Silylene

Silylenes are compounds containing a divalent and dicoordinate silicon atom without any charge. They are considered to be heavier analogues of carbene. In earlier times, they were called "silene ," but this is mistake, as that term means compounds of π-bonded silicon: Si=C. The generic term analogous to carbene is "silicene."

Silylenes have been proposed as reaction intermediates and are so unstable that they usually cannot be isolated. Silylenes typically have a singlet ground state, though in carbenes the triplet state is generally more stable than the singlet state, because the energy gap between the 3s and 3p orbitals of the silicon atom is very large and so the singlet-triplet gaps are enormous.

History

The first experimental chemical observation of silylene as an intermediate was demonstrated by P. S. Skell and E. J. Goldstein in 1964.

The first stable silylene was synthesized and isolated as a diamino silylene, N,N’-di-tert-butyl-1,3-diaza-2-silacyclopent-4-en-2-ylidene, in 1994 by Michael Denk et al.

Synthesis

Silylenes are generally synthesized by thermolysis or photolysis of polysilanes, by silicon atom reactions (insertion , addition or abstraction ), by pyrolysis of silanes, or by reduction of 1,1-dihalosilane.

Properties

Simple silylenes are very reactive species and easily dimerize to give disilenes, so their lifetime is very short. Bulky substituents are used to prevent this dimerization and stabilize silylenes effectively for long-term survival in dilute solution and even in crystals.

The reactivities of silylenes are similar to those of carbenes, and insertion to polar bonds, addition to multiple bonds, and dimerization occur. But, in contrast to carbenes, insertion to C-H bonds and C-C bonds generally does not happen and complexation to Lewis base takes place.