Chemical Bonding in Hypervalent Molecules Revised. 2. ² Application of the Atoms in Molecules Theory to Y 2 XZ and Y 2 XZ 2 (Y ) H, F, CH 3 ;X ) O, S, Se; Z ) O, S) Compounds J. A. Dobado, ‡ Henar Martı ´nez-Garcı ´a, ‡ Jose ´ Molina Molina,* ,‡ and Markku R. Sundberg* ,§ Contribution from the Grupo de Modelizacio ´ n y Disen ˜ o Molecular, Instituto de Biotecnologı ´a, Campus FuentenueVa, UniVersidad de Granada, E-18071 Granada, Spain, and Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014, UniVersity of Helsinki, Helsinki, Finland ReceiVed August 7, 1998. ReVised Manuscript ReceiVed NoVember 6, 1998 Abstract: The atoms in molecules theory has been applied to analyze bonding properties in potentially hypervalent structures with chalcogen (O, S, or Se)-chalcogen (O or S) bonds. The topological analyses [based upon the electron charge density F(r), its Laplacian ∇ 2 F(r), bond ellipticity, and local energy density E d (r)] and the charges clearly displayed the dependence of the bonding properties with the central atom: (a) When the central atom is oxygen, the main electron charge concentration remains in the surroundings of the central atom, yielding a very weak coordinate bond. (b) Bonding to the central sulfur and selenium atoms is consistent with a model of a highly polarized σ-bond, its strength depending mainly on electrostatic interactions, so no evidence was found for double bonding, which has so far been the conventional way to describe the interaction in these systems. The equilibrium geometries were optimized by both density functional theory with a hybrid functional (B3LYP) and ab initio methods at the MP2(full) level, using the 6-311+G* basis set. I. Introduction The bonding nature in hypervalent molecules has been controversial for years, including pnicogen or chalcogen (groups 15 and 16 in IUPAC nomenclature, respectively) compounds. 2-18 The description of the structure and bonding in these hypervalent compounds was connected with the possible involvement of virtual d orbitals in the bonding. For the first-row atoms, the d basis functions in the ab initio calculations play a role as polarization functions augmenting the quality of the sp basis set. However, for transition metals this function provides a description for the valence d orbitals. For the second-row elements, there appears to be no clear demarcation with use of d functions between normal octet and hyperValent species. The majority of accurate ab initio calculations 2,3,7,16,18-38 now agree that the d function acts mostly as a polarization function for second-row atoms, compensating for the inflexibility of the sp basis set. 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