Planar pentacoordinate carbon in [XC₇H₂]²⁺ (X = Be and Mg) and its derivatives

The planar pentacoordinate carbon (ppC) atom is theoretically established here in [XC₇H₂]²⁺ and [XSi₂C₅H₂]²⁺, where X = Be and Mg, using density functional theory. Inclusion compounds with alkali and alkaline earth metal ions are identified with the monomer units of tricyclic C₇H₂ and Si₂C₅H₂ isomers with a planar tetracoordinate carbon (ptC) atom. While all alkali and some alkaline earth metals (Ca²⁺, Sr²⁺, and Ba²⁺) stabilize the ptC isomer in both cases, Be²⁺ and Mg²⁺ ions form a bond directly with the ptC atom, thus making it a ppC atom. The theoretical binding energies computed at the PBE0-D3/def2-TZVP level of theory are ∼-9.68, -10.42, -5.85, and -5.47 eV for [BeC₇H₂]²⁺, [BeSi₂C₅H₂]²⁺, [MgC₇H₂]²⁺, and [MgSi₂C₅H₂]²⁺, respectively.