Structure and Stability of Y-Conjugated Silylium Cations [Si(XH)₃]⁺(X = O, S, Se, and Te)

Abstract

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.

π-Donation and Stabilizing Effects of Pnicogens in Carbenium and Silicenium Ions: A Theoretical Study of [C(XH2)3]+ and [Si(XH2)3]+ (X=N, P, As, Sb, Bi)

“…[4] These studies show that the stabilization for the series of disubstituted (n 2) and triply (n 3) substituted carbenium ions follows the order O b Te b S b Se. The trend for the corresponding monosubstituted (n 1) carbenium ions is less pronounced, but the stabilization energies calculated according to Equation (1) show that the heaviest chalcogen, tellurium, stabilizes this cation even better than oxygen; [9] the results suggest that this is in agreement with the trend of analogous halogen-substituted complexes. [3,8] Further calculations for the corresponding silicenium cations show a fully reversed, yet clear-cut trend.…”

“…How does this behavior found for the fully heteroatomsubstituted pnicogen carbenium (curve C1, Figure 6) and silicenium ions (curve Si1, Figure 6) compare with that found [4,6,9], respectively. for those substituted by chalcogens (curves C2 and Si2, Figure 6) or halogens (curves C3 and Si3, Figure 6), respectively.…”

“…In this case, the tendency of increased stabilization ranges from O to Te without exceptions. [9] In order to explain why an increasing p(p)-donor ability results in a decreasing stabilization, we conclude that it is not reasonable to reduce the nature of the stabilizing effects of the pnicogens and chalcogens only to their p-donor abilities. Therefore, other contributions than the p-interaction alone must be taken into account in order to predict the overall stabilization of these cations.…”

“…The p(p)-donation as well as thermodynamic stabilization increase in the order O`SS e`Te. [9] Note, that the chalcogens stabilize the carbenium ion more effectively than the silicenium ions. As pnicogens are considerably better p-stabilizing substituents than chalcogens, it was, however, shown that the stabilization of the silaguanidinium cation [Si(NH 2 ) 3 ] by amino groups reaches up to 40 % of the corresponding resonance stabilization of the ªrock-stableº guanidinium cation.…”

“…From these plots, it becomes evident that the p(p)-donor ability of the pnicogen substituents (C1) follows the same trend that was found for the corresponding trichalcogen and trihalogen cations (C2 and C3, respectively). [6,9] The relative p(p)-donor abilities of pnicogens and halogens are, therefore, almost equal to each other! In particular, the dominant increase in p(p)-donation from substituents with elements of the second period to elements of the third period is noteworthy.…”