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

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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, [Co 3 @Ge 6 Sn 18 ] 5–, which can be rationalized as the fusion of three [Co – @Ge 3 Sn 6 4– ] units via a Ge 3 face. Theoretical studies have revealed that two σ-electrons are delocalized across the entire trimer, with the spherical aromaticity of each [Co@Ge 3 Sn 6 ] unit and the global σ-aromaticity of [Co 3 @Ge 6 Sn 18 ] 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 H 3 + and Li 3 + . Our findings suggest the potential for synthesizing cluster-of-cluster analogs of discrete all-metallic aromatic species, such as Al 4 2–, and further enhance our understanding of chemical bonding.