The [Dy(tta)3(L)] complex behaves as a single ion magnet both in its crystalline phase and in solution. Experimental and theoretical magnetic anisotropy axes perfectly match and lie along the most electro-negative atoms of the coordination sphere. Both VSM and MCD measurements highlight the robustness of the complex, with persistence of the memory effect even in solution up to 4 K.

The two mononuclear complexes [Ln(tta)3(L)]·C6H14 {Ln = DyIII and YbIII} (tta– = 2‐thenoyltrifluoroacetonate, L = 2‐{1‐methylpyridyl‐4,5‐[4,5‐bis(propylthio)tetrathiafulvalenyl]‐1H‐benzimidazol‐2‐yl}pyridine) are isostructural. The lanthanide ions adopt a distorted‐square‐antiprism coordination polyhedron (D4d symmetry). The DyIII compound behaves like a single‐molecule magnet (SMM), whereas no out‐of‐phase component of the magnetic susceptibility is measured for YbIII. The crystal‐field splitting of the 2F7/2 and 6H15/2 ground multiplets of YbIII and DyIII, respectively, were determined by means of ab initio calculations and confirmed by the emission energy lines of the 2F5/2→2F7/2 luminescence spectrum in the case of YbIII. The nature of the MJ ground states indicates that YbIII is not an Ising system, whereas DyIII is. The experimental and calculated anisotropy axes for DyIII lie along the most negatively charged direction, whereas those for YbIII lie almost perpendicularly. Finally, the gz values corroborate the uniaxiality of the anisotropy axis for DyIII.