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)

Quantum chemical calculations at the MP2 and CCSD(T) levels of theory are reported for cations of the general type [A(XH₂)₃]⁺ with A=C, Si and X=N, P, As, Sb, Bi. Population analysis, methyl stabilization energies (MSEs), and structural criteria were used to predict the p(π)-donor ability of and the π-stabilization energy exerted by this series of pnicogens. All of the substituents XH₂ considered in these studies invariably stabilize the triply substituted carbenium as well as the silicenium ions. The calculated data show that the intrinsic p(π)-donation of the group 15 atoms follows the order N < P < As < Sb < Bi. However, the trend of the stabilization energies is fully reversed. The intrinsic stabilization energies of the planar carbenium ions decrease monotonically from 161.2 kcal mol^-1 for X=NH₂ to 98.0 kcal mol^-1 for X=BiH₂. The effective stabilization of the pnicogens in the equilibrium structures, which also includes the energy-demanding pyramidalization of the XH₂ substituents, follows the same trend, although the absolute numbers are reduced to 145.6 kcal mol^-1 for X=NH₂ and 53.2 kcal mol^-1 for X= BiH₂. This seemingly contrasting behavior of increasing p(π) charge donation and decreasing stabilization has already been found for other substituents. Previous studies have shown that carbenium ions substituted by chalcogens up to the fourth row also stabilize C⁺ less effectively with respect to heavier substituents. Of the ions investigated in this study, only the silicenium ions that are stabilized by pnicogens from the third to the sixth row of the periodic system yield increased stabilizing energies that follow the corresponding intrinsic p(π)-donor abilities of the respective substituent.