Ca₂B₁₈-A Bicapped Double-Ring Boron Tube Retaining a Toroidal B₁₈ Cluster Template

Finding novel cluster templates is useful to envisage the formation of stable building blocks with particular functionality. Here, we evaluated the unusual bonding properties of boron clusters toward transition metals, describing the Ca₂B₁₈ cluster as a highly stable double-ring bicapped nonagonal antiprism structure, featuring a D_9d symmetry, confirmed as the global minimum. The resulting structure leads to a significant charge transfer from calcium to the boron framework, enhancing its stability. Energy decomposition analysis of the Ca₂-B₁₈ interaction shows a sizable stabilization, attributed mainly to orbital interactions, supported by electrostatic effects and minor dispersion interactions. The charge distribution in Ca₂B₁₈ gives rise to electron-deficient σ-hole regions at calcium sites, indicating Lewis acid behavior. Adaptive natural density partitioning shows a complex multicenter bonding network with planar aromatic character, supported by magnetic shielding. Overall, this work elucidates the plausible formation of Ca₂B₁₈ as a three-dimensional cluster, offering valuable insights for designing tunable clusters based on the B₁₈ toroid skeleton for achieving suitable molecular building blocks for extended materials.