Nickel nitrate hexahydrate

Nickel nitrate is the inorganic compound Ni(NO3)2 or any hydrate thereof. The anhydrous form is not commonly encountered, thus "nickel nitrate" usually refers to nickel(II) nitrate hexahydrate. The formula for this species is written in two ways. Ni(NO3)2.6H2O and, more descriptively [Ni(H2O)6](NO3)2. The latter formula indicates that the nickel(II) center is surrounded by six water molecules in this hydrated salt. In the hexahydrate, the nitrate anions are not bonded to nickel. Also known are three other hydrates: Ni(NO3)2.9H2O, Ni(NO3)2.4H2O, and Ni(NO3)2.2H2O. Anhydrous Ni(NO3)2 is also known.
It is prepared by the reaction of nickel oxide with nitric acid:
NiO + 2 HNO3 + 5 H2O → Ni(NO3)2.6H2O
The anhydrous nickel nitrate is typically not prepared by the heating the hydrates. Rather is generated by reaction of hydrates with dinitrogen pentoxide or of nickel carbonyl with dinitrogen tetroxide:
Ni(CO)4 + 2 N2O4 → Ni(NO3)2 + 2 NO + 4 CO
The hydrated nitrate is often used as a precursor to supported nickel catalysts.