Adaptive Aromaticity in 18e Metallapentalenes

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hu''ckel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.

Automated summary

Complexes with adaptive aromaticity, which possess aromaticity in both the S0 and T1 states, are rare. In this study, we show that 18e metallapentalenes can exhibit adaptive aromaticity when nitrido and imido ligands are introduced. The strength of the metal-carbon bond is found to play a crucial role in S0 state aromaticity, while T1 state aromaticity is attributed to spin electron localization. These findings are valuable for the development of metallaaromatic chemistry.