Silicenium ions solution studies

The experimental search for free stable silicenium ions in solution seems to have finally succeeded, at least for triaryl-substituted species4,5. However, computational studies will continue to augment, to verify and to guide the interpretations of experimental findings, as well as to suggest new experiments for preparing other silicenium ions. Schleyer and coworkers, for instance, have recently explored a silicenium ion within a cage , namely... 

Running in parallel with the numerous experimental attempts to prepare silicenium ions in recent years, there has been a considerable number of computational studies carried out on them. The computational work is described in detail in the chapter by Schleyer and Maerker and this chapter will describe experimental work, concentrating mainly on solution and solid state experimental work with only occasional reference to calculations when necessary. 

Molecules with trivalent positive silicon (R3Si+) are termed either silicenium ions (in analogy to carbenium ions) or silylenium ions (derived from silylene as carbenium is derived from carbene). Silicenium ions have been known and studied in the gas phase for over 20 years302. In solution, silicenium ions have been sought for more than 40 years258... 

Lambert and his coworkers have made similar claims earlier, based on insufficient evidence in solution . Lambert s 1993 study, for example, showed an organosilyl moiety with no coordination to the anion, but with a toluene molecule close nearby (R(Si—C) = 2.18 A). Lambert and coworkers claimed that in 2 covalent bonding is weak or absent i.e. there is no fourth coordination between the silicon atom and toluene. The elongated Si—distance in 2 led Lambert and his coworkers to the stable silicenium ion description. The X-ray structure of 3, reported by Reed and coworkers, showed a Si+ -Br distance of 2.48 A, 0.24 A longer than a normal Si—Br single bond . 

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