Do Planar Tetracoordinate Fluorine Atoms Exist? Revisiting a Theoretical Prediction

We re-examined the existence of planar tetracoordinate F (ptF) atoms, which was proposed recently by using high-level ab initio methods such as coupled-cluster singles and doubles with perturbative triples (CCSD(T)) with large basis sets. Our calculations indicate that the planar structures of FIn₄⁺ (D_4h), FTl₄⁺ (D_4h), FGaIn₃⁺ (C_2V), FIn₂Tl₂⁺ (D_2h), FIn₃Tl⁺ (C_2V), and FInTl₃⁺ (C_2V) are not the minimum energy states; by contrast, they are transition states. Density functional theory calculations overestimate the size of the cavity formed by the four peripheral atoms, leading to erroneous conclusions regarding the existence of ptF atoms. Our analysis suggests that the preference for non-planar structures in the six cations studied is not due to the pseudo Jahn-Teller effect. Additionally, spin-orbit coupling does not alter the main conclusion that the ptF atom does not exist. If sufficiently large cavity formation by group 13 elements to accommodate the central F^- ion is guaranteed, then the existence of ptF atoms is plausible.