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Time resolved gas-phase kinetic studies have contributed a great deal of fundamental information about the reactions and reactivity of heavy carbenes (silylenes, germylenes and stannylenes) during the past two decades. In this article we trace the development of our understanding through the mechanistic themes of intermediate complexes, third body assisted associations, catalysed reactions, non-observed reactions and substituent effects. Ab initio (quantum chemical) calculations have substantially assisted mechanistic interpretation and are discussed where appropriate. Trends in reactivity are identified and some signposts to future studies are indicated. This review, although detailed, is not comprehensive.

Time-resolved studies of silylene, SiH2, generated by laser-flash photolysis of phenylsilane, have been employed to obtain rate constants for its bimolecular reactions with C2H2 and C2D2. The reactions have been studied in the gas-phase, in the pressure range 1-100 torr (with SF6 bath gas) at five temperatures in the range 291-613 K.Reaction with CzH2 is pressure dependent, consistent with a third body assisted association reaction. However the lack of a simple fit to RRKM calculated values suggests a more complex process with another reaction channel. Reaction with C2D2 is faster than with C2H2, showing a pressure dependent isotope effect. The data are consistent with a rapid isotopic scrambling mechanism. Further RRKM modeling of a three-channel decomposition mechanism for the suggested silirene adduct (intermediate) provides a semi-quantitative fit to the data. Rate constants extracted from the modeling are shown to be consistent with a mechanism leading to formation of both ethynylsilane and vinylsilylene, as previously proposed by O"ea1, Ring et al. from higher temperature studies. An enthalpy surface is shown to be consistent with this mechanism.

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