Tricapped tetrahedron structures

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. 

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. 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

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. 

Fig. 2.27. Regular deltahedra often found in molecular structures of boranes and carboranes. (a) Tetrahedron (b) trigonal bipyramid (c) octahedron (d) pentagonal bipyramid (e) dodecahedron (f) tricapped trigonal prism (g) bicapped archimedean antiprism (h) octahedron (i) icosahedron...

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