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

Abstract

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 (RmX+–YRn−2−). Substituent effects (R = alkoxy, alkyl, phenyl, H, NH2, NMe2, halogens) on the stability of these ylides and their neutral counterparts (Rm−1X–YRn−1) are analyzed across various X → Y bonding scenarios. Fluorine-substituted ylides (e.g., NhO/F, ShO/F, ShS/F, PhO/F) and PhO/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 (RmX = YRn−2). These findings provide insights into the stability and electronic structure of hypervalent molecules, aiding the design of novel compounds.