According to Hückel’s and Baird’s rules, cyclic conjugated species are aromatic either in the ground state or in the excited state only. Thus, species with aromaticity in both states (denoted as adaptive aromaticity) are particularly rare. Here we carry out density functional theory calculations on a series of osmapyridine and osmapyridinium complexes (96 species) and find that two of them display adaptive aromaticity, which was verified by various aromaticity indices including HOMA, ELFπ, MCI, ACIDπ plots and the heat of hydrogenation. Further study reveals that two osmapyridiniums containing one or two phosphonium substituents exhibit the character of the triplet ground state, which was supported by the high-level coupled cluster calculations. Our findings highlight the importance of a transition metal and phosphoniums in achieving adaptive aromaticity and triplet ground state and may aid the design of organometallics for photochemical and molecular magnetism applications.

https://pubs.acs.org/doi/abs/10.1021/jacs.8b11564