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

Nickel K- and L-2,L-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,L-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 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 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.

Automated summary

The Nickel K- and L-2, L-3-edge X-ray absorption spectra (XAS) of 16 complexes and complex ions with nickel centers were studied. It was found that the K-edge XAS alone cannot accurately determine the physical oxidation state of these Ni complexes. On the other hand, the L-2, L-3-edge XAS showed that the physical d-counts of the Ni-IV compounds measured were higher than the d(6) count implied by the oxidation state formalism. Computational analysis of 8 additional complexes confirmed this phenomenon.