Revisiting the Structure and Bonding in Li₅H₆^– and the Exploration of Reactivity: Planar Pentacoordinate Hydrogen

Abstract

e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li 5 H 6 − , featuring C 2v symmetry in the singlet state with two distinct H−Li (center-ring) bond distances. We herein revisited the potential energy surface of Li 5 H 6 − by using a target-oriented genetic algorithm. Our investigation revealed that the lowest-energy structure of Li 5 H 6 − exhibits a ppH configuration with very high D 5h symmetry and a 1 A 1 ′ electronic state. We did not find any electronic effect like Jahn−Teller distortion that could be responsible for lowering its symmetry. Moreover, our calculations demonstrated significant differences in the relative energies of other low-lying isomers. An energetically very competitive planar tetracoordinate hydrogen (ptH) isomer is also located, but it corresponds to a very shallow minimum on the potential energy surface depending on the used level of theory. Chemical bonding analyses, including AdNDP and EDA-NOCV, uncover that the optimal Lewis structure for Li 5 H 6 − involves H − ions stabilized by the Li 5 H 5 crown. Surprisingly, despite the dominance of electrostatic interactions, the contribution from covalent bonding is also significant between ppH and the Li 5 H 5 moiety, derived from H − (1s) → Li 5 H 5 σ donation. Magnetically induced current density analysis revealed that due to minimal orbital overlap and the highly polar nature of the H−Li covalent interaction, the ppH exhibits local diatropic ring currents around the H centers, which fails to result in a global aromatic ring current. The coordination of Li 5 H 6 − with Lewis acids, BH 3 and BMe 3 , instantly converts the ppH configuration to (quasi) ptH. These Lewis acid-bound ptH complexes show high electronic stability and high thermochemical stability against dissociation and, therefore, will be ideal candidates for the experimental realization.