In this account, the synthesis, structure, and reactions of cyclic, spiro-conjugated, and cumulative silicon-silicon doubly-bonded compounds are described. Photochemical and thermal interconversion among Si4R 6 isomers including cyclotetrasilene 1, cyclotrisilene 2, and bicyclo[1.1.0]tetrasilane 7 occur without apparent participation of the corresponding tetrasila-1,3-diene, in contrast to the related interconversion among C4H6 isomers. Whereas parent spiropentadiene, the simplest spiroconjugation system, is known to survive only below -100 °C, spiropentasiladiene 3 is thermally very stable and shows remarkable spiroconjugation between the two ring π systems. The skeleton of isolable trisilaallene 4, the first stable silicon compound with tetracoordinate divalent (formally sp-hybridized) silicon atom, is not linear but remarkably bent. The considerable conjugation between two cumulative Si=Si double bonds was observed. The structural feature of 4 is quite different from that of carbon-based allenes having linear structure without conjugation between two π-bonds.
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