Stabilization of silicenium ions by second row π-donor substituents.An study.

Fluoromethyl Cations and Group XIV Congeners AHnF3 – n+ (A = Si, Ge, Sn, Pb; n = 0–2): From Covalent Structures to Ion‐Molecule Complexes

“…The geometries and harmonic frequencies of the FSiH 2 + isomer 3 and of the H-SiF 2 + isomer 6 of C 2v symmetry are also in good agreement with previous HF, [14][15][16] MP2 [10,17] and CASSCF [18] ab initio studies and are again typical of covalent structures. On the other hand, the bond lengths, harmonic frequencies and dissociation energies of the (Si,H 2 ,F) + isomers 4 and 5, and of the (Si,H,F 2 ) + isomers 7 and 8, point to ion-molecule complexes between ground-state SiF + ( 1 Σ + ) [19] and H 2 (4) or HF (7), and ground-state SiH + ( 1 Σ + ) [20] and HF (5) or F 2 (8).…”

Preparation of the first tricoordinate silyl cation

“…The second strategy we pursued was the use of sulfur instead of silicon as the substituent on the central silicon atom. This approach was based on published calculations of Apeloig et al, 12 who found that sulfur and phosphorus are particularly effective in stabilizing silyl cations in the gas phase. Sulfur not only is highly polarizable, like silicon as a substituent, but also possesses 3p lone pairs.…”

“…Since silylium ions do not possess this stability, extraordinary efforts must be made to protect them. Calculations on silylium ions in the gas phase, going back to 1981, 12 had demonstrated that substituents do provide considerable stabilization, but apparently not enough to lower electrophilicity to the point that solvent and anion no longer complex with silicon.…”

“…Smaller, stronger nucleophiles, however, could react, as the resonance position quickly moved to lower frequencies on the addition of acetonitrile or triethylamine. Reed and Fackler succeeded in making trimesitylsilylium from a sample of allyltrimesitylsilane we supplied them, with closo-7, 8,9,10,11,12-Cl 6 -CB 11 H 6 as the anion, and they measured its chemical shift in benzene to be 225.9. Thus the cation also is insensitive to a change in anion.…”

Silicenium Ions: Quantum Chemical Computations

“…Schleyer, Apeloig and coworkers were the first to report comprehensive investigations on the structure and stability of˛-substituted silicenium cations 51,54 . Although those early computations only employed modest SCF levels, the authors were able to analyse the importance of the various properties of˛-substituents, such as -inductive effects, lone pair -donation, conjugation and hyperconjugation, in stabilizing silicenium ions 51,54 . The relative stabilities of silicenium ions were explored by means of isodesmic reactions 1 to 3.…”