Hardcastle and R. Mason for communicating their results prior to publication, and Ms. E. Boespflug for preparation of some of the complexes. The high-spin tetrakis(pyridine)iron(II) complexes Fe(py),X2, where X is C1, Br, I, NCO, NCS, and NCSe, have a tetragonally distorted trans octahedral structure. Magnetically perturbed Mossbauer 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 r-bonding ligand and is comparable to iodine. The high-spin pseudooctahedral Fe(py),X2 complexes, where X is CI, 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 Mossbauer spectra of the bis(pyridine) chloride, thiocyanate, and selenocyanate complexes reveal spontaneously ordered one-dimensional ferromagnets at 4.2 and 1.3 K. The Mossbauer spectrum at 4.2 and 1.1 K reveals that Fe(py),Br2 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 u-bonding ligand than the halide or pseudohalide ligands. In general, the monodentate nonbridging halides and pseudohalides are better cr-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.
Magnetic phase transitions in the pyridine (pyr) compounds Co(pyr)2C12, Fe(pyr)2C12, Fe(pyr)2(NCS)2 and Ni(pyr)2C12 have been observed at applied magnetic fields of '-'0.7, 0.7, 1.1 and 2.7 kG respectively. These low field phase transitions are observed in the Fe and Ni compounds at T= 4.2 K, and in the Co compound at T< 3 K, and are consistent with metamagnetic behavior. Magnetic saturation is not achieved in any of these compounds for fields of 60 kG, reflecting high anisotropy.FEW magnetically ordered compounds show metaferromagnetic exchange, whereas the other four commagnetic phase transitions. Here we report the obserpounds exhibit ferromagnetic intrachain interactions. vation of low field, apparently metamagnetic, transitions in the pyridine (pyr) compounds Co(pyr)2C12,
1The temperature dependence of the magnetic Fe(pyr) 2C12 ,2 Fe(,pyr)2(NCS)2 ,~and Ni(pyr)2Cl2. susceptibility of Co(pyr)2Cl2 has been studied' in These compounds as well as Mn(pyr)2Cl2 4 and low applied fields (0.1-40 Oe) and the compoundS Cu(pyr) 2Cl2~5 '6have linear chain structureswith was shown to order antiferromagnetically with strong exchange interactions.along the chains, and TN = 3.17 K and with a parainagnetic Curie temperarelatively weak exchange interactions between chains. ture 0 + 5 K. A theoretical fit1 of the susceptibility The Mn and Cu compounds have intrachain antiwith a ferromagnetic Ising model yielded an intrachain interaction I/k = 11.7K. In Fig. 1(a) we plot the magnetic moment, a, as a function of applied magnetic field, H 0, for several selected temperatures . i both above and below the previously reported valuẽ TN =~. 17 K. The magnetic moment data were obtained with a vibrating sample magnetometer adapted to a superconducting solenoid. The data in Fig. 1(a) cover the range where the onset of long range
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