Comparative chemistry of unsaturated compounds of Group 14 elements *

Structures and reactions of a stable dialkylsilylene and various cyclic and acyclic disilenes, which we have recently synthesized, are discussed in detail. The stable silylene was found to be well protected sterically from dimerization but least perturbed electronically as evidenced by UV–vis and NMR spectroscopy. Persilylated acyclic and cyclic disilenes have shown interesting features in the Si=Si bond distances, electronic spectra, and reactions, due to the remarkable electronic and steric effects of trialkylsilyl substituents. Detailed theoretical calculations for the addition of water to disilene has shown that the intramolecular antiaddition pathway is rationalized using the orbital symmetry rule. The reactions of disilenes with haloalkanes were judged to proceed via the rate-determining halogen abstraction of disilenes from haloalkanes. The biradical character of disilenes may be related to the small π bond energy.