Planar tetracoordinate fluorine atom: global minimum with viable possibility

Planar hypercoordinate structures are emerging tremendously. Most of the second-row elements from the periodic table exhibit this remarkable structural feature. Planar tetracoordinate fluorine (ptF) atoms were also predicted in group 13 supported clusters. However, high-level ab initio calculations nullified the fact and established that all these ptFs were not minimum energy structures on the potential energy surface. Thus, a true ptF is still scarce in the literature. Herein, we propose the unprecedented ptF as the global minimum of the C_2V symmetric H₃Li₄F^- cluster. Heavier alkali metals (Na and K) showed similar results. Both density functional theory (DFT) and ab initio calculations revealed that the ptF structure is a real minimum and indeed, the global minimum. Bonding analysis indicates that the central fluorine atom is stabilized by multicentre bonding with four surrounding Li atoms. Natural charge analysis reveals that the fluorine atom is negatively charged, which is strongly attracted by the positively charged surrounding lithium centres, thereby imparting significant electrostatic attraction. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in the gas phase.