The bonding picture in hypervalent XF (X = Cl, Br, I, At) fluorides revisited with quantum chemical topology.

Hypervalent XF (X = Cl, Br, I, At) fluorides exhibit T-shaped C equilibrium structures with the heavier of them, AtF , also revealing an almost isoenergetic planar D structure. Factors explaining this behavior based on simple "chemical intuition" are currently missing. In this work, we combine non-relativistic (ClF ), scalar-relativistic and two-component (X = Br - At) density functional theory calculations, and bonding analyses based on the electron localization function and the quantum theory of atoms in molecules. Typical signatures of charge-shift bonding have been identified at the bent T-shaped structures of ClF and BrF , while the bonds of the other structures exhibit a dominant ionic character. With the aim of explaining the D structure of AtF , we extend the multipole expansion analysis to the framework of two-component single-reference calculations. This methodological advance enables us to rationalize the relative stability of the T-shaped C and the planar D structures: the Coulomb repulsions between the two lone-pairs of the central atom and between each lone-pair and each fluorine ligand are found significantly larger at the D structures than at the C ones for X = Cl - I, but not with X = At. This comes with the increasing stabilization, along the XF series, of the planar D structure with respect to the global T-shaped C minima. Hence, we show that the careful use of principles that are at the heart of the valence shell electron pair repulsion model provides reasonable justifications for stable planar D structures in AX E systems. © 2017 Wiley Periodicals, Inc.