Coordinative interactions in chelated complexes of silicon: XII. Mapping the expansion of coordination of silicon from 4 to 5. Crystal structure of 1,2,3,4-tetrahydro-1-trimethylsilyl-1,10-phenanthroline

Determination of the crystal structure of 1,2,3,4-tetrahydro-1-trimethylsilyl-1,10-phenanthroline has shown that coordination geometry at silicon may be regarded as a strongly distorted trigonal bipyramid or as a distorted tetrahedron, depending on where the boundary between bonded and non-bonded interactions is fixed. An intramolecular Si…N Lewis acid base interaction (2.689(8) Å) is present; the Si…N distance is 54% longer than the SiN “single bond” (1.746(8) Å) in the molecule. The structural details for the title compound and other pentacoordinated silicon complexes are used as a base for mapping the expansion of coordination at silicon from four to five. The correlation diagrams are interpreted in terms of geometrical transformations along the S_N2 inversion pathway. They provide a possible model for the molecular motions of dynamic rearrangements involving an intermediate alteration of coordination number (5 → 4 å 5) at Si in chelated complexes of pentacoordinate silicon.