Li4F4−: Planar tetracoordinate fluorine in a highly viable binary mono-anionic cluster

Planar hypercoordinate fluorine is elusive due to the high electronegativity of fluorine, which restricts electron delocalization. Despite some progress, planar tetracoordinate fluorine (ptF) anionic species with simple components suitable for photoelectron spectroscopy have not yet been reported. Herein, we introduce the first binary mono-anionic Li4F4- cluster containing a ptF center with C2v symmetry. At a given level of theory, it is a global minimum (GM) structure with good dynamic stability and significant barriers to interconversion into the lowest energy isomers. It is noteworthy that the GM of the neutral Li4F4 species adopts a cube configuration similar to that of the typical bulk lithium fluoride crystal structure rather than a planar shape. The stability of the ptF is dominated by electrostatic interactions, supplemented by a weak covalent component rather than any aromaticity. The cluster is a highly viable species and a promising candidate for experimental detection and characterization.