Silylium cations - free at last

Carbocations (carbenium ions) are important intermediates in organic chemistry, and have been intensely studied since as long ago as 1900. The analogy between carbon and silicon is often stressed, but the isolation of a silylium ion, RaSi, took much longer than for carbon chemistry, although as silicon is more electropositive than carbon it should form cations more easily. But it was not until 1986 that Lambert first reported the synthesis of a free silylium cation, by the following reaction [52]  

PhaSiH + [PhaC]+[C104] [Ph3Si]+[C104] + PhaCH 

The resulting solution was found to conduct an electric current. A log-log plot of molar conductance against concentration was used to deduce whether the species formed existed as free ions or an ion pair. When 

These conclusions were questioned [54]. There had been inconsistencies in earlier reports of covalent silyl perchlorates, and it was also suggested that the new eonductivity data could be attributed to traces of water in incompletely dried solvents, which would lead to formation of perchloric acid. Unambiguous proof of the existence of a silylium ion was said to require the observation of a Si resonance that was shifted significantly to high frequency. 

With the NMR evidence pointing firmly in the direction of a covalent molecule, several authors repeated the conductivity experiments under very carefully controlled conditions. The apparent contradiction was resolved when it was shown that donor solvents can liberate [C104] by replacing them in the coordination sphere of silicon, such that 

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