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 d ⁶ count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF ₆ ^2- 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 d ⁶ 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.
Competing Interests: There are no conflicts to declare.
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