Be₂H₃L₂^– (L=CH₃ and F–I): Hyperhalogen Anions with Ultrashort Beryllium-Beryllium Distances

The superalkali cations and superhalogen anions commonly have different type of core moieties. Based on the previous reports that   Be₂H₃L′ ₂⁺ (L′=NH₃ and noble gases Ne−Xe) are superalkali cations, in the present work, we designed the superhalogen anions Be₂H₃L₂⁻ (L=CH₃ and halogens F−I), and both superalkali cations and superhalogen anions can be constructed using Be₂H₃ as the core moiety. The newly designed Be₂H₃L₂⁻ species are much more stable than their isoelectronic cationic counterparts Be₂H₃L′ ₂⁺, as can be reflected by the highly exergonic substitution reaction of L′ ligand in Be₂H₃L′ ₂⁺ with isoelectronic L⁻ to give Be₂H₃L₂⁻. These anionic species possess the well-defined electronic structure, which can be proven by their large HOMO−LUMO gaps of 4.69 eV to 5.38 eV. It is remarkable that Be₂H₃L₂⁻ can be regarded as the hyperhalogen anions due to the extremely high vertical detachment energies (5.38 eV to 6.06 eV) and the Be−Be distances in these species (1.776 Å to 1.826 Å) are short in ultrashort metal-metal distances (defined as d_M−M<1.900 Å) between main group metals. In the designed five small model species, three of them, i.e. Be₂H₃L₂⁻ (L=CH₃, Cl, and Br), are kinetical viable global energy minima, which are the promising target for generation and characterization in anion photoelectron spectroscopy. The analogue molecule t-Bu−Be₂H₃−t-Bu⁻ with bulky protecting tert-butyl (t-Bu) groups is designed as a possible target for synthesis and isolation in condensed states.