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 U-2(-). The threshold of the lowest electron binding energy (EBE) spectral band occurs at 1.0 eV, which corresponds to the electron affinity (EA) of U-2, whereas the vertical detachment energy of U-2(-) is found at EBE similar to 1.2 eV. Electronic structure calculations on U-2 and U-2(-) were carried out with state-of-the-art theoretical methods. The computed values of EA(U-2) and EA(U) and the difference between the computed dissociation energies of U-2 and U-2(-) are found to be internally consistent and consistent with experiment. Analysis of the bonds in U-2 and U-2(-) shows that while U-2 has a formal quintuple bond, U-2(-) 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 U-2 and U-2(-).