Quantifying magnetic anisotropy using X-ray and neutron diffraction.

In this work, the magnetic anisotropy in two iso-structural distorted tetrahedral Co(II) complexes, Co tmtu [ = Cl() and Br(), tmtu = tetra-methyl-thio-urea] is investigated, using a combination of polarized neutron diffraction (PND), very low-temperature high-resolution synchrotron X-ray diffraction and CASSCF/NEVPT2 calculations. Here, it was found consistently among all methods that the compounds have an easy axis of magnetization pointing nearly along the bis-ector of the compression angle, with minute deviations between PND and theory. Importantly, this work represents the first derivation of the atomic susceptibility tensor based on powder PND for a single-molecule magnet and the comparison thereof with calculations and high-resolution X-ray diffraction. Theoretical ligand field theory (AILFT) analysis finds the orbital to be stabilized relative to the and orbitals, thus providing the intuitive explanation for the presence of a negative zero-field splitting parameter, , from coupling and thus mixing of and . Experimental -orbital populations support this interpretation, showing in addition that the metal-ligand covalency is larger for Br-ligated than for Cl-ligated .