Molecular Aromaticity at the Edge of the Periodic Table

The recent discovery of the cyclically delocalized three-center two-electron σ-bonding in the crystalline actinide cluster isolated by Liddle and co-workers has sparked a heated debate on the role of molecular aromaticity on the periphery of the periodic table. It has been suggested that the tri-thorium ring at the heart of the cluster features considerable aromatic stabilization comparable to the heterocyclic π-aromatic rings such as thiophene. In this Communication we challenge previous conclusions by showing from first principles that the tri-thorium bonding does not fulfill the very fundamental requirement by IUPAC of being distinctly stabilized by cyclic delocalization of electrons, and therefore it should be classified as non-aromatic once and for all.