Solid state stepwise nitro–nitrito linkage isomerization of trans- [dinitrobis(ethylenediamine)nickel(II)], [Ni(en)2(NO2)2] (N,N-en), trans-[dinitrobis(N,N′-dimethylethylenediamine)nickel(II)], [Ni(N,N′-dmen) 2(NO2)2] (N,N-dmen), and trans-[dinitritobis(N,Ndimethylethylenediamine)nickel(II)], [Ni(N,N-dmen)2(ONO)2] (O,Oudmen) were investigated by performing non-isothermal differential scanning calorimetry (DSC) analyses and the thermodynamic and thermokinetic parameters of isomerization were determined using DSC curves. Both N,N-en and N,N-dmen DSC curves show an endothermic peak at elevated temperatures, attributed to dinitroto-dinitrito linkage isomerization. Similar exothermic peaks were observed in the cooling cycle, assigned to the reverse dinitrito-todinitro isomerization. The O,O-udmen isomer is more stable than the corresponding dinitro one at ambient temperatures, but an exothermic dinitrito-to-dinitro isomerization occurs upon cooling down to −70 °C and reverts endothermically to dinitrito isomer upon heating up to ambient temperature. The overlapping DSC peaks associated with stepwise linkage isomerization were resolved using nonlinear fitting method. The peak temperature and the enthalpy changes of linkage isomerization are inversely dependent on the steric hindrance provided by diamine ligands, which increases in the order udmen > dmen > en. The results showed that the steric factor of the co-ligands also influences the kinetic parameters of isomerization, so that more bulky ligand substituent leads to higher isomerization rate constants. A DFT and TD-DFT calculations have been carrid out on both dinitro complexes of this investigation.
