φ-Aromaticity in prismatic {Bi$_6$}-based clusters

The occurrence of aromaticity in organic molecules is widely accepted, but its occurrence in purely metallic systems is less widespread. Molecules comprising only metal atoms (M) are known to be able to exhibit aromatic behaviour, sustaining ring currents inside an external magnetic field along M–M connection axes (σ-aromaticity) or above and below the plane (π-aromaticity) for cyclic or cage-type compounds. However, all-metal compounds provide an extension of the electrons’ mobility also in other directions. Here, we show that regular {Bi$_6$} prisms exhibit a non-localizable molecular orbital of f-type symmetry and generate a strong ring current that leads to a behaviour referred to as φ-aromaticity. The experimentally observed heterometallic cluster [{CpRu}$_3$Bi$_6$]–, based on a regular prismatic {Bi$_6$} unit, displays aromatic behaviour; according to quantum chemical calculations, the corresponding hypothetical Bi$_6$$^{2−}$ prism shows a similar behaviour. By contrast, [{(cod)Ir}$_3$Bi$_6$] features a distorted Bi$_6$ moiety that inhibits φ-aromaticity.