Si—X bonds

The properties and applications of commercially important hydride functional silanes, ie, compounds having a Si—H bond halosilanes, ie, compounds having a Si—X bond and organosilanes, ie, compounds having a Si—C bond, are discussed hereia. Compounds having Si—OSi bonds are called sdoxanes or sihcones. Those having a Si—OR bond are called siUcon esters. Sdoxanes and siUcon esters are discussed elsewhere ia the Eniyclopedia (see Silicon COMPOUNDS, SILICON ESTERS SILICON COMPOUNDS, SILICONES). 

With regard to the stabilizing effect of the a-substituent at the silicon, the following gradation can be inferred from results of x-ray structures O > S > C > Cl. This sequence correlates with known Si-X bond energies. 

Fig. 9. Dependence of the relative lengthening of the Si-X bond in 87 in comparison with organosilanes X-SiRs on the halogen X. (Reproduced from Ref 174, with permission from The Royal Society of...

Shortened M-Si bonds Long Si-X bonds Si-H contacts of 1.8-2.1A Small Si-M-Si angles in bis(silyl)hydride systems X and H substituents in the apical positions in respect to silicon atom Elongated M-H bonds Elongated M-Si bonds Normal Si-X bonds Si-H contacts of 1.7-1.8 A No regulations No regulations Normal M-H bonds... 

Furthermore, a quasicyclic SNi—Si mechanism (/, 2) (Scheme 1) cannot be invoked to explain the retention of configuration. The use of a basic solvent or of a cryptand leads to a shift of the stereochemistry to retention (7, 8) and to a parallel increase of the substitution rate (9). These data are in complete opposition to the SNi—Si process. Because the controlling force of such a mechanism is electrophilic assistance to the cleavage of the Si—X bond, the use of a more basic solvent or of a cryptand would shift the stereochemistry toward inversion. 

A satisfactory explanation also cannot be made in terms of polarizability of the leaving group. The highly polarizable Si—Cl and Si—SR bonds are always displaced with inversion. However, the Si—F bond, which is far less polarizable than Si—OR and Si—H bonds, is more easily cleaved by nucleophiles with inversion of configuration. It is also impossible to explain the closely similar behavior of F and SR groups. However, the relative tendency of Si—X bonds to undergo inversion and the polarizability order run parallel in the same group of the periodic table. The order of polarizability and ability for inversion are... 

The tendency of halosilanes to be racemized by nucleophiles (HMPA, etc.) is parallel to the tendency of the Si-X bonds to be displaced... 

Inversion is observed mainly with Si—X bonds which are able to be stretched under the influence of an attacking nucleophile. For instance, the X-ray crystal structure analysis of the five-membered chloro-(A-chlorodimethylsilylacetamido)methyldimethylsilane (4) shows the chlor-... 

This observation, that the aptitude of X to be displaced with inversion, is parallel to the tendency of the Si—X bond to be stretched under the influence of an attacking nucleophile, does not agree with the apicophilic-ity order stated from dynamic NMR studies on stable pentacoordinate phosphoranes (42), which is as follows ... 

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

This order is parallel to the ability of the Si—X bond to be displaced with inversion... 

Salem s frontier-orbital treatment is consistent with the fact that retention of configuration is a commonly observed stereochemical outcome at silicon (Tables I and II), whereas there is still no proven example of an Sx2 reaction with retention at carbon (69). Because Si—X bonds are significantly longer than C—X bonds, the unfavorable interaction between X and the nucleophile for front-side RN attack is less for silicon than for carbon (Scheme 9). The valence orbitals also change from 2s and 2p for carbon to 3s and 3p for silicon and therefore become more diffuse and capable of better overlap with the nucleophile at longer distances. Consequently, the probability of attack with retention is enhanced. 

At the same time, the Si—X bond shortens, increasing the unfavorable X—Nu overlap in an attack with retention (Scheme 9). 

The two former factors favor a front-side attack leading to the retention. Moreover, numerical calculations indicate that in this example, the Si—X bond shortening does not compensate for the two former effects. Therefore, retention is favored. 

This frontier-orbital approximation does not give a complete picture of the influence of the leaving group on the stereochemistry. For instance, it does not account for the aptitude of the Si—X bond to be stretched under... 

Because of the cage structure of the above bicyclic systems, only frontside attack at silicon is possible. Thus an increase of the reaction rate would be observed for the Si—X bonds that have a high degree of s character. Our analysis is as follows ... 

The formation of Si-X bonds was confirmed by measurements of XPS spectra and flat band potentials. For (100) surface, back-bond oxidation occurs simultaneously with the Si-X formation under the same conditions, though it is negligible for Si fill). The mechanism of the surface reactions can be explained by hole injection by an oxidant, followed by the nucleophilic attack of halide ions or water molecules.32,33) For dihydride (=Si H2) bonds on H-Si (100), for example, the reactions at the initial stage can be expressed as follows ... 

All Si—X bond lengths obtained by X-ray crystal structures of the organosilicon compounds discussed in this review are summarized in Table 32. 

An important feature of the (p-d) jr-bonding model is the lack of conformational requirements. The degree of (p-d) tt overlap is constant, regardless of rotation around the Si—X bond, as a consequence of the symmetry of the five 3d orbitals on silicon. 

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