Bicapped tetrahedron distortion

The final distinctive pair of rforhitals that can be added to an i/ - manifold for a six-coordinate polyhedron is the (z, j z) pair corresponding to a bicapped tetrahedron (Figure 1) similar to that found in the metal cluster Osfi(CO) g. This polyhedron is not favorable for a coordination polyhedron since the two capping vertices are located further from the central metal atom than the remaining four vertices. However, the distortion of a regular H2M(CO)2L2 octahedron toward an M(CO)2L2 tetrahedron in metal carbonyl dihydride derivatives of the type H2Fe(CO>2L2 fL = and PPh(OEt)2 ] is related to bicapped tetrahedral stereochemistry. 

Disubstitution of a silicon center by the 2-dimethylaminomethyl-4,6-dimethylphenyl substituent may result in hexacoordinated silicon compounds. This is manifested in the X-ray structure of difluorosilane 4, which shows a weak Si-N interaction of both amino side chains resulting in Si-N distances of 282 pm and 287 pm (Fig. 4). Both fluorine substituents occupy positions trans to the attacking nitrogen donor atoms, thereby forming N-Si-F angles of 173° and 172°. Thus, the silicon center is weakly hexacoordinated showing a coordination sphere, that may be described as a bicapped distorted tetrahedron [4]. 

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