Planar pentacoordinate carbon in [XC 7 H 2 ] 2+ (X = Be and Mg) and its derivatives.

The planar pentacoordinate carbon (ppC) atom is theoretically established here in [XC 7 H 2 ] 2+ and [XSi 2 C 5 H 2 ] 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 C 7 H 2 and Si 2 C 5 H 2 isomers with a planar tetracoordinate carbon (ptC) atom. While all alkali and some alkaline earth metals (Ca 2+ , Sr 2+ , and Ba 2+ ) stabilize the ptC isomer in both cases, Be 2+ and Mg 2+ 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 [BeC 7 H 2 ] 2+ , [BeSi 2 C 5 H 2 ] 2+ , [MgC 7 H 2 ] 2+ , and [MgSi 2 C 5 H 2 ] 2+ , respectively.