The solid-state interconversion of trans-[Co(cyclam)(ONO)2]- X (X = PF6–, ClO4–; cyclam = 1,4,8,11-tetraazacyclotetradecane, [14]aneN4; dinitrito isomer) and trans-[Co(cyclam)- (NO2)2]X (dinitro isomer) was investigated by differential scanning calorimetry (DSC). The conversion of the dinitrito isomer to the dinitro isomer was accompanied by the appearance of exothermic peaks. However, no isomerization DSC peak was observed for the dinitro isomers. The observed DSC peak for the dinitrito isomer was resolved into two peaks that correspond to two isomerization stages. The low temperature peak was ascribed to the conversion of the metastable dinitrito isomer to an unstable nitro–nitrito one, whereas the high-temperature peak was attributed to the subsequent conversion of the nitro–nitrito isomer to the dinitro isomer. The kinetic parameters of the isomerization were determined by the Kissinger method. The electronic structures and thermodynamic stabilities of the linkage isomers, as well as the nature of the transition states, were investigated by DFT.
