Cerium Dimer Anion and the Contribution of 4f Electrons to Lanthanide Metal–Metal Bonds

Direct metal–metal bonding between lanthanide atoms has been challenging to observe. We report on the first spectroscopic characterization of the cerium dimer anion (Ce₂^–) and its neutral analog (Ce₂) in the gas phase, achieved using photoelectron and ultrafast spectroscopy combined with high-level quantum chemistry calculations. The electron affinity of Ce₂ is 0.24 eV, from which a dissociation energy of 2.21 eV is derived for Ce₂^–. The wave-packet dynamics upon photodetachment are studied and yield vibrational frequencies for electronically excited Ce₂. Ce₂^– exhibits a conventional metal–metal triple bond with minimal contribution from 4f electrons. However, evidence of 4f-electron participation in bonding is identified for the low-energy excited states only 0.1 eV higher. The results challenge the assumption of inert 4f electrons in metal–metal bonding, and we propose a promising strategy for forming stable lanthanide–lanthanide bonds involving significant 4f-electron contributions.