Nickel is a chemical element with the symbol Ni and atomic number 28.The most common oxidation state of nickel is +2, but compounds of Ni0, Ni+, and Ni3+ are well known.The related nickel(0) complex bis(cyclooctadiene)nickel(0) is a useful catalyst in organonickel chemistry because the cyclooctadiene (or cod) ligands are easily displaced.