The first Zintl cluster containing a distorted Bi6 triangular prism, [Bi6Mo3(CO)9]4−, has been synthesized and structurally characterized. Quantum chemical calculations indicated that the distorted cage cluster features multiple local σ-aromaticity.

https://pubs.rsc.org/en/content/articlelanding/2021/CC/D1CC00734C#!divAbstract

Based on Hückel’s and Baird’s rules, species are aromatic either in the lowest singlet state (S0) or the lowest triplet state (T1) only. Thus, species with adaptive aromaticity (with aromaticity in both the S0 and T1 states) is particularly rare. On the other hand, σ-aromaticity in the T1 state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive s-aromaticity in an unsaturated three-membered ring, which is a traditional area dominated by π-aromaticity.

In comparison with the widely recognized π aromaticity, σ aromaticity is a less developed concept in chemistry, especially for unsaturated systems. Moreover, most studies on σ aromaticity have been mainly limited to the ground state of saturated systems; unsaturated species with σ aromaticity in the excited state have never been reported.