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 t he 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 ob served. The overall tetrahedral environment of the silicon (or germani um) atom is retained, and the structure can be described as a ''tricap ped tetrahedron''. In most cases, and despite the large distances enco untered, the nitrogen lone pair is directed toward the silicon atom op posite to an aryl-silicon bond. In the case of the fluorosilane no dat ive bonding opposite to the fluorine atom is observed, and the approac h of the three amino groups occurs exclusively on the frontal faces of the Si-F bond. Steric hindrance appears as the critical factor preven ting closer approach of the three nitrogen lone pairs to the silicon a tom.