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Hypervalent XF3 (X = Cl, Br, I, At) fluorides exhibit T-shaped C2V equilibrium structures with the heavier of them, AtF3, also revealing an almost isoenergetic planar D3h structure. Factors explaining this behavior based on simple 'chemical intuition' are currently missing. In this work, we combine non-relativistic (ClF3), 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 ClF3 and BrF3, while the bonds of the other structures exhibit a dominant ionic character. With the aim of explaining the D3h structure of AtF3, 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 C2v and the planar D3h 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 D3h structures than at the C2v ones for X = Cl − I, but not with X = At. This comes with the increasing stabilization, along the XF3 series, of the planar D3h structure with respect to the global T-shaped C2v 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 D3h structures in AX3E2 systems. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The nature of carbon–astatine bonds involved in some model species that mimic211At-labelled biomolecules, was investigated by means of ELF and QTAIM analyses in a context of two-component relativistic computations. The nature of the bonded carbon atom proved to be decisive. When At is bonded to an ethynyl group, some charge delocalisation with the vicinal triple C–C bond strengthens the At–C bond and gives it a multiple bond character. However, At displays also a large positive charge which may alter thein vivostability of such At–C bonds. In the case of an isopropyl group, the At–C bond is less polarised but also much weaker. In contrast, the bond remains strong whilst retaining a small At positive charge when At is bonded to an sp2carbon atom. Hence, these latter results rationalise why aromatic or aryl groups appear reasonably suited for a priori stable radiolabelling of biomolecules with211At in the context of alpha therapy. [ABSTRACT FROM PUBLISHER]