Advances in accessing rare oxidation states of nickel for catalytic innovation

Nickel catalysis has experienced a renaissance over the past two decades, driven by its ability to access diverse oxidation states (0 to +4) and unique reactivity. This review consolidates the advancements in nickel chemistry, providing an overview of ligands that stabilize specific nickel oxidation states. The stability, reactivity, and catalytic applications of Ni⁰ sources, including in situ generation from air- and moisture-stable Ni^II precursors, are discussed, along with the roles of Ni^I and Ni^III intermediates in catalytic cycles. The progress in synthesizing and utilizing Ni^IV complexes highlights their emerging importance in catalysis. Advances in spectroscopic and theoretical tools have enhanced the understanding of nickel's complex catalytic behavior.