Silicon complexes heptacoordinate

A different synthesis of a heptacoordinate silicon complex (200) is shown in equation 52. The resulting complex had the same basic tricapped-tetrahedron structure (Figure 29)228. 

Formal octacoordination ( 4 + 4 )229 in a silicon complex was recently demonstrated by Corriu s group, who prepared 201 (equation 53). The Si-N distances (Table 27) are long relative to those common in penta- and hexacoordinate compounds, but are in the same range as those found in the heptacoordinate compounds discussed here. Also, the geometry resembles the heptacoordinate complexes, in that it is basically a tetrahedron with dimethylamino donor groups pointing toward the center, in what may be termed a tetracapped tetrahedron . 

Going back to mechanistic studies it is not clear if the reactions of nucleophiles with hexacoordinated silicon compounds are pure nucleophilic substitutions or if they take a more complex route. However there is another challenge to find whether the silicon atom can accept being in heptacoordination. Such a possible situation has been observed with a tricapped tetrahedron structure of a silane which has been proved to be isosteric with the corresponding germane of which the X-ray structure determination has been carried out. 

The first reported synthesis of a heptacoordinate complex of silicon (197) was accomplished by the reaction of IlSiCh with 2-lithio-(dimethylaminomethyl)-benzene227. The crystal structure was determined later and confirmed the tricapped tetrahedron geometry81,82. Replacement of the hydrido ligand by chloro (198) led to the formation of a tetracoordinate silane without any dative bonds, apparently due to the steric bulk associated with the chloro ligand81. 

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