Differential scanning calorimetry (DSC) measurement was performed in the temperature range between 300 K and 150 K (sweep-rate: 5 K min -1 ) for complex 1 (Figure ). For complex 1, a reversible endothermic/exothermic peak was observed upon cooling/heating at around 225 K, indicating a reversible first-order transition due to spin-state switching in 1, which is consistent with the single-crystal X-ray diffraction studies (vide supra) and the magnetic measurements (vide infra). The thermodynamic parameters related to the spin-state switching process have been calculated and are observed as enthalpy (H) = 0.40 kJ mol -1 and entropy (S) = 1.78 J K -1 mol -1 , which is significantly lower than the expected value for spin-state switching in iron(II) complexes, which might be due to incomplete HS to HS: LS transition in the temperature range of 300 -150 K.