Cyclobutadienyl dianion
= Cyclobutadienyl dianion
{title2=$\mathrm{C_4H_4^{2-}}$}
The six-pi-electron ion $\mathrm{C_4H_4^{2-}}$. In an ideal equal-coupling four-site Hückel ring, one bonding and two nonbonding pi orbitals are filled, leaving the highest antibonding orbital empty.