Silyl Substituted Silylium Ions in Solution

The measured 29Si NMR chemical shift of 59 in benzene solution is 111 ppm, which was considered to indicate strong silylium ion character of 59 in solution. [7,9] This was based on the fact that the parent silane, (Me3Si)3SiH, possesses a shift value (-119.6 ppm, Table 8) which is 230 ppm more upfield than that of 59, which is large in comparison to the corresponding difference in shift values for silylium cations and parent silanes observed in the case of trialkyl substituted systems. 

However, all these considerations became rather irrelevant when the IGLO 29Si chemical shift of uncoordinated 59 was calculated at 921 ppm, which is more than 800 ppm downfield from the experimental shift value of 111 ppm [22,45] Ottosson and Cremer [45] calculated that 29Si NMR chemical shift values of cations (H3Si)nH3 nSi+ are shifted downfield by approximately 220 ppm per silyl 

a weakly covalent Si-C bond connects ion 59 and benzene thus leading to a tetracoordinated Si atom. According to the calculated 829Si gas phase value of 59 and the measured 829Si shift value of 53, the complex possesses just 31% silylium ion character. [45] 

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