Tetravalent silicon compounds

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Tetravalent silicon compounds are able to form adducts with Lewis bases, depending on the acceptor strength of the silicon atom. Tetraorganosilanes show only very little... 

Intramolecular complexes of penta-coordinate silicon formed by only one five-membered ring tre listed in Table 6. For identical substituents, the axial bound distances are longer than the equatorial ones and both are pronouncedly elongated in comparison with distances in tetravalent silicon compounds. A change in the covalent bond lengths upon the formation of the cycle points to the tendency to acquire an aromatic character. 

Characterization of these extracoordinate compounds has depended mostly on elemental analysis and differences in IR and UV spectra compared with typical tetravalent silicon compounds. (124)... 

Si NMR offers a unique method for characterization of these complexes since their chemical shifts deviate greatly from the normal range found for most tetravalent silicon compounds. A number of different types of extracoordinate silicon complexes have been prepared and characterized (125) and their Si NMR chemical shifts determined. The chemical shift data are in Table XVIII. 

Summary The synthesis of the pentacoordinated silane (2-Me2NCH2C6Hi)-(CH=CH2)Si(H)2 (1) is described. A comparison of the chemical behavior of hypervalent 1 with tetravalent silicon compounds is carried out. 

The silicon catecholates 197 (Figure 29) are typical species in which silicon exhibits a coordination number of six (323). Characterization relies on elemental analysis and differences in IR, UV, and 2 9 Si NMR spectra compared with typical tetravalent silicon compounds (324, 325). The crystal structure of pyridinium tris(o-phenylenedioxy)siliconate, [C6H5NH]2+ [(C6H402)3Si]2 indicates hexacoordinate silicon dianions (326). A similar structure is assumed for 198 (325,327) (Figure 30). 

Bulky substituents at tetravalent silicon centers are the structural key element of sterically overcrowded compounds with extraordinary properties Inter- and intramolecular interactions of the bulky ligands can be studied in detail and allow an assessment of the ligands influence on structure and reactivity. 

The presently known silicon chemical shift range is 990 ppm. This includes the Dsd form of decamethylsilicocene 28 (5 Si = —423 (solid state)), which is the most shielded resonance reported to date and the alkyl-substituted silylene 45, which presently defines the high-frequency end of the spectrum at 5 Si = 567. Most silicon chemical shifts occur, however, in a much smaller range from 5 Si = +50 to —190. This includes hexa-, penta- and tetracoordinated silicon compounds and for trivalent, positively charged silicon a significant low-field shift compared to comparable tetravalent silicon species is expected. 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

This chapter will concentrate mainly on the chemistry of silyllithium compounds, i.e. neutral tetravalent silyl compounds bearing one or more lithium substituents at silicon. In addition, the chemistry of other organosilicon compounds containing alkaline and alkaline earth metals will be outlined in this chapter. 

TABLE 27. X-ray crystallographic and 29Si NMR data for hepta- and octacoordinate silicon complexes and tetravalent model compounds... 

Compounds 4.75 and 4.76 formally contain tetravalent silicon, which is not electron-deficient. It is, however, a strong Lewis acid, and on reaction with F , five-coordinate silicon compounds are obtained. In the case of 4.75, the fluoride anion is localised mainly on the boron atom, although it does display dynamic behaviour involving hopping between boron and silicon. Compound 4.76 as the KF adduct contains two five-coordinate silicon atoms that chelate the F anion. The coordination sphere of the K+ counter-ion is completed by a molecule of [18] crown-6 (Figure 4.32a). 

Synthesis by reduction of compounds with tetravalent silicon atoms 688... 

For l,3,2(A2)-diazasilacyclopentene 83, the following Raman lines at 650, 991, 1178, 1566, 1573 cm-1 were reported in the solid state. The intensive doublet at 1566 and 1573 cm-1 was assigned to the (C=C) stretch vibration. The observed bathochromic shift of this band in the spectra of compound 83 compared to heterocyclic compounds having a tetravalent silicon, for example, 93, and the significant intensity enhancement of this band indicates cyclic conjugation in molecule 83 <2000JST329, 2004JA4114>. 

Silicon in the lower oxidation states is discussed by Burger673. Silicon compounds in the divalent state also exist with formation of Si—Si bonds. In the carbon group the tendency to divalent state increases with higher atomic numbers while the tendency to tetravalence declines. Since silicon atoms are relatively small, the tetra valent state is strictly preferred. 

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