Scrutinizing Formally NiIV Centers through the Lenses of Core Spectroscopy, Molecular Orbital Theory, and Valence Bond Theory

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 13 molecules with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS is shown to be an unreliable metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62– is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F– is incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes in C–C bond forming reactions is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.