PrB 7 − : A Praseodymium‐Doped Boron Cluster with a Pr II Center Coordinated by a Doubly Aromatic Planar η 7 ‐B 7 3− Ligand

The structure and bonding of a Pr‐doped boron cluster (PrB 7 − ) are investigated using photoelectron spectroscopy and quantum chemistry. The adiabatic electron detachment energy of PrB 7 − is found to be low [1.47(8) eV]. A large energy gap is observed between the first and second detachment features, indicating a highly stable neutral PrB 7 . Global minimum searches and comparison between experiment and theory show that PrB 7 − has a half‐sandwich structure with C 6v symmetry. Chemical bonding analyses show that PrB 7 − can be viewed as a Pr II [η 7 ‐B 7 3− ] complex with three unpaired electrons, corresponding to a Pr (4f 2 6s 1 ) open‐shell configuration. Upon detachment of the 6s electron, the neutral PrB 7 cluster is a highly stable Pr III [η 7 ‐B 7 3− ] complex with Pr in its favorite +3 oxidation state. The B 7 3− ligand is found to be highly stable and doubly aromatic with six delocalized π and six delocalized σ electrons and should exist for a series of lanthanide M III [η 7 ‐B 7 3− ] complexes.