Researchers have always been intrigued by one-step selective hydroxylation of aromatic compounds since achieving adequate selectivity toward the target product and avoiding overoxidation are challenging. However, the naturally available oxygenase enzymes efficiently and selectively perform such transformations under normal atmospheric conditions. Inspired by naturally occurring oxygenase enzymes, researchers have undertaken numerous attempts to catalyze the oxidation of arenes using homogeneous and heterogeneous catalysts. In this regard, the first-row transition metal complexes are considered as candidates due to their abundance and redox characteristics. Therefore, this review focuses on catalyst design strategies to improve the efficiency and selectivity of arene oxidation and illustrates the distinctive homogeneous approaches reported in this area by employing first-row transition metal complexes.

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Oxygen atom transfer (OAT) reactions catalyzed by metal complexes have been a subject of intensive research over the century, owing to the prevalent involvement of OAT in organic transformations mediated by several biologically important enzymes such as methane monooxygenases, cytochrome P450, etc as well as in synthetic chemical reactions. In biomimetic model chemistry, iron and manganese complexes are the most exploited catalysts in the realm of oxygen atom transfer reactions and many of these metal complexes produce very short-lived reactive metal.oxygen intermediates during the catalytic reactions. Characterization of such reactive intermediates of numerous heme and non-heme iron and manganese complexes and comparing them with their natural enzyme analogous have emerged as a promising approach toward understanding several intricate enzymatic mechanisms. Considerable research advancements in the studies of OAT reactions involving late transition metal complexes such as cobalt, nickel, and copper have also been recognized in the past few years. In this account, various reports have been published, demonstrating catalytic oxidation of organic substrates by the active nickel-oxygen species generated either via heterolysis or homolysis of O-O bond of oxidant bound nickel complexes. This book chapter aims at a comprehensive summary of noteworthy attempts contributed towards nickel catalyzed OAT reactions and various implicated or well-characterized nickel.oxygen active intermediates. The effect of stereoelectronic properties of ligand architecture on catalytic efficiency and various characterization techniques used to identify the catalytically active nickel-oxygen species are also discussed.

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