Planar pentacoordinate carbon in CGa₅⁺ derivatives

We report a family of systems having a planar pentacoordinate carbon (ppC) based on the next heavier analogue of CAl₅⁺, the ppC system par excellence. Although because of the larger size of Ga, the ppC isomer is not even a local minimum in CGa₅⁺, a single isoelectronic substitution of Ga by smaller sized Be maximizes the bonding in the ppC form. Retaining the 18 valence electron rule, the global minimum structures of CGa₄Be, CGa₃Be₂^-, CGa₂Be₃^2-, and CGaBe₄^3- clusters and their corresponding lithium salts have a ppC.