Quasi-triple bonds and ultrashort Be-Be distances in binary superhalogen clusters [X-Be2(BeX)3-X]- (X = Cl, Br, I)

In computational design, clusters with minimal elemental composition are prioritized for superior gas-phase synthetic accessibility, yet most reported main-group metal clusters exhibiting ultrashort metal-metal distances (USMDs, dM-M < 1.900 Å) require complex ternary or quaternary systems, severely limiting experimental realization. We demonstrate that binary superhalogen clusters [X-Be2(BeX)3-X]- (X = Cl, Br, l) achieve the desired USMD via formation of a quasi-triple bond composed of three 3c-2e bonds between two axial beryllium atoms, resulting in compressed Be-Be distances ranging from 1.832 to 1.851 Å. Their exceptional superhalogen characteristics result from vertical detachment energies (VDEs) of 4.65-4.70 eV, surpassing the threshold (VDE = 3.62 eV) by over 1.00 eV and indicating good stability. Consistently, [X-Be2(BeX)3-X]- (X = Cl, Br, l) are identified as dynamically stable global energy minima exhibiting wide HOMO-LUMO gaps (6.38-6.62 eV), which establish them as promising candidates for experimental validation of USMDs between main-group metals.