Mixed Localized and Delocalized Bonding in a Linear‑B₃‑Centered B₃Mg₆ ^- Cluster

The structural and bonding properties of the boron-magnesium alloy cluster B₃Mg₆ ^- were investigated using quantum chemical calculations. The global minimum is identified as a highly symmetric D _3h structure featuring an unusual linear B₃ core embedded in a Mg-rich framework. Bonding analyses show that the cluster contains three peripheral Mg-Mg two-center two-electron (2c-2e) σ bonds, two B-B 2c-2e σ bonds, and two orthogonal three-center two-electron (3c-2e) π bonds associated with the linear boron chain. In addition, the framework exhibits further four-center two-electron (4c-2e) and eight-center two-electron (8c-2e) σ delocalization, indicating that both local bonding and global skeletal delocalization contribute to its stability. Natural population analysis reveals substantial charge transfer from Mg atoms to the boron skeleton, suggesting a mixed ionic-covalent bonding picture. The calculated highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of 2.30 eV supports the good electronic stability of the closed-shell anion. Electron localization function (ELF), nucleus-independent chemical shift (NICS), and iso-chemical shielding surface (ICSS) analyses further support appreciable σ delocalization associated with the BMg₃-containing framework. The present results provide a clear bonding picture for B₃Mg₆ ^- and offer additional insight into the stabilization of atomically thin boron-chain motifs in alloy cluster environments.