Theoretical and Experimental Study of Tri- and Tetrahalodiorganostannate(IV) Salts. Solvent Dependence in the Reaction of Dimethyltin Dibromide with Tetraethylammonium 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 thermochemical cycle that includes the lattice
enthalpies of both compounds, the solvation enthalpies of the Et4N+ and Br- ions, and the
gas-phase dissociation enthalpy of [SnMe2Br4]2- into [SnMe2Br3]- and Br-. Both compounds
have been characterized in the solid state by IR, Raman, and 119Sn Mössbauer and MAS
NMR spectroscopy and in solution by 1H, 13C, and 119Sn NMR spectroscopy. The X-ray crystal
structures of 1 and (Me4N)[SnMe2Br3] (3) are reported. The crystallographic study of 3
provides the first X-ray crystal structure 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 the 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- anions are unstable in the gas phase toward dissociation
into [SnR2X3]- and X-, while cis-[SnR2X4]2- species (R = Me, Et) are unstable with respect
to 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° as the electronegativity of X increases.
The pentacoordinated [SnR2X3]- anions show a trigonal-bipyramidal geometry with the R
groups in equatorial positions (C−Sn−C angles in the range 128.7−133.5°) and longer Sn−X
distances for the axial bonds than for the equatorial ones. When one takes into account that
calculated distances are longer than the experimental ones, the present results strongly
support the accuracy of the structural predictions from ab initio MO calculations.