Stable Pentacoordinate Siliconium Complexes

We make a distinction between two types of ionic pentacoordinate complexes those which are in dynamic equilibrium with neutral hexacoordinate complexes have been dealt with in Sections III.A.4, III.A.5.ii, and III.B.2. The second group includes those pentacoordinate siliconium-ion salts which are formed as such and are stable and do not equilibrate (to a noticeable extent) with their hypothetical neutral hexacoordinate counterparts. The present section discusses this group of persistent salts of pentacoordinate silicon cations. 

Free tricoordinate silicon cations (silicenium) are exceedingly unstable, and have only recently been first realized.74,75 A silicenium ion can be stabilized by coordination with two intramolecular donors, to form a stable (pentacoordinate) siliconium complex. These have been reported with 

N Si coordination (76, 7776 78a77 ) and most recently also with oxygen (78b,78 7 979), sulfur (78c),78 and phosphorus (78d)78 donor-ligands. The present section describes the chemistry of donor-stabilized silyl cations with hydrazido chelates. 

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