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 (<b>1</b>; 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 <i>C</i>_3<i>v</i> symmetry, on account of the lifting of Ni 4<i>p</i>-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 <i>C</i>₃ axis of the complex, which may be attributed to delocalized, multicenter aromatic NiBe₆ bonding.