A Crystalline NiX₆ Complex

High-valent nickel species are implicated as intermediates in industrially relevant chemical transformations and in the catalytic cycles of metalloenzymes. Although a small number of tetravalent NiX₄ complexes have been crystallographically characterized, higher nickel valence states have not been identified. Here we report a stable, crystalline NiX₆ complex, Ni(BeCp)₆ (1; cyclopentadienyl anion (Cp)), formed by the insertion of zerovalent nickel into three Be-Be bonds. This 16-electron species features an inverted ligand field, is diamagnetic, and exhibits C₃ symmetry, on account of the lifting of Ni 4p-orbital degeneracy in this molecular geometry. Single-crystal X-ray diffraction and quantum chemical calculations both reveal a toroidal band of electron density perpendicular to the C₃ axis of the complex, which may be attributed to delocalized, multicenter aromatic NiBe₆ bonding.