Theoretical and experimental study of tri- and tetrahalodiorganostannate (IV) salts. Solvent dependence in the reaction of dimethyltin dibromide withtetraethylammonium bromide

The reaction of SnMe2Br2 with Et4NBr, in a 1:2 molar ratio, yields (Et4N)(2 )[SnMe2Br4] (1) in CHCl3/hexane mixtures but (Et4N)[SnMe2Br3] (2) in water. This remarkable solvent dependence is explained by means of a thermochemic al cycle that includes the lattice enthalpies of both compounds, the solvat ion enthalpies of the Et4N+ and Br- ions, and the gas-phase dissociation en thalpy of [SnMe2Br4](2-) into [SnMe2Br3](2-) and Br-. Both compounds have b een characterized in the solid state by IR, Raman, and Sn-119 Mossbauer and MAS NMR spectroscopy and in solution by H-1, C-13 and Sn-119 NMR spectrosc opy. The X-ray crystal structures of 1 and (Me4N)[SnMe2Br3] (3) are reporte d. The crystallographic study of 3 provides the first X-ray crystal structu re containing an [SnR2Br3](-) anion. The structures of SnMe2X2, [SnMe2X3](- ), trans-[SnMe2X4](2-) (X = F, Cl, Br, I), cis-[SnR2Cl4](2-) (R = Me, Et), SnEt2Cl2, [SnEt2Cl3](-), and trans-[SnEt2Cl4](2-) have been optimized, at t he SCF level, by ab initio MO methods. The gas-phase formation of [SnR2X3]( -) anions from SnR2X2 and X- is an exothermic process, but [SnR2X4](2-) ani ons are unstable in the gas phase toward dissociation into [SnR2X3](-) and X-, while cis-[SnR2X4](2-) species (R = Me, Et) are unstable with respect t o their trans isomers by ca. 79 kJ/mol. The optimized gas-phase structures of SnMe2X2 show C-Sn-C angles increasing from 117.7 to 124.9 degrees as the electronegativity of X increases. The pentacoordinated [SnR2X3](-) anions show a trigonal-bipyramidal geometry with the R groups in equatorial positi ons (C-Sn-C angles in the range 128.7-133.5 degrees) and longer Sn-X distan ces for the axial bonds than for the equatorial ones. When one takes into a ccount that calculated distances are longer than the experimental ones, the present results strongly support the accuracy of the structural prediction s from ab initio MO calculations.