Experimental Investigations andabInitioStudies on Hexacoordinated Complexes of Dichlorosilane

Dichlorosilane, SiH 2 Cl 2, forms two different kinds of coordination compounds with pyridine and 3-picoline: the simple adduct trans -SiH 2 Cl 2 (L) 2 and the ionic complex [SiH 2 (L) 4 ]Cl 2 ·4CHCl 3 (L = pyridine, 3-picoline). The adducts trans -SiH 2 Cl 2 (py) 2, 1, and trans -SiH 2 Cl 2 (3pic) 2, 2, form directly from the reaction of bis(dichlorosilyl)methylamine, NMe(SiHCl 2 ) 2 with pyridine (py), and 3-picoline (3pic). Reaction of 1 with an excess of pyridine in chloroform yielded [SiH 2 (py) 4 ]Cl 2 ·4CHCl 3, 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, C 2/ 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, P 1̄): 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, C 2/ 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 -SiH 2 Cl 2 (py) 2 and [SiH 2 (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 SiH 2 Cl 2 and pyridine in the gas phase is slightly endothermic (Δ H 298 = 17.2 kJ mol - 1 ) and endergonic (Δ G 298 = 113.6 kJ mol - 1 ). The significance of the Lewis acidity of SiH 2 Cl 2 is thus questioned and intermolecular interactions (e.g. hydrogen bonds) are held responsible for the existence of 1 in the solid state.