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@Ih(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@D5h(1)-C70, An2@Ih(7)-C80, and An2@D5h(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@Ih(7)-C80 features two OETC Pu–Pu
π bonds without any evidence of a corresponding σ bond.
In the Pu2@D5h(1)-C90 with rPu–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.