Carbon-silicon double bonds lengths

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... 

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]. 

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... 

The Si=C bond lengths in the recently synthesized novel stable 1-silaallenes 571s and 572259 are relatively short (1.704 A and 1.693 A in 571 and 572, respectively). This might be expected for a Si=C double bond formed from a sp-hybridized carbon and a sp2-hybridized silicon. Both 1-silaallenes are slightly bent at the central carbon [wiSiC C2) 173.5° and 172.0° in 571 and 572, respectively]5,259 and 571 is trans bent about the Si=C1 bond in agreement with theoretical predictions264. 

The crystal structure shows the expected E geometry of the double bond. The length of the silicon-potassium bond (3.45 A) lies within the range observed for similar anionic compounds [3]. The length of the carbon-carbon olefinic bond is not significantly affected by the adjacent anionic silicon atom, in contrast to the spectroscopic behavior of this bond (see below). 

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