Dimetalloendofullerene U2@C60 has a U-U multiple bond consisting of sixfold one-electron-two-center bonds

Endohedral metallofullerenes (EMFs) have been extensively studied since their discovery in 1985. Metal-metal bonds, nevertheless, have never been explicitly observed in EMFs synthesized so far. In this contribution, we show by means of all-electron relativistic density functional computations that the dimetalloendofullerene, U-2@C-60, has an unprecedented U-U multiple bond consisting solely of sixfold ferromagnetically coupled one-electron-two-center bonds with the electronic configuration (5f pi(u))(2)(5f sigma(g))(1)(5f delta(g))(2)(5f phi(u))(1), which are dominated by the uranium 5f atomic orbitals. This bonding scheme is completely distinct from the metal-metal bonds discovered thus far in the d- and f-block polynuclear metal complexes. This finding initiates a connection of the metal-metal multiple bonding chemistry and the fullerene chemistry.