Optimized Structures and Ring Strain Energies of Isoelectronic Homo- and Heterosiliranes c‑AX₂SiR₂SiR₂^<i>q</i> (A/<i>q</i> = Al/1–, Si/0, P/1+): Unexpected Effects of Charge and Size

Ring strain energies (RSEs) estimated for a series of isoelectronic three-membered ring homo- and heterosiliranes c-AX₂SiR₂SiR₂^<i>q</i> (A/<i>q</i> = Al/1–, Si/0, P/1+; X = H, Me, F, CF₃; R = H, Me) fall into two categories. Those where the X substituent is electron-withdrawing show RSE trends where the aluminatadisilirane and trisilirane differ only slightly, while that for the phosphoniodisilirane is significantly larger. This arises because the positive charge/small size of the phosphorus in the last species is poorly accommodated in the ring and more readily accommodated in one of the chain molecules used to calculate RSE. In contrast, rings where the X substituent is neutral or an electron donor show V-shaped trends in RSE. This holds despite the fact that the bond distances and angles exhibit expected linear trends. Quantum theory of atoms in molecules (QTAIM) calculations suggest that this arises because the charged rings have the greatest difference between bond lengths and bond path distances, the latter being the paths of most significant electron density