pi-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 r eported for cations of the general type [A(XH2)(3)](+) with A = C, Si and X =N, P, As, Sb. Pi. Population analysis, methyl stabilization energies (MSEs ), and structural criteria were used to predict the p(pi)-donor ability of and the pi -stabilization energy exerted by this series of pnicogens. All o f the substituents XH2 considered in these studies invariably stabilize the triply substituted carbenium as well as the silicenium ions. The calculate d data show that the intrinsic p(pi)-donation of the group 15 atoms follows the order N < P < As < Sb < Bi. However, the trend of the stabilization en ergies is fully reversed. The intrinsic stabilization energies of the plana r 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 pnicog ens 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 = NM and 53.2 kcal mol(-1) for X = BiH2. This seemingly contrasting behavior of increasi ng P(pi) charge donation and decreasing stabilization has already been foun d for other substituents. Previous studies have shown that carbenium ions s ubstituted by chalcogens up to the fourth row also stabilize C+ less effect ively with respect to heavier substituents. Of the ions investigated in thi s 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(pi)-donor abilities of t he respective substituent.