I think cyclobut‐3‐ene‐1,2‐diide is non-aromatic since there are two sp3 carbons (carrying negative charge), but my source says it is aromatic. Why?



1 Answer 1


Just because a carbon atom is negatively charged and has a lone pair, it need not be $sp^3$ hybridized. The lone pair could be on a $\pi$-bonding $p$ orbital with the atom hybridized $sp^2$. If you allow for this possibility, you find that the given ion can satisfy the $4n+2$ rule for aromaticity, which the text apparently is claiming.

Whether the ion is actually a stable aromatic species is more complicated. Formation of a highly strained ring, the high charge density, and a relatively low amount of resonance stabilization for four-member aromatic rings all work against forming a strongly stabilized ring (the cyclopropenyl cation is also highly strained, but it has less charge density and greater resonance stabilization). Known cyclobutadiene-dianion compounds are stabilized by using substituents that allow some dispersion of the charge and closely associating the counterions above or below the ring. See for instance References 1 and [2].

enter image description here

Structure of a lithium cyclobutadienide complex, from Ref. [2]. Unlabeled atoms in the ring substituents and solvent molecules are carbon; hydrogen atoms are not shown.


  1. Akira Sekiguchi, Tsukasa Matsuo & Hidetoshi Watanabe (2001). "Cyclobutadiene Dianion Dilithium: a New Aromatic Ring System**. Phosphorus, Sulfur, and Silicon and the Related Elements 168:1, 51-58, https://doi.org/10.1080/10426500108546530.

  2. Akira Sekiguchi, Tsukasa Matsuo, and Hidetoshi Watanabe (2000). "Synthesis and Characterization of a Cyclobutadiene Dianion Dilithium Salt:  Evidence for Aromaticity". J. Am. Chem. Soc. 122, 23, 5652–5653. https://doi.org/10.1021/ja0004175


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.