Its layered structure resembles that of the brucite polymorph of nickel(II) hydroxide, but with half as many hydrogens. The oxidation state of nickel is 3+. It can be prepared by the reaction of nickel(II) hydroxide with aqueous potassium hydroxide and bromine as the oxidant:
: 2 Ni(OH) + 2 KOH + Br  →  2 KBr + 2 HO + 2 NiOOH