[Co₃@Ge₆Sn₁₈]^5-: A Giant σ-Aromatic Cluster Analogous to H₃⁺ and Li₃

Aromaticity is one of the most important concepts in chemistry and has been successfully extended to all-metal clusters. However, the study of all-metallic aromatic clusters remains in its early stages, with σ-aromatic clusters mostly limited to small sizes (≤12) that often require external stabilization. In this work, we report the first Ge/Sn-based trimer, [Co₃@Ge₆Sn₁₈]^5-, which can be rationalized as the fusion of three [Co^-@Ge₃Sn₆^4-] units via a Ge₃ face. Theoretical studies have revealed that two σ-electrons are delocalized across the entire trimer, with the spherical aromaticity of each [Co@Ge₃Sn₆] unit and the global σ-aromaticity of [Co₃@Ge₆Sn₁₈]^5- further supported by its electron delocalization and magnetic behavior. As a result, this trimer can be viewed as a giant σ-aromatic counterpart to triatomic H₃⁺ and Li₃⁺. Our findings suggest the potential for synthesizing cluster-of-cluster analogs of discrete all-metallic aromatic species, such as Al₄^2-, and further enhance our understanding of chemical bonding.