ANOMALOUS SPIN STATES OF IRON (II) IN MÖSSBAUER EMISSION SPECTRA OF [⁵⁷ Co(2-CH₃-phen),] (ClO₄)₂.2 H₂O AND [⁵⁷Co(2-CH₃O-phen)₃] (ClO₄)₂.H₂O

Abstract

In a previous investigation we had studied the Mössbauer emission spectra of the complexes [57Co(phen)2(NCS)2](phen = 1.10-phenanthroline) and [57Co(bpy)2(NCS)2](bpy = α, α'-bipyridyl) and observed anomalous 5T2 states at sufficiently low temperatures, where the absorption spectra of the corresponding iron compounds show the coexistence of high-spin (S = 2) and low-spin (S = 0) ground states. In the present work we have extended these studies to systems with purely aromatic ligands. It has been well established [1,2] by various physical methods that the complex [Fe(2-CH3-phen)3](ClO4)2(2-CH3-phen = 2-methy-1.10-phenanthroline)shows a temperature dependent high-spin (5T2) ↔ low-spin (1Al) transition, because the ligand field splitting parameter l0Dq and the mean spin pairing energy π are of the same order of magnitude. We recently found [3] by variable temperature Mössbauer absorption spectroscopy, far infrared spectroscopy and magnetic susceptibility measurements that the compound [Fe(2-CH3O-phen)3](ClO4)2.H2O(2-CH3O-phen = 2-methoxy-1.10-phenanthroline) also shows a temperature dependent spin transition. We have measured the Mössbauer emission spectra of [57Co(2-CH3-phen)3](ClO4)2.2H2O and [57Co(2-CH3O-phen)3](ClO4)2.H2O versus K4[Fe(CN)6].3H2O (293 K) as absorber in the temperature range 293 - 4.6 K and compared with the corresponding absorption spectra of the synthesised complexes. The emission spectra demonstrate the absence of resonances arising from the 1Al state, which is the dominating electronic state of iron (II) in the absorption spectra at the same temperature. Even at liquid helium temperature the 5T2 state is the only one visible in the emission spectra. The isomer shift and the quadrupole splitting of the high-spin resonances in the emission spectra of [57Co(2-CH3-phen)3](ClO4)2.2H2O agree well with those of the high-spin resonances in the absorption spectra of the system [57Fe0.01Co0.99(2-CH3-phen)3](ClO4)2.x H2O. From the calculated temperature dependence of the quadrupole splitting of the high-spin resonances in both emission and absorption spectra we have derived the nature of the electronic high-spin state to be 5A1 under D3 symmetry. The calculation was based on a ligand field model including trigonal distortion, spin-orbit coupling and covalency. The mechanism of the formation of the anomalous spin states will be discussed.