The Curious Case of the Crystalline Tri-Thorium Cluster: Cyclic Delocalization Without Aromatic Stabilization

The discovery of the delocalized tri-thorium σ-bonding in the crystalline actinide cluster isolated by Boronski et al. has been hailed for extending the concept of aromaticity to the record seventh row of the periodic table, which sparked a heated debate on the magnitude of aromatic stabilization and its actual impact on the thermodynamic stability of the cluster. Most recently, Lin and Mo demonstrated that the experimentally observed delocalization of charge in the Th3 ring may be associated with considerable aromatic stabilization effect, comparable with archetypical heteroaromatics like thiophene and pyrrole. In the following, we first challenge this conclusion by showing that the highly ionized model clusters studied by Lin and Mo dramatically exaggerate σ-bonding and charge delocalization, and thus they cannot be used to justify σ-aromaticity in the real actinide cluster. Next, we demonstrate that the Th3 bonding is actually too weak to fulfill the very fundamental requirement (by IUPAC) of being distinctly stabilized by cyclic delocalization of electrons compared to the linear isomer, and therefore it should be classified as non-aromatic once and for all.