This study elucidates the distinct structural evolution of Eu-doped boron clusters, progressing from planar to half-sandwich and finally to tubular/drum-like configurations. Notably, EuB₂₂ exhibits a record coordination number (CN = 22) in lanthanide - doped boron chemistry. Chemical bonding analysis reveals that the exceptional stability of the EuB₈ and EuB_{2 2} clusters is underpinned by their pronounced aromaticity.

In recent years, lanthanide-doped boron clusters have attracted considerable attention. However, the structural evolution and stability of medium-sized boron clusters doped with Eu atoms remain largely unknown. In this study, a combination of particle swarm optimization and density functional theory is employed to systematically investigate the structural evolution and stability of Eu-doped boron clusters, EuB _n (n = 1–24). The results show that the most stable configurations of the EuB _n clusters transitioned from planar geometries to half-sandwich molecular structures as the cluster size increased. Furthermore, for larger cluster sizes, such as n = 18, 20, and 22, the lowest-energy configurations transitioned to tubular and drum-type molecules featuring double B rings. Notably, the EuB₂₂ cluster has been identified as a twenty-two-coordinate complex, which is the highest coordination number achieved in lanthanide-doped boron-cluster chemistry. The research not only uncovers the structural evolution of Eu-doped boron clusters but also delves into their electronic characteristics, offering valuable insights into the chemistry of rare-earth-element-doped boron clusters.