Silylium ion

A silylium ion is a reactive silyl-containing cation with the formula SiR+3.

Early efforts to generate these cations produced salts of the pyridine complex [(CH3)3Si-NC5H5]+, the hydride-bridged species [(Et3Si)2H]+, and the toluene complex [(mes)3Si(toluene)]+.

These cations are related to trityl (C(C6H5)+3), with the extra methyl groups providing steric protection, compensating for the greater size of Si vs C. Its 29Si NMR chemical shift is 225.5 ppm, downfield of TMS, which indicates that the cation is quite "naked".

Trimethylsilyl trifluoromethanesulfonate (Me3SiOTf), normally considered a source of electrophilic silicon, has a 29Si NMR shift of 43 ppm.

Weakly coordinating anions are essential for the isolation of these highly electrophilic cations.