Aromaticity at the Edge of the Periodic Table: Cyclic Delocalization Without Aromatic Stabilization

The discovery of the three-center two-electron σ-bonding Th3 in the crystalline actinide cluster isolated by Boronski et al. has been hailed for extending the range of aromaticity to the record seventh row of the periodic table. However, it has recently been shown that high symmetry and unusual thermodynamic stability of this cluster is determined by the unique charge-shift bonding (ThCl2)3 rather than σ-aromaticity. In the most recent follow-up investigation, Lin and Mo has argued that the experimentally observed charge delocalization in the tri-thorium core may be associated with considerable aromatic stabilization after all. In the following we challenge this conclusion by showing that the theoretical models used by the authors dramatically exaggerate charge delocalization, while the Th3 σ-bonding in the real crystalline cluster is so weak that it does not fulfill the very fundamental requirement of being distinctly stabilized by cyclic delocalization of electrons. Thus, the σ-bonding in Th3 should be classified as non-aromatic once and for all.