Silicon coordination number

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. 

Finally, a group of hypercoordinated silicon compounds, the decamethyl silicocenes, in which the formal silicon coordination number is ten, is worthy of mention in connection with this chapter. These analogs of ferrocene have been studied extensively232, and are described in detail in Jutzi s chapter in this volume. 

Theoretical Concepts of an Increase in the Silicon Coordination Number. . . Ill... 

Intermolecular coordinate interactions involving an expansion of the silicon coordination number were revealed by X-ray diffraction mainly for monosubstituted silanes of the HjSiX type, in whose molecules the central Si atom is sterically accessible. Since all the compounds of this type are gases or volatile liquids, their association was observed only at low temperatures. There are only few exceptions (Table 11). 

It is clear that NMR spectroscopy is a suitable analytical tool to ascertain the silicon coordination number in the solution state. 

The axial Si-F bonds in the fluorosilicate anions listed in Table 21 are between 0.040 and 0.080 A longer than the respective equatorial Si-F bonds. An increase in the silicon coordination number is associated with an elongation in the Si-X bond distances, as exemplified by a comparison of SiF4311 and [SiF5] The equatorial Si-F bonds in the pentacoordinated anion are respectively 0.023 and 0.046 A longer than the Si-F bonds in the tetrahedral molecule SiF4. 

Tasker classification, as, asymmetric surface Caj, surface calcium coordination number rel, relaxed surface energy Si, surface silicon coordination number unrel, unrelaxed surface energy. 

In spite of the fact that the Si—N, Si—F and Si—Cl bonds are reinforced, they (and also the Si—Br and Si—I bonds) can easily be changed into the Si—O bond in a solvolysis reaction. Hydrolysis or alcoholysis proceeds rapidly by an Sn2 reaction mechanism. A transition state is generated involving the vacant silicon d orbitals and thus increasing the silicon coordination number. The hydrolysis of the C—Br and C—I bonds, which are easily polarised, also proceeds by an 8 2 reaction mechanism, and is also rapid. For example, the hydrolysis constant K[CH3Br] [OH ] in an ethanol/water mixture (4 1) at 55°C is 0.0021 1-mol see [6]. 

So far, no crystallographic evidence for adducts of silanes with aldehydes, ketones, esters, or acyl halides has been reported [81]. Dimethylformamide [86-88] and tetramethylurea [89], however, are known to enter the silicon coordination sphere (e.g., in 13 and 14, respectively). In a similar manner amine-M-oxides (e.g., in 15) [90], phosphine oxides (e.g., in 16 and 17) [90,91], and phosphoric amides [92-94] form hypercoordinated Si complexes. Although dimethyl sulfoxide (DMSO) increases the silicon coordination number (as shown Si NMR spectroscopy) [49], crystallographic evidence for a siUcmi complex with DMSO Ugand(s) is still lacking [81]. 

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

As the silicon coordination number can be understood as the number of atoms located in proximity of the Si atom within the sum of the van der Waals radii and a large number of compounds without lone pair dmiation from atoms within this kind of coordination shell would fall into this section (such as those listed in Sect. 5.1), we focus on silicon compounds with five or six formal lone pair donors in the Si coordination sphere (Sects. 5.2-5.T). 

---