Large magnetic anisotropy in mononuclear metal complexes

Abstract

This review examines mononuclear metal complexes with high magnetic anisotropy and the theoretical approaches used to rationalize their magnetic properties. Electronic structure calculations based on CASSCF (or CASPT2/NEVPT2) methods provide a quantitative agreement of the zerofield splitting parameters either for mononuclear transition metal complexes or for equivalent lanthanide systems. To produce a more qualitative tool for predicting the magnetic anisotropy of metal complexes, we have developed a set of simple models. For transition metal systems, a simple model based on the splitting of the d orbitals, considering the coordination mode of the metal and its electronic configuration, is enough to qualitatively predict the system's magnetic anisotropy. A similar approach does not work with the f orbitals of the lanthanide complexes. As an alternative, we studied the electrostatic field generated by the ligands and found that this magnitude controls the shape and the orientation of the anisotropic electron density. This procedure allows us to rationalize and to predict whether the system will have a strong axial character, and also to determine the direction of the magnetic moment.

Trigonal antiprismatic Co(ii) single molecule magnets with large uniaxial anisotropies: importance of Raman and tunneling mechanisms

“…It should be mentioned that using a positive D value cannot reproduce the experimental data but it is difficult to obtain accurate values of D and E , as has been noted for other compounds with huge negative D values, which are too large to be determined by high-field EPR spectroscopy. The D values are among the largest negative ones reported for mononuclear transition metal complexes 3–5 and comparable to those for the Co( ii ) ion in a trigonal prismatic geometry. 29,31,32 …”

“…These challenges notwithstanding, a great deal of effort is being directed at both experimental and theoretical analysis of transition metal mononuclear SMMs and the field is expanding rapidly. 3–6 …”

Late First-Row Transition-Metal Complexes Containing a 2-Pyridylmethyl Pendant-Armed 15-Membered Macrocyclic Ligand. Field-Induced Slow Magnetic Relaxation in a Seven-Coordinate Cobalt(II) Compound

“…66 That is why we also analyzed relaxation times with eq 8 containing the Raman term τ = − CT n (8) for data in temperature range between 2.2 and 4.0 K, which resulted in C = 2.12 × 10 −5 and n = 2.84 ( Figure 12). The fitted n-value is lower than n = 4.9 or 8.7 found in other sevencoordinate Co(II) complexes with easy plane anisotropy.…”

Halogen atom effect on the magnetic anisotropy of pseudotetrahedral Co(II) complexes with a quinoline ligand

“…Mononuclear Co(II) complexes attract considerable attention due to their single-molecule magnet (SMM) behavior [1,2]. Among them, tetrahedral complexes possess a strong magnetic anisotropy D that plays a key role in creating a spin reversal barrier [3].…”