High-coordination-number silicon examples

Beyond any application purposes, silicon compounds with high coordination numbers ( hypervalent compounds) are of general interest and examples with four-membered rings, as are formed by chelating amidinate ligands, are almost unknown. Finally, with the aid of these ligands, low-valent complexes of these elements (Si°, Ge ) seem accessible. 

An increase in pressure generally leads to a decrease in the volume of the unit ceU and an increase in coordination number. In addition to the examples already given, we should also mention the high pressure form of SiOa (stishovite), where silicon has a coordination number of 6, as opposed to its normal coordination number of 4. 

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal-planar coordinated positively charged silicon atom in the cation created a series of stabilized sUyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacify the high electron demand of the silyl cation. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (Scheme 2) [9]. Examples for siliconium ions 8 in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules are known but are less frequently observed. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10 for both modes of stabilization have been structurally characterized [9, 28, 29]. 

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