High-coordination-number silicon compounds

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. 

Compounds with High Coordination Numbers at Silicon Models for the Investigation of the Nucleophilic Substitution Reaction at Silicon Centers... 

The C vs P reactivity of phosphinomethanides towards silicon centers may be turned by the substitution pattern of both, the phosphinomethanide and the chlorosilane. Novel heterocycles, skeleton rearrangements and high coordinations numbers may be achieved. 9 and 14 potentially be precursors for a specific type of unsaturated silicon compounds, adding to the growing class of compoimds at the interface between organophosphorus and organosilicon chemistry. 

The majority of the NMR data reported involve derivatives of Si(IV). The most deshielded silicon (+268.7 ppm) has been reported for [2], the most shielded for Si (-351.7 ppm), but the majority of shifts are found between -200 and 150 ppm. As with the other heavier nuclei, the chemical shift depends primarily on the coordination number of the silicon in such a way that a low number of substituents around the silicon leads to deshielding (e.g. disilylenes 13 to 175 ppm) and a high coordination number gives high negative numbers (e.g. sixfold coordination -198 to -135 ppm). The regions of chemical shifts for some important classes of silicon compounds are depicted in Figure 2. 

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

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