Silaguanidinium cation

U. Pidun, M. Stahl, and G. Frenking, Chem. Eur. J., 2, 869 (1996). The Silaguanidinium Cation and the Search for a Stable Silylium Cation in Condensed Phases. 

A silylium ion Y3Si+ with Y = NR2, OR, etc. possesses partial SiY double bonds (Scheme 3). For example, the carbocation analogues of ions 5 and 6 in Scheme 3 are considered to possess just little trivalent carbenium ion character. [87a] An amino substituent completely changes the nature of the LUMO of a silylium cation. For the silaguanidinium cation, (NH2)3Si+ (5), the next-LUMO rather than the LUMO itself possesses the expected 3pjt(Si) character. Moreover, the energies of both LUMO and next-LUMO are high, and therefore, its electrophilicity is much lower than that of an alkylsilylium cation. 

Of course, a NMR chemical shift value of 225 ppm can be interpreted in different ways. One can assume that 61 represents a free silylium cation in solution that because of considerable -conjugation between 3pn(Si) orbital and the three phenyl rings is internally stabilized and, therefore, does not interact with solvent molecules. The question in this case would be to which extent 11-conjugation has reduced the silylium cation character and how much positive charge is spread over the C framework. It could be that 61 represents more a carbenium ion in the same way as the silaguanidinium ion represents more an ammonium ion (see section 4.1). 

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