A crystalline tri-thorium cluster with σ-aromatic metal-metal bonding.

Metal-metal bonding is a widely studied area of chemistry1–3, and has become a mature field spanning numerous d transition metal and main group complexes4–7. In contrast, actinide-actinide bonding is predicted to be weak8, being currently restricted to spectroscopically-detected gas-phase U2 and Th29,10, U2H2 and U2H4 in frozen matrices at 6-7 Kelvin (K)11,12, or fullerene-encapsulated U213. Conversely, attempts to prepare thorium-thorium bonds in frozen matrices produced only ThHn (n = 1-4)14. Thus, there are no isolable actinide-actinide bonds under normal conditions. Computational investigations have explored the likely nature of actinide-actinide bonding15, concentrating on localised σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium-thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground-state with a valence-delocalised three-centre-two-electron σ-aromatic bond16,17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalised σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborating actinide-actinide bonding.