TEMPERATURE-INDUCED OR PRESSURE-INDUCED STRUCTURE CHANGES OF A SPIN CROSSOVER FE(II) COMPLEX - [FE(BPY)2(NCS)2]

The crystal structures of polymorph II of diisothiocyanatobis(2,2'-bipyridine)iron(II), [Fe(bpy)2(NCS)2], in the high-(298 K) and low-spin states (175, 110 K) were determined by an X-ray diffraction method. The crystal is orthorhombic, space group Pcnb and Z = 4 over the entire temperature range. At 298 K, a = 13.2044 (6), b = 16.4820 (7), c = 10.1086 (7) angstrom, U = 2200.0 (2) angstrom 3, D(c) = 1.462 g cm-3 and at 110 K, a = 12.9842 (9), b = 16.1067 (12), c = 9.8938 (8) angstrom, U = 2069.1 (3) angstrom 3, D(c) = 1.555 g cm-3. The structures were refined to R = 0.047, 0.044, and 0.042 at 298, 175, and 110 K, respectively. The difference in the average Fe-N bond lengths between high- and low-spin states is 0.17 angstrom. The pressure dependence of the absorption spectra was also measured up to 8.5 GPa using a diamond anvil cell. These results indicate that with increasing pressure the high-spin state at 1 atm starts to change to the low-spin state at 0.3 GPa and that its low-spin state reverts again to the high-spin state at around 3.3 GPa.