Temperature-dependent IR studies were carried out on complex 1 in KBr in the temperature range of 400-200 K in both heating and cooling modes (Figures and). An expected alteration observed is the magnificent decrease or increase in the intensity of the (C=O) band coming from the coordinated bik* ligand in complex 1 in the heating or cooling cycle. The selected region of IR spectra showing characteristic (C=O) for 1 is displayed in Figure . Upon cooling, the intensity of the (C=O) peak at around 1650 cm -1 decreased, which is the characteristic stretching vibration for HS iron(II) state, while the intensity of the ( C=O) peak at around 1635 cm -1 increased which is the characteristic stretching vibration for LS iron(II) state. These results indicate a thermal conversion of the HS state to the LS state in 1. It is essential to mention that these changes are reversible upon heating, indicating a reversible thermal spin-state switching in 1 (Figures and).