# Why are antiaromatic compounds unstable?

Antiaromaticity is so destabilizing that it can cause compounds such as cyclobutadiene to elongate or manipulate their orbitals so that the pi system is no longer aromatic. I understand that aromatic compounds are stabilized because they have a full HOMO and that antiaromatic compounds have a half filled HOMO, which according to this question is the reason that they are unstable. I don't agree with this explanation because in other compounds, half filled orbitals cause compounds to be stabilized. For example, triplet oxygen is significantly more common than singlet oxygen in nature. Also, some transition metals will promote electrons from their S orbitals to D orbitals so that they can have these shells half filled. I know that full orbitals are more stable than half filled orbitals, but are half filled orbitals destabilizing in alkenes while they are stabilizing in oxygen and metals? If not, why are antiaromatic compounds so unstable?

• chemistry.stackexchange.com/questions/18557/… May 8 '16 at 1:03
• First of all, they are not unstable, just less stable and less symmetric, because the Jahn-Teller effect distorts their geometry to remove degeneracy. With $\ce{O2}$ it would not work that way, since you can't really distort a 2-atom molecule; no matter what you do, its symmetry is still the same. May 8 '16 at 6:55
• But, correct me if I am wrong, the Jahn-Teller effect distorts their geometry so that they are no longer antiaromatic. Compounds distort their orbitals to avoid the instability associeated with antiaromaticity. The Jahn-Teller effect is a symptom of antiaromatic instability, but I don't think it is it's cause. May 8 '16 at 16:55
• chemistry.stackexchange.com/questions/30927/… May 8 '16 at 17:28
• Re. your first comment, you are wrong about that. The antiaromatic instability is an instability with respect to a structure where the double bonds are localised, i.e. instability with respect to a Jahn-Teller distortion. The Jahn-Teller distortion leads to the adoption of exactly such a geometry: one with alternating bond lengths and that is not necessarily planar. So, the JT instability and the "antiaromatic instability" are one and the same. May 9 '16 at 18:46