Adaptive σ aromaticity in the rhenacyclopropene rings.

Species generally exhibit one-state aromaticity either in the lowest singlet state (S 0 ) or the lowest triplet state (T 1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S 0 and T 1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T 1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S 0 state could hold in the T 1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.