Revisiting the bonding nature of pyramidane: an analogue of the CO molecule

Pyramidane (C(C₄H₄)) and its derivatives have garnered considerable interest in organic and synthetic chemistry due to their distinctive pyramidal geometry. Nevertheless, the non-classical bonding pattern between the pyramidal apex and base remains insufficiently elucidated. This work firstly developed a two-dimensional (2D) superatom-atom super bonding framework, providing new insights into the bonding nature of C(C₄H₄). Specifically, the π-conjugated C₄H₄ unit acts as a 2D ^◊O superatom with four π-electrons, enabling interaction with the apical carbon atom to form a CO-type superatomic molecule via a super triple bond, satisfying the electron closed shell for both ^◊O and C. Subsequently, a series of coordination complexes, Pd[C(C₄H₄)]_n (n = 1-4), are designed to further explore the chemical bonding abilities, wherein each C(C₄H₄) interacts with the Pd center via a σ bond and several multicenter d-π* bonds. Moreover, we design two stable 2D all-carbon monolayers derived from pyramidane-based assemblies, which exhibit good stability, feasible synthetic accessibility, and moderate band gaps under certain strain conditions, suggesting potential electronic applications. This work revisits the bonding paradigm of C(C₄H₄) and broadens our understanding of chemical interactions, offering a new strategy for the design of clusters and materials via 2D superatom-atom bonding.