Silicenium ions substituted

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

Equation 1 compares the stability of substituted silicenium ions versus the parent SiH3+ (1). Equation 2 compares the stabilities of the correspondingly substituted carbocations versus CH3+. Equation 3 compares the stabilities of silicenium ions with those of the corresponding carbocations. The calculated energies of equations 1-3 are presented in Table 1, and the following conclusions can be drawn51 ... 

Let us first have a closer look at trialkyl-substituted silicenium ions2,3 since, four years ago, the X-ray structures of these species started a lively debate on silicenium ions. 

Quantum theory sheds more light on the character of such silicenium ion-Lewis base complexes, e.g. on the nature of the bonds formed. Are they covalent Do d-orbitals contribute Flow large are the complexation energies and how do they change upon substitution Can free tricoordinate silicenium ions be formed at all in 7r-donor or aromatic solvents ... 

Reed s group reported the X-ray structures of trialkyl-silicenium ions R3ST6 complexed to low-nucleophilic hexahalo carborane anions90,91. The authors varied systematically both the alkyl substituents R (R = Me, Et, z -Pr and t-Buj and the halogen substitution of the XgCBnlL (X = Cl, Br, I) counterions. The degree of silicenium ion character in the... 

The experimental search for free stable silicenium ions in solution seems to have finally succeeded, at least for triaryl-substituted species4,5. However, computational studies will continue to augment, to verify and to guide the interpretations of experimental findings, as well as to suggest new experiments for preparing other silicenium ions. Schleyer and coworkers, for instance, have recently explored a silicenium ion within a cage , namely... 

The cationic product in Gq. (127) is a substituted silicenium ion attempts to produce a corresponding species chemically by abstraction of R from RsSiM(CO)n or R2SitM(CO)n]2 have all been unsuccessful [see Section III,C and Ref. (24)]. Thus addition of the chloride-ion acceptor SbCl5 to ClaSiRetCO) results in Si-Re bond cleavage (32). 

An important conclusion from all the calculations is that the same types of substituents stabilize both silicenium ions and carbenium ions. However, as substituents stabilize carbenium ions more effectively than silicenium ions, the much higher inherent stability of H3Si+ relative to CH3 is reduced significantly in the substituted cations (equation 46). For example, SiHj is more stable than CH3 by 73.5 kcal mol", but H2SiNH2 is more stable than H2CNH2+ by only 18.0 kcal mol"1 (STO-3G)310a. 

---