Since the concept of aromaticity was first introduced in transition metal complexes, metals have become a crucial component for modulating aromaticity, leading to a variety of structural frameworks. Initial studies successfully achieved aromatic pentalene dianions through metal-ion coordination, and recent advancements in bridgehead metallapentalenes have demonstrated the transformation of antiaromaticity into aromaticity.

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.

Aromaticity is a fundamental chemical concept of ever-increasing diversity. According to Hückel’s and Baird’s rules, cyclic conjugated species with 4n+2 π-electrons are aromatic in the singlet electronic ground state (S0) and antiaromatic in the lowest triplet state (T1), and vice-versa. Thus, species with aromaticity in both states have not yet been reported.