text: Thermal perturbations are the most common type of external stimulus used to induce SCO.  One example is [Fe(tmphen)][Co(CN)] trigonal bipyramid (TBP), with the Fe centers in the equatorial positions. The HS Fe remains under 20% i the range of 4.2 K to 50 K, but at room temperature about two-thirds of the Fe ions in the sample are HS, as shown by the absorption band at 2.1 mm/s, while the other third of the ions remain in the LS state. The thermally induced spin transition is an entropy driven process. Around 25% of the total entropy gain from the LS to HS transition originates from the increase in spin multiplicity according to the relationship:
  and the larger contribution arises from vibrational effects, since the metal-ligand bond distances are larger in the HS state.