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 D3h 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.