Trialkylsilylium Ions in Aromatic Solvents

This description seemed to be confirmed just months later when Lambert and co-workers succeeded in crystallizing Et3Si+TPFPB in toluene. [8] The X-ray diffraction investigation showed that counterion TPFPB is well separated from Si+ (closest contact distance 4.18 A) thus excluding any coordination between silylium cation and the anion. However, the structure determination also revealed that the unit cell contains eight toluene molecules, one of which has a contact distance of 2.18 A to the silylium cation. [8] Lambert and co-workers 

Reed and co-workers published X-ray crystal structures of tris(f-butyl)silylium and tris(i-propyl)silylium ions, for which the counterion Br6CBnFl6 was found to be located at Si-Br distances of 2.47 and 2.48 A, respectively. [15] Schleyer and coworkers showed with the help of ab initio calculations that the Br atom of the counterion interacts so strongly with the positive Si atom in this situation that little silylium ion character remains. [6] All these results suggested that Lambert and Reed had too little information from their experimental studies to draw 

Comparison of calculated properties of complexes R3Si(S)+ (S = solvent). a 

Complex Cation Solvent 50 H3Si+ (1) benzene 51 Me3Si+ (4) benzene 52 Et3Si+ (58) toluene 53 (Me3Si)3Si+ (59) benzene 54 (9-BBN)3Si+ (60) benzene 55 H3Si+ (1) methane 56 H3Si+ (1) He 57 Me3Sl+ (4) He 

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