Aliphatic CÀH bond functionalization by various metalactive oxygen species M m /O n such as superoxo, peroxo, highvalent oxo-metal complexes, etc. is of great importance for understanding the reaction mechanisms of metalloenzymes and utilizing metal complexes as oxidation catalysts. [1][2][3][4] In some cases, aliphatic C À H bond functionalization starts from hydrogen abstraction, and the resulting alkyl radicals undergo a variety of subsequent reactions. [5][6][7] For example, a bis(malkylperoxo)dicopper(ii) complex was suggested as an intermediate in the reaction of a copper(i) complex, [Cu(iPr 3tacn)] + , [8] with O 2 in the presence of 2,4-tert-butylphenol. [5] Very recently, an alkylperoxopalladium(ii) complex was also reported which was produced from the reaction of a hydroperoxopalladium(ii) complex in the presence of a copper(ii) complex as a catalyst. [6] Although various transition-metalalkylperoxo complexes are known, [9] the alkylperoxo complexes generated as reaction intermediates are limited. Previously, we found that the reaction of [Ni 2 (OH) 2+ , and a bis(m-superoxo)dinickel(ii) complex, [Ni 2 (O 2 ) 2 (Me 3 -tpa) 2 ] 2+ . [10] The decomposition of [Ni 2 (O) 2 (Me 3 -tpa) 2 ] 2+ and [Ni 2 (O 2 ) 2 (Me 3 -tpa) 2 ] 2+ under O 2 caused the oxidative conversion of a methyl group of the Me 3 -tpa ligand to give carboxylate and alkoxide ligands. Furthermore, the carboxylate ligand has been shown to be not derived from the autoxidation of the alkoxide ligand, and some other reaction pathways have been suggested. Herein, we report the formation, structural characterization, and reactivity of a novel bis(m-alkylperoxo)dinickel(ii) complex as a reaction intermediate in the oxidative conversion of the methyl groups of the Me 2 -tpa ligand into carboxylate and alkoxide ligands; this nickel complex is the first example of a structurally characterized dinuclear transition-metal complex with bridging alkylperoxides, although a crystal structure of a main-group-metal complex with a bis(m-alkylperoxo)Zn II 2 core was reported very recently.