John S. Haynes

Mössbauer spectroscopic study of the electronic and magnetic properties of iron(II) para-toluenesulphonate

Abstract 57 Fe Mossbauer spectra have been obtained for Fe( p -CH 3 C 6 H 4 SO 3 ) 2 between 2.3 and 300 K in zero field, and at 2.3 and 4.2 K in longitudinal applied magnetic fields ranging from 1.1 to 5.6 T. The complex is a fast-relaxing paramagnet under all conditions studied and there is no evidence of antiferromagnetic exchange coupling. The FeO 6 chromophore is distorted by a trigonal elongation and the orbital ground state is the [( 2 3 ) 1 2 |±2〉 ∓ ( 1 3 ) 1 2 |∓1〉] doublet. The temperature dependence of the quadrupole splitting has been analysed via a crystal-field model to provide estimates of the axial field splitting parameter Ds = -93 cm -1 , spin-orbit coupling constant λ = -70 cm -1 , and fine structure constant D σ = -28 cm -1 . The magnetic properties of the complex are described by treating the ground state as a non-Kramers doublet with fictitious spin S = 1 2 . Five separate Mossbauer-Zeeman spectra can be fitted in this spin-hamiltonian approximation with identical values of the g - and A -tensor components, viz. g ⊥ = 1.0, g u = 9.0; A ⊥ ≈ 2.0 mm s -1 . A u = -1.79 mm s -1 . The trigonal z axis, the z axis of the electric field gradient tensor, and the easy axis of magnetisation are collinear, and the saturation value of the internal hyperfine field along this axis is +13.0 T.

Structural isomerism in iron(II) methanesulphonate

Abstract Fe(CH 3 SO 3 ) 2 has been prepared in two isomeric forms. Mossbauer spectra show that the FeO 6 octahedron is compressed along the trigonal axis in the α isomer and elongated in the β form, α-Fe(CH 3 SO 3 ) 2 remains paramagnetic down to 4.2 K, with evidence from susceptibility measurements for some magnetic exchange. β-Fe(CH 3 SO 3 ) 2 orders antiferromagnetically at ≈23 K.

Studies of the pyrazine-bridged antiferromagnets Fe(pyz)mX2 (m=1, X=CF3SO3−, NCO−; m=2, X=NCS−)

Mössbauer and magnetic susceptibility data are reported for the title complexes. The data demonstrate that these pyrazine-bridged complexes represent examples of low-dimensional lattices with anisotropic magnetic exchange arising from different interaction strengths of the bridging ligands.

Metal hexafluorophosphate and hexafluoroarsenate complexes with pyrazine

Abstract PF6− and AsF6− are known to exhibit very weak coordinating tendencies towards transition metals and have been used as counter anions in the synthesis of a number of complex cationic species. We report hefe the synthesis of the complexes [CuL4](EF6)2 (where L is pyrazine (pyz) or methylpyrazine and E is P or As). Spectroscopic and magnetic studies indicate the presence of [CuL4]2+ complex cations containing monodentate pyrazine ligands and non-coordinated EF6− anions. Thermal decomposition, under vacuum, of the [CuL4] (AsF6)2 complexes leads to the decomposition of the fluoroanion with the formation of donor-acceptor complexes, L·AsF5, plus mixtures of copper(II) fluoride complex species. Anion decomposition is avoided when the complexes are heated in an atmosphere of AsF5. Under these conditions, complexes of composition CuL2(AsF6)2 are obtained. Spectroscopic studies suggest that these complexes contain bridging pyrazine ligands and weakly coordinated AsF6− ions. Magnetic susceptibility studies on Cu(pyz)2(AsF6)2 and a structurally related mixed anion complex, Cu(pyz)2(NO3)(AsF6), show the presence of significant antiferromagnetic coupling between metal centres. The role of the AsF6− anion in enhancing magnetic concentration in pyrazine complexes will be discussed. Some recent studies on related iron(II) complexes will also be reported.