σ*-Aromaticity of Substituted 1H-Phosphirenium Cations and Substituted Silacyclopropenes^†

The influence of the electronegativity of the ligands Y (F, Cl, Br, I, OH, NH₂, CH₃, H) on the strength of the σ*-aromatic effect in gem-disubstituted 1H-Phosphirenium cations and substituted 3-silacyclopropenes has been investigated with ab initio theory. Phosphorus and silicon provide a low lying p-type σ*-orbital antibonding to the ligands Y. This interacts with the ring double bond analogously to a third p-orbital in a 2π Hückel system. These systems have been compared to the analogous saturated rings. Calculations at the RHF/6-31G* and MP4/6-31+G* levels, NBO analyses of the effect of the ligands Y and comparisons of the relative stabilities of the saturated and unsaturated compounds suggest a weak but significant σ*-aromatic effect dependent on the electronegativity of Y.