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.