Star-Like CBe₅Au₅⁺ Cluster: Planar Pentacoordinate Carbon, Superalkali Cation, and Multifold (π and σ) Aromaticity

We report on the computational design of star-like CBe₅Au₅⁺ cluster with planar pentacoordinate carbon (ppC), which is also classified as a superalkali cation. Relevant isovalent CBe₅Au_n^n-4 (n = 2-4), BBe₅Au₅, and NBe₅Au₅²⁺ clusters with ppC/B/N are studied as well. Global-minimum structures of the clusters are established via computer global searches. The species feature a pentacoordinate pentagonal XBe₅ (X = C, B, N) core, with Au occupying outer bridging positions. Molecular dynamics simulations indicate that they are dynamically stable. Bonding analysis reveals 3-fold (π and σ) aromaticity in CBe₅Au₅⁺, a key concept that overrides the 18-electron rule and should be applicable for (or help revisit existing models of) other planar hypercoordinate systems. Vertical electron affinities of CBe₅Au₅⁺ and its lighter counterparts (CBe₅Cu₅⁺ and CBe₅Ag₅⁺) are calculated to be unusually low, which are below 3.89 eV, the smallest atomic ionization potential of any element in the periodic table. Thus, these three clusters belong to superalkali cations. The merge of ppC and superalkali characters makes them unique chemical species.