Nickel(II) nitrite

Nickel(II) nitrite is an inorganic compound with the chemical formula Ni(NO2)2. Anhydrous nickel nitrite was first discovered in 1961 by Cyril Clifford Addison, who allowed gaseous nickel tetracarbonyl to react with dinitrogen tetroxide, yielding a green smoke. Nickel nitrite was the second transition element anhydrous nitrite discovered after silver nitrite.

Properties
Nickel nitrite decomposes when heated to 220°C, however it can be heated up to 260°C in argon. The nitrite is covalently bonded to nickel, and the material is slightly volatile. The infrared spectrum of the solid has absorption bands at 1575, 1388, 1333, 1240, 1080, and 830 cm−1. Liquid dinitrogen tetroxide oxidises nickel nitrite to nickel nitrate.

In solution
When nickel nitrite dissolves in water, different mixed nitro-aqua complexes form such as Ni(NO2)2(H2O)4, Ni(NO2)3(H2O)3(-), and Ni(NO2)(H2O)5(+).

The aqueous complex Ni(NO2)2(H2O)4 forms when an alkali metal nitrite is added to a nickel salt solution:


 * Ni(H2O)6(2+) + 2 NO2(-) Ni(NO2)2(H2O)4 + 2 H2O; K = 0.16 at standard conditions

The complex is a much more intense emerald green colour than the Ni(H2O)62+ ion. Brooker claims that intense light photocatalyses the destruction of the ionic nitro complexes, leaving only Ni(NO2)2(H2O)4.

Nickel nitrite slowly decomposes slightly in aqueous solution due to disproportionation:


 * 3 NO2− + 2 H+ → 2 NO(g) + NO3– + H2O

Complexes
In the presence of additional ligands, nickel nitrite can change colors substantially. These color changes may arise from the new molecular orbitals, or linkage isomerism, in which the nitro ligands rearrange to a nitrito (-ONO) complex. Sorted by color, known compounds include:
 * Violet
 * Ni((Me2N)Pyr)2(ONO)2


 * Reddish blue-violet
 * Ni((MeNH)Pyr)2(NO2)2


 * Blue-gray
 * Ni(C(MeNH)H(CH2)3(CMeH)NH)2(ONO)2


 * Blue
 * Ni(Pyr)4(ONO)2
 * Ni(N,N-Et2en)2(NO2)2
 * Ni(C(Me2N)H(CH2)4NH)2(ONO)2
 * Ni((MeNH)(Me)Pyr)2(ONO)2


 * Blue-green
 * Ni(R$n$en)2(ONO)2
 * Ni(N,N'-Et2en)2(NO2)2


 * Green
 * Ni(α-MePyr)2(NO2)2
 * Ni(C9H7N)2(NO2)2


 * Red
 * Ni(NO2)2(NH3)4
 * Ni(en)2(NO2)2
 * Ni(R-en)2(NO2)2
 * Ni((NH2CH2)(NH2)iPr)2(NO2)2
 * Ni(Et-en)2(NO2)2
 * Ni(rac-Ph2en)2(NO2)2
 * Ni(C(MeNH)H(CH2)4NH)2(NO2)2
 * Ni(N,N'-Me2en)2(NO2)2•H2O
 * Ni(N,N-Me2en)2(NO2)2


 * Pink
 * Ni(Me-en)2(NO2)2

Double salts
The nitronickelates are related compounds where more nitro groups are attached to nickel to yield an anion. They could be described as nickel double nitrites.