Bonding in compounds of Groups IV and

Pyrolysis of dimethylsilacyclobutane at 600°C gives a mixture of ethene and dimethylsilaethene. The latter species has been detected in the gas phase by mass spectrometry. It has also been trapped in a solid argon matrix at lOK and its infrared spectrum measured. It is very reactive, dimerizing readily and adding to dienes in a Diels-Alder reaction  

The introduction of very bulky substituents hinders dimerization and in the following example also prevents intermolecular rearrangement of small Sij rings to larger oligomers (Ar = 2, 4, 6-trimethyl-phenyl). 

The bulkier the substituents the more stable the silene becomes. While Me2Si= SiMe2 has been observed only in a frozen matrix at low temperatures, Bu2Si= SiBu2 persists for a few hours under ambient conditions. Ar2Si=SiAr2 and 

Bu ArSi=SiArBu are crystalline materials whose structures have been determined by X-ray diffraction (Ar = 2,4,6-trimethylphenyl). The latter has a planar 281=8102 skeleton with a 8i=8i bond distance of 2.14 A. This bond length may be compared with a C=C distance of 1.33 A in trans-1,2-diphenylethene and a 8i=C distance of 1.70 A in Me28i=C(8iMe3)8iMeBu2 8ilabenzene and silatoluene have been obtained as highly reactive intermediates from dehydrochlorination of silacyclohexadienes. They can be trapped as Diels-Alder adducts. 

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