Aromaticity of Four-Membered-Ring 6π-Electron Systems: N ₂S₂ and Li₂C₄H₄

N₂S₂ is a four-membered-ring system with 6π electrons. While earlier proposals considered N₂S₂ to be aromatic, recent electronic structure calculations claimed that N ₂S₂ is a singlet diradical. Our careful reexamination does not support this assertion. N₂S₂ is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the π electrons). These characteristics as well as the electronic structure of N₂S₂ are compared with those for an isoelectronic π system, Li₂C₄H₄, motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N₂S₂ and Li₂C₄H₄ are both essentially 2π-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6π electrons. This is because four of the six π electrons occupy the nonbonding π HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N₂S₂ and Li₂C ₄H₄, respectively.