Complexes containing 4f elements are of great interest in molecular magnetism. They can give rise to single molecule magnets (SMM) with easy-axis type anisotropy, high-energy barrier, and finally slow magnetic relaxation at low temperature. These features seem to originate from the competition between spin-orbit coupling inherent in the rare-earth element and the strength and geometry of the crystal field created by the ligands surrounding the lanthanide ion. So far, few general rules for building SMM have been formulated,[1] but no fundamental magneto-structural relationship explaining the behavior of lanthanide-based SMM has been proposed yet. To this end, ab initio calculations (CASSCF/PT2/RASSI-SO) [2] are the most appropriate tool to get a reliable insight into i) the electronic structure of these compounds, ii) the mechanisms behind the magnetic properties and iii) if any, the correlation between magnetism and luminescence.

Herein, we report on our recent findings using such an approach.[3] Among others, examples on [Ln(tta)3L] with Ln = Dy (III) and Yb(III), tta = 2-thenoyltrifluoroacetonate and L = 4,5-bis(propylthio)-tetrathiafulvalene-2-(2-pyridyl)benzimidazole-methyl-2-pyridine (see Figure) will be presented.