Hexacoordinate silicon compounds

The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

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 main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

Pathways B and C describe the process that could occur to form the symmetrical silyl ketal. Formation of silyl ketal-metal complex VII could occur in one step via an associated pathway B, or in two steps via a dissociative pathway C. Pathway B could occur by attack on the pentacoordinated silyl ketal-metal complex IV by alcohol ROH, forcing the dissociation R OH leading to complex VII. The associative pathway would precede through a hexacoordinated silicon species. Hexacoordinated silicon compounds have been shown to be very reactive.36,23d The choice of the alcohol that dissociates would be determined by electronic and/or steric factors. 

Molecular and Electronic Structure of Penta-and Hexacoordinate Silicon Compounds... 

Table 2. Calculated Bond Distances (pm) for Tetra-, Penta-, and Hexacoordinate Silicon Compounds...

Molecular and Electronic Structure of Penta- and Hexacoordinate Silicon Compounds Table 3. Calculated Mulliken Charges on the Atoms of Simple Silicon Complexes... 

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