POSSIBILITY OF HEPTACOORDINATION AT SILICON - SYNTHESIS AND X-RAY CRYSTAL-STRUCTURES OF TRIS[8-(DIMETHYLAMINO)NAPHTHYL]SILANE AND TRIS[2-((DIMETHYLAMINO)METHYL)PHENYL]FLUOROSILANE

The title compounds exhibit an imperfect C3 geometry: in addition to the four sigma-bonds (Si-H or Si-F and three Si-C), three weak donating Si<--N interactions ranging from 2.87 to 3.47(1) angstrom are also observed. The overall tetrahedral environment of the silicon (or germanium) atom is retained, and the structure can be described as a ''tricapped tetrahedron''. In most cases, and despite the large distances encountered, the nitrogen lone pair is directed toward the silicon atom opposite to an aryl-silicon bond. In the case of the fluorosilane no dative bonding opposite to the fluorine atom is observed, and the approach of the three amino groups occurs exclusively on the frontal faces of the Si-F bond. Steric hindrance appears as the critical factor preventing closer approach of the three nitrogen lone pairs to the silicon atom.