A reactive high-valent dinuclear nickel(IV) oxido bridged complex is reported that can be formed at room temperature by reaction of [(L)(2)Ni(II)(2)(mu-X)(3)]X (X = Cl or Br) with NaOCl in methanol or acetonitrile (where L = 1,4,7 - trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-mu-oxido-bridged structure as [(L)(2)Ni(IV)(2)(mu-O)(3)](2+). Its structure and its reactivity with organic substrates are demonstrated through a combination of UV-vis absorption, resonance Raman, H-1 NMR, EPR, and X-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods, The identification. of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidations.