Boron‐Based Mg₂Be₂B₇⁻ Ternary Cluster Stabilizes a Monocyclic [B₇]⁵⁻ Heptagonal Ring: Chemical Bonding, Double 6π/6σ Aromaticity, and Dynamic Structural Fluxionality

Incorporating metals into boron clusters through doping or alloying can effectively tailor the structural, electronic, and bonding properties of boron‐based compound clusters. In this contribution, we report on the computational design of a ternary Mg₂Be₂B₇⁻ cluster, which assumes a three‐layered geometry between Be─Mg─Mg trimer, monocyclic B₇ ring, and an isolated Be atom. Chemical bonding analysis reveals double 6π/6σ aromaticity in the system, whose π/σ electron counting conforms to the (4n + 2) Hückel rule. Heptagonal boron ring is a nonclassical structural motif, which is formally in the [B₇]⁵⁻ charge‐state, except for secondary covalent interaction with Be₂ component. The Be─Mg─Mg trimer is held together by dative σ bonds with relative low bond orders. The system shows dynamic structural fluxionality, as demonstrated by the Born–Oppenheimer molecular dynamics (BOMD) simulation at near room temperature. The intramolecular rotation barrier is only 0.15 kcal mol⁻¹ at the single‐point CCSD(T) level. A concept is proposed that a complex molecular system with mutually inert components may facilitate dynamic structural fluxionality. This work also highlights the potential to develop a heptagonal [B₇]⁵⁻ ring as inorganic ligand in boron chemistry.