The activation of carbon-hydrogen (C-H) bonds is the first step of functionalizing hydrocarbons. This transformation is a key step in many biological and synthetic processes. Soluble methane monooxygenase (sMMO) is a representative example of nonheme dinuclear iron enzymes that hydroxylate strong aliphatic C-H bonds at mild conditions by a high-valent FeIV2(μ-O)2 “diamond core” intermediate. One of our research thrusts is to develop bio-inspired high-valent M2(μ-O)2 complexes of late-transition metals for activating and functionalizing inert aliphatic C-H bonds. We have generated a mixed-valent CoIII,IV2(μ-O)2 species by one-electron oxidation of its CoIII2(μ-O)2 precursor, and characterized it using combined spectroscopic and computational approaches. This high-valent species cleaves strong aliphatic C-H bonds with decent rate constants at cryo-temperatures to afford the oxygenated products. Interestingly, CoIII,IV2(μ-O)2 can interact with Lewis bases to open up its diamond core and convert to an open core species, which is a much more powerful oxidant with substrate C-H bonds than the diamond core itself. Beyond activating aliphatic C-H bonds, our Co2(μ-O)2 complexes have been found capable of carrying out a variety of novel bond breaking and bond forming processes.