 or log *β11 = −3.54 ± 0.01, log *β22 = −4.60 ± 0.02, log *β21 = −8.98 ± 0.01, log *β32 = −9.76 ± 0.06, and log *β43 = −10.40 ± 0.05. The tendency of dimethyltin dichloride to form polycondensed species in aqueous solution is slight, and there is no evidence for the formation of large rings or chains. The solid stannoxane which precipitates from chloride solution was studied by infrared and X-ray powder photography and is essentially the same as the compound obtained by reaction of (CH3)2SnO and (CH3)2SnCl2 in a 1:1 mole ratio in 95% ethanol. This corresponds to a slightly different product than that obtained by a 1:1 reaction of these compounds in benzene which gives ((CH3)2SnCl)2O. This is in accord with the observation that precipitation occurs only at a pH higher than that at which ((CH3)2SnOH)2+2 has attained the maximum concentration, and hence precipitation is occasioned by the formation of larger polycondensed species. A comparison of the perchlorate and chloride data also indicates that the tendency to form polycondensed species is reduced by the presence bf chloride ion. This is partially an ionic strength effect and partially a result of chloride complexing of the simple aquodimethyltin (IV) ion, and the first chloro-complexing constant was Estimated to be 28 1. mole−1.