Planar tetracoordinate lithium in LiLi4F4+ cluster

Over the past 50 years, planar tetracoordinate carbons have attracted significant attention for challenging the classical tetrahedral bonding model proposed by van't Hoff and Le Bel. While the planar framework has been extended to all other first-row elements beyond carbon, stable ptLi (lithium) compounds remain notably missing. Here, we conducted extensive structural searches on the potential energy surfaces of A5Ha4+ (A = alkali metals; Ha = halogens) to explore planar tetracoordinate alkali metals. Our results show that while A5Ha4+ species exhibit grid-like, square-planar A-centered ptA structures (A©A4Ha4+) in closed-shell states, most are transition states. Notably, we identified a rare, stable ptLi species, Li©Li4F4+. In this structure, strong charge transfer from Li to F atoms, forming a Li+©[Li+]4[F-]4 complex, provides strong electrostatic interactions. In addition, donor-acceptor covalent interactions Li4F4 → ptLi offer significant electronic stabilization, contributing to the overall stability of the intriguing ptLi structure.