Stable hexacoordinated neutral complexes between silyl halides and two water or two ammonia molecules:: SiX4Y2 (X = H, F, Cl; Y=H2O, NH3)

Potential energy surfaces for SiH(4-n)Hal(n) + 2NH(3) (I) and SiH(n)Hal(4-n) + 2H(2)O (II) Systems (n = 2, 4; Hal = F, Cl) were explored using the B3LYP and MP2 methods with 6-311+G(d,p) and DZP+diff basis sets in search of stable tightly bound complexes with hexacoordinate silicon. Despite discrepancies in values of the complexation energies, all methods employed predict a similar order of stability of isomers. In contrast to monohalosilanes, where tight complexes are metastable, the energies of all low-entropy complexes in I are below those of the reactants. However, only for the SiH2Cl2, SiF4, and SiCl4 complexes were the tight structures found to be global minima. The predicted SiN bond lengths in these species are in a good agreement with experimentally determined parameters for complexes of these three halosilanes with pyridines. In the water systems (II), the tight octahedral complexes (with water molecules in axial and equatorial positions) are predicted to lie below reactants only for the SiF4 + 2H(2)O system. A loosely bound complex is the global minimum for this system, as well as for complexes of SiH2F2 and SiH2Cl2 with two water molecules. No minimum with energy below those of the reactants was found for the SiCl4 + 2H(2)O system.