Chemical Bonding in Hypervalent Molecules Revised. Application of theAtoms in MoleculesTheoryto Y₃X and Y₃XZ (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 (∇ 2 ρ( 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.