When Three Electrons Are Not A Crowd

If a three-electron bond sounds weird, it is. The bonding picture can be thought of as a resonance between a pair of electrons on one atom and a lone electron on the partner atom—a type of radical cation. Examples involving π-delocalized electrons, for example in bonds involving sulfur atoms in heterocyclic rings, are established. But cases where the electrons largely remain localized in a σ bond between two sulfur atoms had not been pinned down until now. Xinping Wang and coworkers at Nanjing University, in China, report the first crystal structure evidence for a three-electron σ bond in substituted naphthalenes with two sulfur atoms (shown) in close proximity on adjacent rings (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja507918c). Sulfur-based three-electron σ-bonded radicals are of interest because they are key intermediates in organic reactions and biochemical processes and play important roles in materials science.