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