Lanthanide-nickel molecular intermetallic complexes featuring a ligand-free Ni^2- anion in endohedral fullerenes

Transition metals (TMs) typically exhibit rich redox chemistry and can be found in various oxidation states. In most cases, TMs are positively charged. Strong π-accepting ligands have been shown to stabilize molecular complexes with TMs in formal negative oxidation states. By contrast, organic-ligand-free TM anions remain rare, limited to intermetallic compounds based on third-row TMs such as gold or platinum. Here we report the synthesis of air-stable lanthanide-nickel molecular intermetallic complexes featuring a ligand-free Ni^2- confined within fullerenes, namely, Tb₂Ni@C₈₂. The charged Tb₂Ni lanthanide nickelide cluster forms metal-only Lewis pairs, featuring strongly polarized Tb-Ni covalent bonds with short bond lengths in the range of 2.50-2.57 Å. X-ray absorption spectroscopy supports the -2 oxidation state of Ni with 3d¹⁰4s² electron count, in line with the spectroscopic and magnetic measurements, and theoretical study. This finding opens up an efficient way to stabilize intermetallic clusters with elusive nucleophilic TM anions by confining them inside molecular carbon cages.