σ*-Aromaticity of Substituted 1<i>H</i>-Phosphirenium Cations and Substituted Silacyclopropenes<sup>†</sup>

The influence of the electronegativity of the ligands Y (F, Cl, Br, I, OH, NH<sub>2</sub>, CH<sub>3</sub>, H) on the strength of the σ*-aromatic effect in gem-disubstituted 1<i>H</i>-Phosphirenium cations and substituted 3-silacyclopropenes has been investigated with <i>ab initio</i> 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.