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 similar to-9.68, -10.42, -5.85, and -5.47 eV for [BeC7H2](2+), [BeSi2C5H2](2+), [MgC7H2](2+), and [MgSi2C5H2](2+), respectively.

Automated summary

The existence of planar pentacoordinate carbon (ppC) atom is theoretically observed in [XC7H2](2+) and [XSi2C5H2](2+) molecules using density functional theory. Inclusion compounds with alkali and alkaline earth metal ions are formed with the tricyclic C7H2 and Si2C5H2 isomers with a planar tetracoordinate carbon (ptC) atom.