HLi₄Cl₄^-: a planar tetracoordinate hydrogen superhalogen anion

Both planar tetracoordinate hydrogen (ptH) species and superhalogen anions are exotic. Herein we have designed a ternary ptH D_4h HLi₄Cl₄^- star, by replacing the peripheral H auxiliary atoms of C_4v HLi₄H₄^- with four Cl bridges. The ptH HLi₄Cl₄^- cluster consists of an H center, middle Li₄ square, and four outer Cl bridges. It is a true global minimum via unbiased isomeric searches. Born-Oppenheimer molecular dynamics simulations reveal that the ptH structure is relatively robust against isomerization or dissociation at 300 and 600 K. Bonding analyses indicate that there are twelve lone pairs (LPs) for four Cl auxiliary atoms, four peripheral 3c-2e Li-Cl-Li σ bonds, and one delocalized central 5c-2e σ bond. Multicenter ionic bonding dominates the stability of the system, rather than σ aromaticity. The ptH HLi₄Cl₄^- cluster has a large vertical detachment energy (7.33 eV) at the CCSD(T) level and can be seen as a superhalogen anion. The current work not only proposes an "altering the auxiliary atoms" strategy for designing new ptH species, but also provides an important bridge between ptH chemistry and superhalogens.