Aromaticity of four-membered-ring 6π-electron systems:: N2S2 and Li2C4H4

N2S2 is a four-membered-ring system with 6.pi electrons. While earlier proposals considered N2S2 to be aromatic, recent electronic structure calculations claimed that N2S2 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 T electrons). These characteristics as well as the electronic structure of N2S2 are compared with those for an isoelectronic pi system, Li2C4H4, motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N2S2 and Li2C4H4 are both essentially 2pi-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6pi electrons. This is because four of the six pi electrons occupy the nonbonding pi HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antiboncling effect can be said to reduce the aromatic character by approximately 7% and 4% for N2S2 and Li2C4H4, respectively.