Chemical Bonding in Hypervalent Molecules Revised. 3. Application of the Atoms in Molecules Theory to Y3X−CH2(X = N, P, or As; Y = H or F) and H2X−CH2(X = O, S, or Se) Ylides

Abstract

The electron pair density in conjunction with the AIM theory and calculated NMR chemical shifts were used to characterize the bonding properties for nine pnicogen and chalcogen ylide structures. The hybrid B3LYP and MP2 methods were employed with the 6-311+G* basis set. No evidence was found to support a banana (Ω) bonding scheme. Instead, different bonding schemes were found to be dependent on the electronegativity of the X atom in the C−X bond. When X is a highly electronegative atom (N,O), the C−X bond is weaker than a single bond, due to electrostatic repulsion. When the X atom has electronegativity similar to carbon, a covalent, yet significantly polar interaction results, and its strength is determined mainly by electrostatic interactions, with a small contribution of negative hyperconjugation.