Silicon complexes coordination geometry

Compared to the sum of covalent radii, metal-silicon single bonds are significantly shortened. This phenomenon is explained by a partial multiple bonding between the metal and silicon [62]. A comparison of several metal complexes throughout the periodic table shows that the largest effects occur with the heaviest metals. However, conclusions drawn concerning the thermodynamic stability of the respective M —Si bonds should be considered with some reservation [146], since in most cases the compared metals show neither the same coordination geometries nor the same oxidation states. 

Fig. 6. Coordination geometry at silicon structures of the complexes 4 (left) and 5 (right)...

Figure 3.1 Illustration of the linear coordination geometry in the monomeric complex [(Cf,HiMes2-2,6)Be N(SiMe ,)2/]. The beryllium, nitrogen and silicon atoms are shown as black spheres and carbon atoms are white...

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

Summary Binuclear hexacoordinate silicon complexes with two N-+Si dative bonds (per Si atom) have been prepared. The crystal stracture conforms to a distorted octahedron, with the N— Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one Si Cl bond in CD2CI2 and CDCI3 solution, to form a binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N- Si bonds are among the shortest coordination bonds of this kind. 

A few examples have been reported in which presumably silicon has a coordination number greater than six . All of the hepta- and octa-coordinated silicon complexes for which single-crystal X-ray diffraction structures were obtained have a basic tetrahedral geometry, with three or four distantly coordinated ligand groups positioned against faces of the tetrahedron, in analogy to the bicapped tetrahedron structure mentioned earlier (Section VI.A.2). 

Finally, an example of an x-ray structure of a cationic complex shall be mentioned. From the data for 12, a surprisingly weak coordination (Si —N 1.932(8) A [146, 147]) of the acetonitrile donor to the silicon is inferred. The deviation from a pure tetrahedral geometry at the silicon is the largest yet observed (Table 4). 

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