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

• Published in: Chemical science (Cambridge) 2023-06, Vol.14 (25), p.6915-6929
• Main Authors: DiMucci, Ida M, Titus, Charles J, Nordlund, Dennis, Bour, James R, Chong, Eugene, Grigas, Dylan P, Hu, Chi-Herng, Kosobokov, Mikhail D, Martin, Caleb D, Mirica, Liviu M, Nebra, Noel, Vicic, David A, Yorks, Lydia L, Yruegas, Sam, MacMillan, Samantha N, Shearer, Jason, Lancaster, Kyle M
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d3sc02001k

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. Nickel K- and L 2,3 -edge X-ray absorption spectra (XAS) are discussed for coordinated nickel centers spanning formal oxidation states from II to IV. The ensuing analysis indicates that a physical "+4" oxidation state is untenable for coordinated Ni.