The magnetization density of hexacyanoferrate(III) ion measured by polarized neutron diffraction in Cs2KFe(CN)6

A polarized neutron diffraction experiment on Cs2KFe(CN)6 gave 292 unique magnetic structure factors. These were analysed by using a model for the magnetization density of multipoles and valence functions on the iron and ligand atoms, with the dipole approximation for orbital effects. Neither the ligand nor the iron atom densities retain the cubic symmetry of the free ferricyanide ion. The natural axes of quantization of the iron atom are rotated by significant amounts from the Fe-CN vectors. The iron electronic configuration was found to be d-0.64(8)xyd0.78(6)xzd0.72(5)yzd-0.06(6)z2d0.17(7)x2-y2 corresponding to the cubic t52g configuration of the low-spin d5 FeIII ion perturbed to put all spin in the dxz and dyz orbitals. The negative spin in the dxy orbital and the 6% of negative spin on the carbon atoms conform with the qualitative predictions of previous ab initio theoretical calculations, although for dxy there are large manifestations of spin polarization. The 12% of spin delocalized onto nitrogen atoms reflects covalence. The ligand populations depart considerably from those for cubic symmetry, and can be understood in terms of spin occupation of molecular orbitals involving 3d-t2g orbitals with coefficients dxy < dxz < dyz. These observations can be rationalized by an empirical model in which the ligand field components exerted by the cyanide groups are influenced by details of the crystal structure.