An unusual iridium dimer 1, [(4-cpbo)Ir(μ-Cl)(μ-O)Ir(4-cpbo)] (4-cpbo = 4-chlorophenylbenzoxazolato-N,C2), was obtained by the oxidative addition of oxygen and reductive elimination of chlorine from a precursor [(4-cpbo)2Ir(μ-Cl)]2. This iridium dimer 1 has a metastable structure with an asymmetric bridge, can spontaneously release metalloradicals and coordinatively unsaturated species in solution at room temperature, and exhibits high catalytic ability for synthetic applications.