text: Due to the changes in magnetic properties that occur from a spin transition - the complex being less magnetic in a LS state and more magnetic in a HS state - magnetic susceptibility measurements are key to characterization of spin crossover compounds. The magnetic susceptibility as a function of temperature, (χT) is the principal technique used to characterize SCO complexes.
  Another very useful technique for characterizing SCO complexes, especially iron complexes, is Fe Mössbauer spectroscopy. This technique is especially sensitive to magnetic effects. When spectra are recorded as a function of temperature, the areas under the curves of the absorption peaks are proportional to the fraction of HS and LS states in the sample.
  SCO induces changes in metal-to-ligand bond distances due to the population or depopulation of the e orbitals that have a slight antibonding character.  Consequently X-ray crystallography above and below transition temperatures will generally reveal changes in metal-ligand bond lengths.  Transitions from a HS to a LS state cause a decrease in and a strengthening of the metal-ligand bond. These changes are also manifested in FT-IR and Raman spectra.
  The spin crossover phenomenon is very sensitive to grinding, milling and pressure, but Raman spectroscopy has the advantage that the sample does not require further preparation, in contrast to Fourier Transform Infrared spectroscopy, FT-IR, techniques; highly colored samples may affect the measurements however. Raman spectroscopy is also advantageous because it allows perturbation of the sample with external stimuli to induce SCO.  Thermally induced spin crossover is due to the higher electronic degeneracies of the LS form and lower vibrational frequencies of the HS form, thus increasing the entropy.  The Raman spectrum of an iron(II) complex in the HS and LS state, emphasizing the changes in the M-L vibrational modes, where a shift from 2114 cm to 2070 cm corresponds to changes in the stretching vibrational modes of the thiocyanate ligand from a LS state to a HS state, respectively.
  SCO behavior can be followed with UV-vis spectroscopy.  In some cases, the absorption bands obscured due to the high intensity absorption bands caused by the Metal-to-Ligand Charge Transfer (MLCT) absorption bands.