Iron-oxo clusters are a very important class of materials. First of all they provide some insight into the mechanisms of formation of inorganic cores which are observed in iron biomineralization. The structure of the clusters seems to be dominated at an early stage by the tendency of the oxygen atoms to define close-packed structures, even when they are bound to the organic part of the ligands. The synthetic strategies developed so far are still rather naive and more work is needed in this area in order to learn how to grow larger clusters, to control their structures. From the magnetic point of view the reported clusters provide large variability, within the general scheme of antiferromagnetic exchange interactions. The cyclic [Fe_6 ring] and [Fe_(10)] clusters are excellent models of the magnetic behaviour of one-dimensional antiferromagnetic materials, and they have provided for the first time a detailed knowledge of the energies of the excited states. More ring structures would be desirable, in order to clarify some points which remain unclear, such as the detailed behaviour of the susceptibility observed at low temperature, below the maximum.
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