Dear Colleagues,

In molecular magnetism, 3d and/or 4f complexes are usually associated with a specific geometry, which is then linked to a particular symmetry. The definition of correct geometry has been an important aspect of molecular magnetism, since the arrangement of ligands in a given geometry determines their magnetic properties. For polynuclear systems, the experimental data have shown that adequate geometry of the 3d ion, together with geometrical distortions of the bridging ligands, affects the magnetic overlap and therefore superexchange interactions, as reflected by Kahn–Briat theory. An interesting system is that of triangular 3d complexes, whose magnetic properties can change depending on their geometrical arrangements and, moreover, anisotropic effects appear at low temperature due to existence of distortions in C₃ symmetry. Moreover, the discovery by Gatteschi et al. of single molecule magnets (SMMs) with the Mn₁₂ complex revolutionized the molecular magnetism community. These SMMs present slow relaxation of the magnetization, and this is the origin of the anisotropy found to exist in Mn^III ions due to Jahn–Teller distortion. 

Almost 10 years later, Ishikawa et al. presented the existence of coordination compounds based on a single 4f ion, which also present slow relaxation of magnetization. These are the so-called single ion magnets (SIMs) based on lanthanides (Ln), which can be defined as the second generation of this type of magnetic materials. The slow magnetic relaxation is due to the anisotropy of the 4f orbitals, and the relaxation mechanisms are defined by thermal processes (Orbach, Raman, and direct) and by quantum phenomena (quantum tunnelling of magnetization (QTM)), which is thermally independent. For these types of complexes, geometry (and, therefore, symmetry) plays a key role in the design of high performance Ln^III-SIMs, since changes in the geometry of Ln^III ions will modify their crystal field parameters and thereby influence the dynamic magnetic properties. In this sense, heptacoordinated Dy^III systems with pentagonal bipyramidal geometry (D_5h symmetry) with negatively charged ligands in the axial positions present the highest energy barriers reported in the literature. By comparison, octacoordinated complexes can present large Ueff values, but not as high as the ones reported for pentagonal bipyramidal geometry complexes. This clearly shows that geometry and therefore symmetry play a key role in the magnetic properties of these types of materials.

This Special Issue of Magnetochemistry aims to publish a collection of research contributions highlighting the recent achievements in the relation of geometry and symmetry with the magnetic properties of 3d and/or 4f complexes. 

Dr. Walter Alberto Cañón Mancisidor
Prof. Dr. Diego Venegas Yazigi
Guest Editors

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• molecular magnetism
• 3d complexes
• 4f complexes
• single molecule magnets (SMMs)
• single ion magnets (SIMs)