Novel aspects of the optical spectra of dichlorotetrakis(thiourea)iron(II)

The low-temperature optical spectra of dichlorotetrakis(thiourea)iron(II) contain strong bands at 16.24 and 24.66 kK. The former gives an intense green color to freshly prepared crystals of this material. Upon long standing at room temperature or when briefly heated to about 150 °C the crystals become colorless and both bands disappear. The bands are assigned to allowed metal-to-ligand electron-transfer processes in an FeCl2(thiourea)2(disulfide) impurity, which is postulated to be present in minute amounts. Most freshly prepared crystals also show ligand field bands ca. 8 kK which are identical at room temperature and 10 K. Disorder in the crystals is presumed to set up an unsymmetrical ligand field which provides the intensity-gaining mechanism for these forbidden transitions. Well-aged crystals, or fresh crystals that have been heated for 15 min at 150 °C, are more ordered and show typical vibronic spectra with the bands decreasing greatly in intensity as the temperature is lowered. The ground state of the complex is shown to be 5B2g.