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 [H 2 Si(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.