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Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation reactions. While Ni(0), Ni( i ), and Ni( ii ) intermediates are most relevant in these transformations, recently Ni( iii ) and Ni( iv ) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni( ii ) and Ni( iii ) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni( ii ) complexes with various other oxidants led to exclusive C–C bond formation in very good yields, the use of O 2 or H 2 O 2 as oxidants led to formation of appreciable amounts of C–O bond formation products, especially for the Ni( ii ) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.