The solid-state linkage isomerization of cis-[Co(NH3)4(NO2)2]X (dinitro) and cis-[Co(NH3)4(ONO)2]X (dinitrito) (X = Cl, NO3 , ClO4 ) was investigated by differential scanning calorimetry (DSC). All three salts of the dinitrito isomer show an exothermic peak associated with linkage isomerization, while only the chloride salt of the dinitro isomer exhibits an endothermic DSC peak. The results imply that the chloride salts of both isomers are metastable at ambient temperature and can convert to a stable equilibrium mixture of both isomers at elevated temperatures, but the NO3 and ClO4 salts of the dinitro isomer are in stable state even at high temperatures. The DSC peak temperature and the enthalpy changes of the dinitrito to dinitro isomerization depend upon the nature of counterion as follows: NO3 < Cl < ClO4 . The stepwise enthalpy changes of the linkage isomerization of the dinitro (only for the Cl salt) and dinitrito complexes were obtained through mathematical resolution of the observed peak into two peaks, assuming a two-stage reversible isomerization for the Cl salt and a similar irreversible process for the NO3 and ClO4 salts. The nature of the counterion has little effect on the kinetics of the linkage isomerization. The electronic structures and transition states of the linkage isomers were optimized by DFT and their electronic spectra calculated by TDDFT/PCM methods.
