Exploring the bonding in alkaline earth halides AeX⁻ (Ae = Be–Ba, X = F–I) from Fermi hole localization and QTAIM perspectives

A theoretical description of various [AeX]^- (Ae = Be-Ba, X = F-I) systems, some of which have been reported in the literature to bear an unusual quadruple bond between the metal and the halogen, is provided based on both (i) the localization of the Fermi hole and (ii) the topological analysis of the one-electron density. Insights into the bond order of various [AeX]^- systems are inferred on the basis of the number of electrons localized in the bond basin, the topology of the Fermi hole information computed along the bond axis, and the delocalization index. The results suggest that the [AeX]^- molecules present a bond with attributes closer to a classical dative bond than to a multiple one, being characterized by large stabilization due to the electrostatic interaction between the polarized metal and the halogen anion. The provided interpretation is supported by means of a comparison with the [Re₂Cl₈]^2- complex considered as a prototype of quadruple bonds.