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Silicon and carbon belong to group IVB of the periodic table, and both have the same constitution of valence shell electrons:C ls22s22p2, Si:ls22s22p63s23p2. Most compounds of silicon have a formal valence number of four due to the transition of one 3s electron to a 3p orbital. Both silicon and carbon readily form single bonds resulting from sp3 orbital overlap. However, despite the great abundance of stable multiple-bonded compounds of pr-pT type for carbon, there has been a complete absence of them for silicon, although silicon does have d orbitals available for pT-d"-bonding.1 <2 Until recently silicon analogs of unsaturated compounds were believed to be nonexistent.1 In this Account we describe the evidence for generation of this new class of organosilicon compounds and discuss their synthetic utility. Our main emphasis is on compounds of silicon which are analogous to olefins and ketones.3 This means that formally, at least, their structure can be described in terms of -bonded silicon. At present there are no data unequivocally indicating this type of chemical bond with silicon: alternative structures may also be correct.R2Si-CH2 -*-* R2Si=CH2R2Si-Ch2We do not consider such 7r-bonded intermediates as :SiO and :SiS, which are discussed in the fairly recent review4 on divalent radicals of group IVB.The first structures with silicon double bonds were assigned to compounds obtained in reactions of ammonia with silicon tetrachloride, trichlorosilane,5 etc. (HN=Si=NH; N=SiNH2). In 1912 Schlenk and Renning6 described the synthesis of a stable compound with a silicon-carbon double bond by dehydration of diphenylmethylsilanol. However, their work was reexamined by Kipping,7 who proved that they had been dealing with a mixture of diphenyldimethylsilane, diphenylmethylsilanol and diphenyl. By the end of the second decade of this century, it had become clear that earlier works in which structures with multiple bonds to silicon were assigned to stable compounds were wrong and reactions which lead to formation of double bonds with carbon are Leonid E. Gusel'nikov was born in Novosibirsk in 1939, graduated from Gubkin Institute for Petrochemical and Gas Industry, and worked for his Ph.D. degree under the supervision of Professor N. S. Nametkin. In 1967 he was on a year's postdoctoral fellowship at Southampton University. His research interests center around reactions of reactive intermediates of group IVB elements.

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