Crystallographic evidence for a free silylium ion

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)(3)Si][H-CB11Me5Br6].C6H6 ( where Mes is 2,4,6-trimethylphenyl). Free (Mes)(3)Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The down field Si-29 nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in gas phase calculations, indicating that three-coordination can be preserved in all phases.