Unveiling the Hypervalent Electronic Structure of Main Group Zwitterions: Ylides or Ylenes? A <scp>DFT</scp> Study

This study uses density functional theory (DFT) calculations at the M06-2X/6-311 + G(d,p) level to explore the potential of main group non-metals (X = O, N, S, P) to form hypervalent ylides (R_mX⁺-YR_n _-2 ^-). Substituent effects (R = alkoxy, alkyl, phenyl, H, NH₂, NMe₂, halogens) on the stability of these ylides and their neutral counterparts (R_m- ₁X-YR_n _-1) are analyzed across various X → Y bonding scenarios. Fluorine-substituted ylides (e.g., N^hO_/F, S^hO_/F, S^hS_/F, P^hO_/F) and P^hO_/OMe exhibited the highest stability compared to their neutral forms. Bond length and Wiberg bond order analyses reveal significant double bond character of the X-Y bond, confirming the hypervalent structure of the molecule. Molecular electrostatic potential (MESP) analysis shows reduced charge separation and delocalized electron density of the molecule, supporting a hypervalent ylene resonance form (R_mX = YR_n _-2). These findings provide insights into the stability and electronic structure of hypervalent molecules, aiding the design of novel compounds.