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 (μ-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-μ-oxido-bridged structure as [(L) 2 Ni(IV) 2 (μ-O) 3 ] 2+ . Its structure and its reactivity with organic substrates are demonstrated through a combination of UV-vis absorption, resonance Raman, 1 H 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

The Ubbo Emmius fund of the University of Groningen, the European Research Council (StG, no. 279549, W.R.B.), NWO for a VIDI grant (723.014.010, D.J.M. and M.T.), The Netherlands Ministry of Education, Culture and Science (Gravity program 024.001.035, W.R.B.), MINECO (CTQ2014-59212-P and CTQ2015-70851-ERC, M.S.), Gen- Cat (2014SGR1202, M.S.), FEDER (UNGI10-4E-801, M.S.), and COST action CM1305 “ECOSTBio” (W.R.B., COSTSTSM- CM1305-29045) are acknowledged for financial support