Silicon bond order

A short digression is in order at this juncture. The electrophilic substitution of a vinylsilane is a very useful process because it is generally both regio- and stereospecific. The carbon-silicon bond is strongly polarized due to the high electronegativity of carbon (2.35) relative to silicon (1.64).67b A unique and important conse-... 

Silicon, in order to bond four hydrogen atoms to itself, must exhibit sp3 hybridization. 

Consequently, the bond is fully saturated for A sp = 0 with a bond order of 1, but it is only partially saturated by the time the gap closes for AEap/2 h = 1 (cf eqn (7.92)) when the bond order equals 0.76. This simple second moment model has been extended to include the compound semiconductors. The resultant values of the bond order are given in Table 7.2. We see that the bonds in tetrahedral carbon and silicon are almost fully saturated, but those in zinc selenide and cadmium telluride are only about 75% saturated due partly to the mismatch in the sp orbitals between chemically distinct atoms. 

Direct reaction of iron pentacarbonyl with trimethylsilyl isocyanide ( C=N—SiMe3) at 65°-75° yields an air-sensitive substitution product Me3Si—N=C Fe(CO)4 in 93% yield, with elimination of carbon monoxide (152). It was shown by infrared spectroscopy (38) that complex formation lowers the N=C bond order for Me3Si—N=C , whereas it raises the N=C bond order for Me3C—N=C , presumably as a result of interaction between dv orbitals of silicon with the metal d orbitals. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

An alternative SN2-Si mechanism may be considered, without a penta-coordinate intermediate. In this alternative, the silicon is rehybridized from spy to sp2. If bond order is conserved, then it is reasonable to ascribe a bond order of 0.5 to the Si—O bond of both the entering nucleophile and the leaving group [52], It has been shown that the bond order is related to the bond distance [45]. Even though there are five substituents in the vicinity of the silicon in an SN2-Si type mechanism, two of the substituents are significantly further away from the central atom (silicon) than the other three substituents. A looser transition state structure than that for an SN2 -Si or an SN2 -Si process results. The steric effects of the alkyl group bonded to silicon should, therefore, be considerably less in an SN2-Si type mechanism. 

Hyperconjugation should raise the bond order between the ipso and benzylic carbons, and lower the bond order between the benzylic carbon and the silicon atom. As the ipso-benzylic bond length decreases, the 13C-13C coupling constant should increase, and as the benzylic carbon-silicon bond length increases, the 13C-29Si coupling constant should... 

Silicon-silicon multiple bonds with a bond order greater than two are still unknown. According to theoretical calculations, voluminous and concomitantly electropositive substituents are the most promising candidates with the potential to stabilize disilynes having a formal Si Si triple bond. When and by what synthetic route this challenge for experimental chemistry will be realized is a question that only time can answer. 

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