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.
formulanamestructureRemarksreferencesRb3[Ni(NO2)5]trirubidium pentanitronickelate(II)Cs3[Ni(NO2)5]tricaesium pentanitronickelate(II)dark red, formed from caesium nitrite and nickel nitrite in waterK3[Ni(NO2)5]·KNO2potassium pentanitronickelate potassium nitritemonoclinic, 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; hygroscopicK4[Ni(NO2)6]·H2Opotassium hexanitronickelate monohydratehexagonal, a = 7.634 Å, c = 22.45 Å, Z = 3; nitro groups bonded to Ni via N; water not coordinatedformed from KNO2 and NiCl2 mix in water; orange brown; stable in vacuumK2Ca[Ni(NO2)6]potassium calcium hexanitronickelatecubic, a = 10.31 Å, Z = 4buffK2Sr[Ni(NO2)6]potassium strontium hexanitronickelatecubic, a = 10.5063 Å, Z = 4, density = 2.867 g/cm3brown yellowK2Ba[Ni(NO2)6]potassium barium hexanitronickelatecubic Fm3 (and m3m), a = 10.7800 Å, Z = 4, density = 2.918 g/cm3brown yellowK2Pb[Ni(NO2)6]potassium lead hexanitronickelatecubic Fm3, a = 10.577 Å, Z = 4, density = 3.480 g/cm3: No NO2 group disorderbuff colourN(CH3)4[Ni(NO2)3]tetramethylammonium nickel nitrite TMNINhexagonal perovskite Pm1, 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 ONOyellow brown; one-dimensional Heisenberg antiferromagnet; Haldane material; phase transitions at 409.1 and 428.4 K due to rotation of N(CH3)4
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