Nickel(III) oxide
Nickel oxide is the inorganic compound with the formula Ni2O3. It is not well characterized, and is sometimes referred to as black nickel oxide. Traces of Ni2O3 on nickel surfaces have been mentioned.
A related, more well-characterized Ni-based material is nickel oxide hydroxide, which is likely the reagent employed in organic synthesis since it is generated in aqueous media.
