Structure and stability of Y-conjugated silylium cations [Si(XH)₃]⁺ (X = O, S, Se, and Te)

Quantum mechanical ab initio calculations at the HF and MP2 levels of theory with valence basis sets up to TZ+2P quality are reported for the Y-conjugated silylium cations [Si(XH)₃]⁺ and the di- and monosubstituted analogues [HSi(XH)₂]⁺ and [H₂Si(XH)]⁺ (X = O-Te). The X→Si p(π) donation and the thermodynamic stabilization increase in the order O < S < Se < Te. This trend is given by the calculated complexation energies of the water complexes and the reaction energies of isodesmic reactions. A comparison with the respective carbenium ions shows that the chalcogen substituents stabilize the carbenium ions more than the silylium ions. While the stabilization of the carbenium ions by chalcogens from O to Te remains nearly the same, it strongly varies within the series of silylium ions in the order Te> Se > S > O. The silylium ions and the carbenium ions are more strongly stabilized by the chalcogens than by the respective halogen atom. The analysis of the Si-X bonds in [Si(XH)₃]⁺ shows that the covalent character increases from Si-O to Si-Te, which has a nearly unpolar bond.