Tetracoordinate silicon compounds

The crystals of both compounds contain pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 88 and 89 are listed in Table XIV. Similar crystal structures were observed for the corresponding oxygen analogs 53 and 77, respectively (see Sections III,D and III, E). As can be seen from the Si-O [1.7600(14)-1.815(4) A], Si-S [2.144(2)-2.1638(9) A], and Si-C distances [1.900(5)-1.906(2) A], the Si02S2C frameworks of 88 and 89 are best described as being built up by five normal covalent bonds rather than a bonding system in the sense of a 4+1 coordination. The Si-S distances are very similar to those observed for tetracoordinate silicon compounds with Si-S bonds. 

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. 

TABLE 24. Comparison of 29Si NMR data for hexa- and tetracoordinate silicon compounds ... 

The Si—N(CS) bond of tetracoordinate silicon compounds is very sensitive towards alcoholysis324. By contrast, 1-isothiocyanatosilatrane (29) reacts with methanol only in the presence of a strong base such as biguanidine (B) to afford 1-methoxysilatrane (65) (equation 94)68. 

The reaction of benzo[A]-l,3-diazasilole 85 with lithium alkyls yields the insertion product 112. It was suggested that the initial step of this reaction is the formation of the donor-acceptor complex 113 (Scheme 10) <2002JOM272, 2002JOM150>. The tetracoordinated silicon compounds 112 might have synthetic potential as silylene transfer reagents. 

Section 7.16 Atoms other than carbon can be chirality centers. Examples include those based on tetracoordinate silicon and tricoordinate sulfur as the chirality center. In principle, tricoordinate nitrogen can be a chirality center in compounds of the type N(x, y, z), where x, y, and z are different, but inversion of the nitrogen pyramid is so fast that racernization occurs vit -tually instantly at room temperature. 

The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

Si—Si bonds in compounds with tetracoordinate silicon atoms. 197... 

Most of the organosilicon compounds contain bonds between the silicon and carbon atom. In the following paragraph the structural chemistry of the Si—C single bond is discussed, mostly in compounds with tetracoordinate silicon and tetracoordinate carbon atoms. The structural chemistry of the Si—C bond in compounds where the carbon coordination state is different, is also discussed. The Si—C bond is markedly polarized and the increase of the bond ionicity by attaching different substituents to either the silicon or the carbon atoms may affect its length. The electronic and steric effects are discussed later. 

FIGURE 11. Histogram of Si—N bond lengths in compounds with tetracoordinate silicon and dicoordinate nitrogen atoms... 

FIGURE 13. Elistogram of the Si—P bond lengths in compounds containing tetracoordinate silicon and tricoordinate phosphorus atoms... 

The 3276 bond lengths obtained by XRD have been used to calculate the average Si—O bond length in compounds containing tetracoordinate silicon atom bonded to a dicoordinate oxygen atom. The average Si—O bond length is 1.629 A (s.d. 0.03 A, s.m. 

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