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

Metal-metal bonding is a widely studied area of chemistry^1-3, and has become a mature field spanning numerous d transition metal and main group complexes^4-7. By contrast, actinide-actinide bonding, which is predicted to be weak⁸, is currently restricted to spectroscopically detected gas-phase U₂ and Th₂ (refs. ^9,10), U₂H₂ and U₂H₄ in frozen matrices at 6-7 K (refs. ^11,12), or fullerene-encapsulated U₂ (ref. ¹³). Furthermore, attempts to prepare thorium-thorium bonds in frozen matrices have produced only ThH_n (n = 1-4)¹⁴. Thus, there are no isolable actinide-actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide-actinide bonding¹⁵, concentrating on localized 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-delocalized three-centre-two-electron σ-aromatic bond^16,17 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 delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide-actinide bonding.