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-GH(2) (X = O, S, or Se) ylides

The electron pair density in conjunction with the AIM theory and calculated NMR chemical shifts were used to characterize the bonding properties for n ine pnicogen and chalcogen ylide structures, The hybrid B3LYP and MP2, meth ods were employed with the 6-311+G* basis set. No evidence. was found to be dependent on the banana (Omega) 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 bon d 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 electr ostatic interact ions, with a small contribution of negative hyperconjugati on.