Reversible nitro–nitrito inter-conversion in a simple mono-nuclear nickel(II) complex [NiII{C6H4(NH2)2}2(NO2)2] in the solid state

A simple mono-nuclear nickel(II) complex [NiII{C6H4(NH2)2}2(NO2)2] (1) has been synthesized from an aqueous solution containing nickel chloride, sodium nitrite and ortho-phenylenediamine (opda). When crystals of compound 1 are heated in the temperature range of 110–120 °C for 5 min, their brown color changes to blue, that is accompanied by the conversion of nitro group (N-coordinated) in compound 1 into nitrito group (O-coordinated) in compound [NiII{C6H4(NH2)2}2(ONO)2] (2). On cooling, the nitrito compound 2 goes back to parent nitro-compound 1.