Silenes length

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... 

TABLE 12. Structural parameters of silenes and related compounds (bond length in A)... 

The calculated Si=C bond lengths in trans-bent silynes are distinctively shorter than Si=C bond lengths in silenes (i.e. 1.632 A and 1.714 A in HSi=CH and in H2Si=CH2, respectively, at MP2/6-31G )2970. A theoretical analysis in the framework of natural bond theory predicts for 668b and substituted derivatives bond orders for the SiC bond which are intermediate between double and triple bonds with considerable ionic contributions29711 0. 

The stable silathiirane 21 results from the silene and elemental sulphur on photolysis. It has an Si—S bond of 212.9 pm, slightly longer than that found in the silathiirane formed from Mes2Sil addition to tetramethyl-2-indanethione62. In addition, the cyclodisilthiane results and can also be made from the disilabicyclo[l.l.l] pentane (20) on hydrolysis. It possesses Si-S bonds of length 217-219 pm (Scheme 4)59. 

The intramolecularly donor-stabilized silenes 2a-d are thermally stable compounds. 2a, 2c and 2d are colorless solids 2b is an oil. X-ray structural analyses unambiguously revealed that in the solid state only one dimethylamino group of the ligand is coordinated to the silene silicon atom. Important structural parameters, e.g., the Si=C bond lengths and the Si-N distances, are in good agreement with data obtained for other intramolecularly amine-coordinated silenes [3]. 

A gas-phase electron diffraction study120 of pyrolytically produced 1,1-dimethylsilene yielded a Si-C bond length of 183 pm. Since this value is significantly longer than those from X-ray structure determination on more complicated stable silenes and also disagrees with quantum-chemical calculations, the interpretation of the electron diffraction data is most likely incorrect121-123. 

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