Experimental Investigations and ab Initio Studies on Hexacoordinated Complexes of Dichlorosilane

Dichlorosilane, SiH₂Cl₂, forms two different kinds of coordination compounds with pyridine and 3-picoline:  the simple adduct trans-SiH₂Cl₂(L)₂ and the ionic complex [SiH₂(L)₄]Cl₂·4CHCl₃ (L = pyridine, 3-picoline). The adducts trans-SiH₂Cl₂(py)₂, 1, and trans-SiH₂Cl₂(3pic)₂, 2, form directly from the reaction of bis(dichlorosilyl)methylamine, NMe(SiHCl₂)₂ with pyridine (py), and 3-picoline (3pic). Reaction of 1 with an excess of pyridine in chloroform yielded [SiH₂(py)₄]Cl₂·4CHCl₃, 3. The molecular and crystal structures of 1−3 were investigated by single-crystal X-ray diffraction. The Si atoms of all three compounds are hexacoordinated and lie on centers of inversion. The basic structural parameters are the following:  1 (monoclinic, C2/c):  Si−N 196.9(1) pm, Si−Cl 228.8(1) pm; N−Si−Cl 90.0(1)°, C−N−C 119.0(2)^o; Cl−Si−N−C 102.4(1)°. 2 (triclinic, P1̄):  Si−N 197.5(1) pm, Si−Cl 229.2(1) pm; N−Si−Cl 90.3(1)°, C−N−C 118.8(1)°; Cl−Si−N−C 101.2(1)°. 3 (monoclinic, C2/m):  Si−N 196.2(4) and 197.0(4) pm, Si···Cl 419.9 pm; N−Si−N 90.0°. All three complexes, neutral and cationic, exhibit very similar lengths of the dative Si−N bond. Hartree−Fock calculations with 3-21G(d) and 6-31G(d) basis sets and density functional calculations with the B3LYP functional and a 6-31G(d) basis set were employed for geometry optimizations of trans-SiH₂Cl₂(py)₂ and [SiH₂(py)₄]²⁺. The optimized structures revealed similar Si−N bond lengths and geometric parameters of the pyridine moieties for both types of complexes as well. Single point energy calculations [B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)], including corrections for the basis set superposition error, showed that the formation of 1 from SiH₂Cl₂ and pyridine in the gas phase is slightly endothermic (ΔH²⁹⁸ = 17.2 kJ mol^-1) and endergonic (ΔG²⁹⁸ = 113.6 kJ mol^-1). The significance of the Lewis acidity of SiH₂Cl₂ is thus questioned and intermolecular interactions (e.g. hydrogen bonds) are held responsible for the existence of 1 in the solid state.