Molecular motions in the solid state: the thermochromic nitro-nitrito interconversion in nickel(II) bis(diamine) complexes

The NO2- ion, in the trans-octahedral [NiII(N,N'-dimethylethylene-diamine)2(NO2)2](H2O) complex, coordinates the metal through thenitrogen atom (nitro form). On heating the solid complex, the anionrotates to give nitrito coordination (oxygen bound), according to areversible process. The coordination mode of NO2- to NiII is related to the steric interplay between the anion and the alkyl substituents on the diamine.