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.

Treatment of the osmabenzyne Os(equivalent to CC(SiMe3)=C(Me)C(SiMe3)=CH)Cl-2(PPh3)(2) (1) with 2,2'-bipyridine (bipy) and thallium triflate (TlOTf) produces the thermally stable dicationic osmabenzyne [Os( equivalent to CC(SiMe3)=C(Me)C(SiMe3)=CH)(bipy)(PPh3)(2)](OTf)(2) (2). The dicationic osmabenzyne 2 reacts with ROH (R = H, Me) to give osmabenzene complexes [Os(=C(OR)CH=C(Me)C(SiMe3)=CH)(bipy)(PPh3)(2)]OTf, in which the metallabenzene ring deviates significantly from planarity.