Cyclobutadienyl dianion (${\mathrm{C}}_4{\mathrm{H}}_4^{2-}$)

The six-pi-electron ion ${\mathrm{C}}_4{\mathrm{H}}_4^{2-}$. In an ideal equal-coupling four-site Hückel ring, one bonding and two nonbonding pi orbitals are filled, leaving the highest antibonding orbital empty.