Chemical Bonding in Hypervalent Molecules Revised. Application of the Atoms in Molecules Theory to Y3 X and Y3 XZ (Y = H or CH3; X = N, P or As; Z = O or S) Compounds

Atoms in Molecules Theory has been applied to analyze bonding properties, in potentially hypervalent pnicogen (N, P or As)−chalcogen(O or S) bonds within the framework of three plausible models: (i) one σ bond and two π back-bonds (negative hyperconjugation), (ii) one σ bond and three π back-bonds, and (iii) three Ω (banana) bonds. The topological analyses (based upon the electron charge density (ρ(r)), its Laplacian (∇2ρ(r)), bond ellipticity, etc.) and the charges were consistent with a highly polarized σ bond, with bond strength dependent on the electrostatic interactions. The equilibrium geometries were optimized by both density functional theory with a hybrid functional (B3LYP) and by ab initio methods at the MP2(full) level, using the 6-311G basis set augmented by polarization and/or diffuse functions.