Manifestations of Boron-Alkali Metal and Boron-Alkaline-Earth Metal Romances

ConspectusThe electron deficiency of boron promotes the formation of multicenter σ and π bonds that endow its clusters and solids with exceptional structural diversity. While bulk boron favors cage-like frameworks, clusters often adopt planar or quasi-planar motifs composed of triangles that evolve into tubular and cage-like architectures as their size increases. Many of these clusters are stabilized by delocalized σ and π bonds that are associated with fluxional behavior and multiple aromaticity.Metal doping enriches this chemistry. Transition metals use their d or f orbitals to couple with the boron framework, generating metal-centered rings, metallo-boron nanotubes, and metalloborophenes. In contrast, alkali and alkaline-earth metals have long been viewed as simple counterions, yet recent findings reveal that they can orchestrate deep structural reorganizations by combining charge transfer with efficient orbital overlap. Lithium, for example, leads to a quasi-planar → tubular → cage evolution in B₁₂ clusters via strong electrostatic attraction to the boron framework, whereas beryllium engages in pronounced covalent Be-B interactions that yield rare architectures such as the Archimedean Be₄B₁₂⁺ cage, the B-Be sandwich B₇Be₆B₇, and four-ring tubular forms like Be₂B₂₄⁺.In heavier alkaline-earth systems, the participation of (n-1)d orbitals (Ca, Sr, Ba) introduces transition-metal-like covalent interactions, producing highly symmetric rings and tubular clusters. This Account summarizes how electrostatic and covalent interactions jointly control geometry and bonding in boron-metal systems, defining the rich landscape of boron chemistry.