Halogen, Chalcogen, and Pnicogen Bonding Involving Hypervalent Atoms.

The additional substituents arising from hypervalency present a number of complicating issues for the formation of noncovalent bonds. The XF5 molecule (X=Cl, Br, I) was allowed to form a halogen bond with NH3 as the base. Hypervalent chalcogen bonding is examined by way of YF4 and YF6 (Y=S, Se, Te), and ZF5 (Z=P, As, Sb) is used to model pnicogen bonding. Pnicogen bonds are particularly strong, with interaction energies approaching 50 kcal mol-1 , and also involve wholesale rearrangement from trigonal bipyramidal in the monomer to square pyramidal in the complex, subject to a large deformation energy. YF4 chalcogen bonding is also strong, and like pnicogen bonding, is enhanced by a heavier central atom. XF5 halogen bond energies are roughly 9 kcal mol-1 , and display a unique sensitivity to the identity of the X atom. The crowded octahedral structure of YF6 permits only very weak interactions. As the F atoms of SeF6 are replaced progressively by H, a chalcogen bond appears in combination with SeH⋅⋅⋅N and NH⋅⋅⋅F H-bonds. The strongest such chalcogen bond appears in SeF3 H3 ⋅⋅⋅NH3 , with a binding energy of 7 kcal mol-1 , wherein the base is located in the H3 face of the Lewis acid. Results are discussed in the context of the way in which the positions and intensities of σ-holes are influenced by the locations of substituents and lone electron pairs.