Hexacoordinate silicon compounds cationic complexes

Relative to carbon, silicon has a much smaller tendency to form compounds of coordination number less than four, such as silenes, silanones or silicenium ions, but a pronounced capacity for the enlargement of the coordination sphere. Complex formation by silicon tetrafluoride was first observed1 early in the nineteenth century, when Gay-Lussac and Davy reported the formation of the adduct SiF4 2NH3. A hexacoordinate cationic complex, Si(acac)3 HCI2 was described by Dilthey2 in 1903, and represented a new structural type for the element. 

In contrast to the well-established chemistry of anionic and cationic hexacoor-dinate silicon(IV) complexes with SiOe skeletons, neutral hexacoordinate silicon(IV) species with SiOe frameworks are significantly less well explored. Compounds and 60 are examples of this type of compound. 

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

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