Scrutinizing formally Ni^IV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory

Nickel K- and L_2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L_2,3-edge XAS reveals that the physical d-counts of the formally Ni^IV 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-donors are incapable of supporting a physical d⁶ Ni^IV center. The reactivity of Ni^IV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.