ReB₁₈^-: a sandwich-structured magic cluster with dual aromaticity

Transition metal-doped boron clusters have attracted significant attention due to their unique bonding patterns and promising applications in catalysis and spintronics, yet medium-sized systems remain underexplored. Here, density functional theory (DFT) calculations are performed to investigate the geometric and electronic properties of low-lying energy isomers of ReB_n^- (n = 3-20). The global minimum of ReB₁₈^- is identified as a highly symmetric C_9v sandwich structure, which is confirmed to be a magic cluster with exceptional stability. Energy decomposition analysis shows that orbital interactions dominate the stabilization, followed by electrostatic contributions. In particular, δ* donation from the π* orbitals of Re-B to the vacant d orbital of Re plays a key role, indicating strong coordination bonding, while B-B bonding favors localized three-center two-electron (3c-2e) σ interactions. The distinctive electronic structure gives rise to dual σ and π aromaticity, along with significant magnetic shielding effects. This work provides fundamental insights into the bonding mechanisms of transition metal-doped boron clusters and offers a design guide for boron-based functional materials.