High-coordination-number silicon

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. 

A fairly large number of silyl-substituted phosphines of type B has been prepared [16], but only in a few cases a (not necessarily intramolecular) phosphorus silicon donor-acceptor interaction has been diagnosed [17], Using specifically the or///o-metallated benzylphosphines C and D as ligands, we investigated whether these anionic phosphines are able to enforce high coordination numbers at Si by (preferrably... 

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. 

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. 

It can be readily confirmed thaf by decreases as the number of bonds N increases and/or llieir length (r ) decreases. This relationship between the bond strength and the number of neighbours provides a useful way to rationalise the structure of solids. Thus the high coordination of metals suggests that it is more effective for them to form more bonds, even though each individual bond is weakened as a consequence. Materials such as silicon achieve the balance for an infermediate number of neighbours and molecular solids have the smallest atomic coordination numbers. 

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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