From π Bonds without σ Bonds to the Longest Metal–Metal Bond Ever: A Survey on Actinide–Actinide Bonding in Fullerenes

Actinide–actinide bonds are rare. Only a few experimental systems with An–An bonds have been described so far. Recent experimental characterization of the U₂@I_h(7)-C₈₀ (J. Am. Chem. Soc. 2018, 140, 3907) system with one-electron two-center (OETC) U–U bonds as was predicted by some of us (Phys. Chem. Chem. Phys. 2015, 17, 24182) encourages the search for more examples of actinide–actinide bonding in fullerene cages. Here, we investigate actinide–actinide bonding in An₂@D_5h(1)-C₇₀, An₂@I_h(7)-C₈₀, and An₂@D_5h(1)-C₉₀ (An = Ac–Cm) endohedral metallofullerenes (EMFs). Using different methods of the chemical bonding analysis, we show that most of the studied An₂@C₇₀ and An₂@C₈₀ systems feature one or more one-electron two-center actinide–actinide bonds. Unique bonding patterns are revealed in plutonium EMFs. The Pu₂@I_h(7)-C₈₀ features two OETC Pu–Pu π bonds without any evidence of a corresponding σ bond. In the Pu₂@D_5h(1)-C₉₀ with r_Pu–Pu = 5.9 Å, theory predicts the longest metal–metal bond ever described. Predicted systems are thermodynamically stable and should be, in principle, experimentally accessible, though radioactivity of studied metals may be a serious obstacle.