Revisiting the Structure and Bonding in Li<sub>5</sub>H<sub>6</sub><sup>–</sup> and the Exploration of Reactivity: Planar Pentacoordinate Hydrogen
Li-juan Cui, Yahui Li, Luis Leyva‐Parra, William Tiznado, Sudip Pan, Zhong‐hua Cui
Abstract
Recently, Guha and co-workers (Sarmah, K.; Kalita, A.; Purkayastha, S.; Guha, A. K. Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride. Angew. Chem. Int. Ed. 2024, e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li 5 H 6 –, featuring C 2 v 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 5 h 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.