Carbon-silicon bonds lengths

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

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... 

There have now been four experimental determinations of a silicon-carbon double bond length. The first of these was a gas phase electron diffraction study of 1,1-dimethylsilene (173). This study was the subject of much controversy since the experimentally determined bond length, 1.83 A, was much longer than the one predicted by ab initio calculations (1.69-1.71 A, see below) (159). Since the calculations were carried out at a relatively high level of theory and the effects of electron correlation on determining the Si=C bond length were considered, the validity of the data extracted from the electron diffraction study is in serious doubt. 

The second determination of a silicon-carbon double bond length came from the X-ray crystal structure of l,l-bis(trimethylsilyl)-2-(trimethyl-siloxy)-2-(l-adamantyl)-l-silaethene (1) (122). Again, the experimentally... 

In the same group (IVA) as carbon, silicon is very much like carbon in atomic structure. It forms silicon-silicon covalent bonds, but since silicon is over double the size of carbon, the silicon bond lengths are longer and weaker. It is like a bridge between two river banks. The bridge across a 4 meter (12 foot) wide stream will be much stronger and more stable than one across an 8 meter (28 foot) stream, when the middle is not supported. 

The first 4-silatriafulvene derivative 94 (R = Me) was obtained by Kira s group as a reactive intermediate using the sila-Peterson reaction [52]. By use of the bulky tert-butyldimethylsilyl groups rather than trimethylsilyl (TMS) substituents in 94, the first stable 4-silatriafulvene 95 was synthesized by the same group (Scheme 6.22). An X-ray analysis reveals that 95 has an almost planar skeleton with bond alternation all skeletal carbon and silicon atoms are located almost in a plane and the silicon-carbon double bond length is 1.755 A, which is close to that of tert-butyldimethylsilyl-(trimethylsUyl)adamantylidenesilane 96 (1.741 A) [53]. 

In Figure 3b and c the absolute atomic concentrations of carbon and silicon, respectively, are shown as a function of the carbon fraction. As expected, the carbon concentration increases upon alloying. In contrast, the silicon content decreases rapidly, which implies that the material becomes less dense. As it was reported that the Si—Si bond length does not change upon carbon alloying [116], it thus... 

The stable silene Me2Si=C(SiMe3(SiMe(r-Bu)2) was first reported as a stable complex with THF,34 and its crystal structure showed the length of the silicon-carbon double bond as 1.747 A. Subsequently, it was possible to remove the THF and isolate the uncomplexed silene, which had a noticeably shorter Si=C bond length of 1.702 A.29 Further investigation showed that stable complexes of this or closely related silenes with trimethyl- or ethyldimethylamine, pyridine, and fluoride ion were also readily formed and moderately stable.31141... 

Fig. l.The large radius of the silicon 3p orbital, compared with that of the carbon 2p orbital, is the origin of the extended Si=Si bond length, compared with that of C=C. 

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