Persistent Planar Tetracoordinate Carbon in Global Minima Structures of Silicon-Carbon Clusters

Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H⁺ ) from the cyclopentadienyl anion (C₅ H₅⁻ ) or the pentalene dianion (C₈ H₆²⁻ ) by three or four E²⁺ dications (E = Si–Pb), respec-tively. The silicon derivatives of these series are the Si₃ C₅ and Si₄ C₈ clusters. Here we show that ptC persists in some clusters with an equivalent number of C and Si atoms, i.e., Si₅ C₅, Si₈ C₈, and Si₉ C₉ . In all these species, the ptC is embedded in a pentagonal C₅ ring and participates in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Furthermore, these clusters are π-aromatic species according to chemical bonding analysis and magnetic criteria.