Combining aromaticity and hyperconjugation, two important concepts in organic chemistry, leads to hyperconjugative aromaticity, which was first proposed by Mulliken in 1939. However, previous studies on hyperconjugative aromaticity have mainly focused on substituents containing either main-group elements (group 14) or transition metals in groups 7, 9, 10, and 11. In this study, we perform density functional theory (DFT) calculations on cyclopentadiene and pyrrolium derivatives containing groups 13, 15 and 16 substituents to examine the possibility of achieving hyperconjugative aromaticity.

In general, species could be aromatic in the ground or excited state only according to Hückel’s and Baird’s rules. Thus, adaptive aromatics are particularly rare as they can be aromatic in both the lowest singlet and triplet states (S0 and T1). Here, we carry out density functional theory calculations on a series of rhenium oxo complexes to examine their structure, bonding, and aromaticity. It is found that all these complexes are aromatic in the S0 state. In contrast, their T1 states could be antiaromatic, nonaromatic, or aromatic depending on the ligands.

Osmapentalyne and osmapentalene complexes, now termed as carbolong species, have attracted considerable attention due to their novel structures, reactivities, chelating properties as well as Möbius and adaptive aromaticity. On the other hand, boron monofluoride (BF), a 10-electron diatomic molecule isoelectronic to carbon monoxide (CO), is unstable below 1800°C in the gas phase, and preparation of its metal complex is particularly challenging.