Mössbauer, Electronic, and Structural Properties of Several Bis- and Tetrakis(pyridine)iron(II) Complexes


The high-spin tetrakis(pyridine)iron(II) complexes Fe(py)4X2, where X is Cl, Br, I, NCO, NCS, and NCSe, have a tetragonally distorted trans octahedral structure. Magnetically perturbed Mössbauer spectra for each of these compounds indicate a positive electric field gradient tensor and a nondegenerate orbital ground term. An angular overlap analysis of the quadrupole interaction in these compounds indicates that, relative to chlorine and bromine and the pseudohalides, pyridine is a poor π-bonding ligand and is comparable to iodine. The high-spin pseudooctahedral Fe(py)2X2 complexes, where X is Cl, Br, NCO, NCS, and NCSe, have polymeric linear-chain structures with bridging anions and trans pyridine ligands. The thiocyanate and selenocyanate complexes have both nitrogen and sulfur or selenium coordinated to adjacent iron atoms whereas the cyanate anion bridges via a three-center bond at the nitrogen atom. The Mössbauer spectra of the bis(pyridine) chloride, thiocyanate, and selenocyanate complexes reveal spontaneously ordered one-dimensional ferromagnets at 4.2 and 1.3 K. The Mössbauer spectrum at 4.2 and 1.1 K reveals that Fe(py)2Br2 is paramagnetic. No spontaneous ordering is observed at 4.2 K in a 6-T applied field. The electronic spectra at room temperature and at 23 K for all of these complexes have been evaluated in terms of the angular overlap model. The results indicate that pyridine is a better σ-bonding ligand than the halide or pseudohalide ligands. In general, the monodentate nonbridging halides and pseudohalides are better σ-bonding ligands than are the bridging ligands. The infrared and powder X-ray diffraction results - which indicate many isomorphisms with the analogous cobalt and nickel complexes - are consistent with the above structural assignments. All evidence indicates that Fe(py)2I2 has a pseudotetrahedral structure.



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