Binary mono-anions with unprecedented anti-aromatic planar tetracoordinate carbon and nitrogen atoms.

Global minima (CBe 4 - and NBe 4 - ) with planar tetracoordinate carbon and nitrogen atoms are stabilized by multicentric bonds. Their unusual anti-aromaticity is due to the strong localized Be-Be bonds preventing the full delocalization of the electrons in the CBe 4 /NBe 4 skeleton. These binary mono-anions are expected to be experimentally realized in the gas phase.