Metal–Metal Bonding in Actinide Dimers: U₂ and U₂^–

Understanding direct metal–metal bonding between actinide atoms has been an elusive goal in chemistry for years. We report for the first time the anion photoelectron spectrum of U₂^–. The threshold of the lowest electron binding energy (EBE) spectral band occurs at 1.0 eV, which corresponds to the electron affinity (EA) of U₂, whereas the vertical detachment energy of U₂^– is found at EBE ∼ 1.2 eV. Electronic structure calculations on U₂ and U₂^– were carried out with state-of-the-art theoretical methods. The computed values of EA­(U₂) and EA­(U) and the difference between the computed dissociation energies of U₂ and U₂^– are found to be internally consistent and consistent with experiment. Analysis of the bonds in U₂ and U₂^– shows that while U₂ has a formal quintuple bond, U₂^– has a quadruple bond, even if the effective bond orders differ only by 0.5 unit instead of one unit. The resulting experimental-computational synergy elucidates the nature of metal–metal bonding in U₂ and U₂^–.