All-metal aromatic ring isolated for the first time

Amazing stuff! Who does not remember Kekule's famous dream of a ring shaped benzine molecule?

"A four-atom bismuth species is the first all-metal ring with aromatic bonding character to have been isolated in the lab. The structure was synthesised by researchers in Germany, who say that their findings raise important questions about the nature of aromaticity in materials composed of heavier elements. ..."

From the abstract:

"Aromaticity in organic molecules is well defined, but its role in metal-only rings remains controversial. Here we introduce a supramolecular stabilization approach of a cationic {Bi4} rhomboid within the symmetric charge sphere of two bowl-shaped dianionic calix pyrrolato indinates. Crystallographic and spectroscopic characterization, quantum chemical analysis and magnetically induced ring currents indicate σ-aromaticity in the formally tetracationic 16-valence electron [Bi4]4+ ring. Computational screening for other p-block elements identifies the planar rhomboid as the globally preferred structure for 16-valence electron four-atomic clusters. The aromatic [Bi4]4+ is isoelectronic to the [Al4]4−, a motif previously observed as antiaromatic in Li3[Al4]− in the gas phase. Thus, subtle factors such as charge isotropy seem to decide over aromaticity or antiaromaticity, advising for caution in debates based on the Hückel model—a concept valid for second-row elements but less deterministic for the heavier congeners."

All-metal aromatic ring isolated for the first time | Research | Chemistry World

Supramolecular trapping of a cationic all-metal σ-aromatic {Bi4} ring (open access)

The bismuth ring is the first of its kind to be isolated in the lab

Fig. 1: Examples of aromatic rings.