Magnetic anisotropy: The orientation is in the details.

Lanthanide compounds make for intriguing molecules that are also useful components in fluorescent, catalytically active and magnetic materials — some have even found their way into medicinal applications, for example as MRI contrasting agents owing to their magnetic properties1. Yet this has not always been the case. They had initially been overlooked, owing to their isostructurality and common +3 oxidation state. This arises because the 4f valence electrons of lanthanide ions are shielded by fully filled 5s2 and 5p6 orbitals, and thus are only weakly affected by coordinating ligands. Pimentel and Sprately depicted this in the 1970s, writing that “Lanthanum has only one important oxidation state in aqueous solution, the +3 state. With few exceptions, this tells the whole boring story about the other 14 lanthanides”2. The beauty and practicality of lanthanide systems, however, lies in their inherent electronic properties. The core 4f orbitals in which the unpaired electrons of lanthanide ions reside may not interact strongly with ligands, but they greatly contribute to their physical properties. Most applications of lanthanide systems rely on their magnetic properties, and it is thus critical to unravel how these arise, and how they are affected not only by the close coordination environments of the lanthanide ions, but also by other subtle effects induced by their nearest neighbours. In this spirit, Sessoli and co-workers have characterized with remarkable precision a dysprosium complex with a tetraaza-cyclododecanetetraacetate ligand (DOTA), and describe in Angewandte Chemie International Edition how subtle structural differences lead to significant changes in magnetic properties3.