In accordance with the constraints by Hückel’s and Baird’s rules, species generally exhibit aromaticity in one state (the lowest singlet state S0 or the lowest triplet state T1). Consequently, species with adaptive aromaticity (being aromatic in both the S0 and T1 states) are particularly rare. In this study, density functional theory (DFT) was employed to investigate adaptive aromaticity in 14e–, 16e– and 18e– metallabenzenes.

Aromaticity, one of the central topics in chemistry, has attracted continuing interest of both experimentalists and theoreticians.

The concept of aromaticity has long played an important role in chemistry and continues to fascinate both experimentalists and theoreticians. Among the archetypal aromatic compounds, heteroaromatics are particularly attractive. Recently, substitution of a transition-metal fragment for a carbon atom in the anti-aromatic hydrocarbon pentalene has led to the new heteroaromatic osmapentalenes. However, construction of the aza-homolog of osmapentalenes cannot be accomplished by a similar synthetic manipulation.

The nonplanarity found in metallabenzene complexes has been investigated theoretically via density functional theory (DFT) calculations. A metallabenzene has four occupied π molecular orbitals (8 π electrons) instead of three that benzene has. Our electronic structure analyses show that the extra occupied π molecular orbital, which is the highest occupied molecular orbital (HOMO) in many metallabenzenes, has antibonding interactions between the metal center and the metal-bonded ring-carbon atoms, providing the electronic driving force toward nonplanarity.