EVIDENCE FOR INTERMEDIATE (S = 1) SPIN STATE STABILIZATION IN FEII(4,4'-DPB)2(NCS)2 AND FEII(4,4'-DPB)2(NCSE)2 (DPB = DIPHENYL-2,2'-BIPYRIDYL) FROM SUSCEPTIBILITY, INFRARED, AND FE-57 MOSSBAUER DATA

Magnetic susceptibility, Fe-57 Mossbauer spectroscopy, and variable-temperature FTIR spectroscopy have been used to characterize the spin states of Fe(II)(4,4'-dpb)2X2 (dpb = diphenyl-2,2'-bipyridyl; X = NCS-, NCSe-) in the temperature range 6 less-than-or-equal-to T less-than-or-equal-to 460 K. Both complexes show a continuous variation of the field-dependent effective magnetic moment over this range. Below approximately 325 K, both the infrared and Mossbauer spectroscopic signatures are insufficient to permit characterization of the spin state(s) of the metal atom in these complexes, but at higher temperatures, the FTIR data can be accounted for by the increasing population of a high (S = 2) spin state, while the Mossbauer spectra can be accounted for by a rapid (on the Mossbauer time scale) relaxation between two spin states. The existence of an intermediate (S = 1) spin state for Fe(II) is a consequence of a major distortion from octahedral symmetry of the metal-based orbitals, permitting a (near) degeneracy of the d(z)2 and d(xy) levels around this atom.