Nitronickelate

The nitronickelates are a class of chemical compounds containing a nickel atom complexed by nitro groups, –NO₂. Nickel can be in a +2 or +3 oxidation state. There can be five (pentanitronickelates), or six, (hexanitronickelates) nitro groups per nickel atom. They can be considered the double nitrites of nickel nitrite.

formula	name	structure	Remarks	references
Rb3[Ni(NO2)5]	trirubidium pentanitronickelate(II)			[1]
Cs3[Ni(NO2)5]	tricaesium pentanitronickelate(II)		dark red, formed from caesium nitrite and nickel nitrite in water	[2]
K3[Ni(NO2)5]·KNO2	potassium pentanitronickelate potassium nitrite	monoclinic, a = 7.240 Å, b = 10.946 Å, c = 7.120 Å, β = 98.086°; Z = 2; 4 nitro groups bonded to Ni by N, 1 by O (–ONO)	formed by heating monohydrate at 100 °C dark purple; hygroscopic	[2][3][4]
K4[Ni(NO2)6]·H2O	potassium hexanitronickelate monohydrate	hexagonal, a = 7.634 Å, c = 22.45 Å, Z = 3; nitro groups bonded to Ni via N; water not coordinated	formed from KNO2 and NiCl2 mix in water; orange brown; stable in vacuum	[2][3][4]
K2Ca[Ni(NO2)6]	potassium calcium hexanitronickelate	cubic, a = 10.31 Å, Z = 4	buff	[5]
K2Sr[Ni(NO2)6]	potassium strontium hexanitronickelate	cubic, a = 10.5063 Å, Z = 4, density = 2.867 g/cm3	brown yellow	[6]
K2Ba[Ni(NO2)6]	potassium barium hexanitronickelate	cubic Fm3 (and m3m), a = 10.7800 Å, Z = 4, density = 2.918 g/cm3	brown yellow	[7]
K2Pb[Ni(NO2)6]	potassium lead hexanitronickelate	cubic Fm3, a = 10.577 Å, Z = 4, density = 3.480 g/cm3: No NO2 group disorder	buff colour	[8]
N(CH3)4[Ni(NO2)3]	tetramethylammonium nickel nitrite TMNIN	hexagonal perovskite P3m1, a = 9.103 Å, c = 7.082 Å, V = 508.2 Å3, Z = 2, density = 1.770 g/cm3; all nitros are bidentate, nickel either links to N or O of the NO2, forming a chain of nickel alternating with ONO	yellow brown; one-dimensional Heisenberg antiferromagnet; Haldane material; phase transitions at 409.1 and 428.4 K due to rotation of N(CH3)4	[2][9][10]