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The principal and mean magnetic moments of Fe acac3 (acac = acetylacetonate) and K3Fe(oxalate),,3H,0 are reported over the temperature range 80--300%. This appears to be the first time that anisotropy of the 6Alp ground-state of the iron( Ill) ion has been detected by crystal magnetic susceptibility measurements. The anisotropy is interpreted in terms of the zero-field splitting of the ground state arising from a low-symmetry ligand-field and spin-orbit coupling ; values of the zero-field splitting are reported and discussed with reference to the molecular structures of the two complexes.THE origin of the zero-field splitting in 3d5 systems to explain ground-state splittings of the order of 0.001 and the effect of such splitting on the magnetic properties cm.-l in nearly cubic systems were based on highof FeIII and MnII have long been of interest in studies order perturbations due to the cubic field and spinof specific heats, adiabatic demagnetisation, and, orbit coupling.lS2 Such an explanation was queried by more recently, paramagnetic resonance. Early attempts Meijer who found that the discrepancy between the 4

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The principal and average magnetic susceptibilities of FeSiF,,6H20 and FeGeF6.6H,0 have been measured in the temperature range 300-80 O K . Theoretical magnetic moments have been calculated for a free-ion 5D basis, perturbed by a trigonal crystal-field and spin-orbit coupling. A crystal field (D,J point-charge calculation was parameterized by the second-and fourth-order radial parameters Cp and D9, and an angular parameter 0. For Cp > D9 the mixing of the two trigonal s€g terms by spin-orbit coupling and the crystal field altered the theoretical principal magnetic moments from those calculated for a 5T2g cubic-field basis set. The low experimental moments of the complexes [as compared to the near tetragonal (NH4),Fe(S04)2,6H,0] derives from the sensitivity of the ground-state splitting, A, to 0 when Cp is large : and this appears to be characteristic of the trigonal nature of the distortion. Assuming A = kA, and D9 = 1000 cm.-l ambiguous fits with ranges of values of Cp, k, and 0 have been found. It was not possible to resolve these ambiguities by use of optical, Mcssbauer, or e.s.r. spectra.The magnetic properties of the 5D term perturbed by a tetragonal crystal-field and spin-orbit coupling, have been briefly investigated and susceptibilities of Fe( NH4),(S0,),,6H,0 discussed.

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