The chemistry of polyoxometalates (POMs) continues to attract much attention, particularly with respect to potential catalytic activity. [1] In the last few years it has also been noted that POMs are ideal models for the study of exchange interactions in magnetic clusters, as they represent a class of well-insulated complexes of controlled nuclearity and topology. [2] The studied clusters have been mainly obtained by filling the vacancies of lacunary Keggin ({XW 11 O 39 }, {XW 9 O 34 }, X = Si IV , P V , As III/V …) or Dawson ({P 2 W 17 O 61 }, {P 2 W 15 O 56 }) type polyoxotungstates with di-or trivalent firstrow transition metals (TMs), which leads to clusters of nuclearity ranging from one to fourteen. [3] Nevertheless, to date, the relevance of POMs in molecular magnetism is limited, partly because only species where the paramagnetic centers are fluoro, oxo, or hydroxo bridged, have been fully characterized; the introduction of larger bridging ligands in a magnetic POM remains a challenge. We are currently exploring the possibility of coupling the paramagnetic ions in the vacancies of POMs by organic or inorganic ligands. Dimeric bis(m-acetato) lanthanide magnetic complexes have already been obtained. [4] Nevertheless, analogous compounds with TMs replacing the rare-earth cations have not been characterized to date. Considering the high affinity of the azido anion N 3 À for transition metals, such as Cu II , Ni II or Mn II , [5] and that N 3
À is certainly one of the most interesting magnetic couplers known in molecular chemistry, we have decided to investigate the interaction of this ligand with magnetic POMs. Very recently, we have shown that the paramagnetic center of a Cu II POM can bind an azido group; [3e] nevertheless, the lack of structural information [**] P.M. is very grateful to Prof. Talal Mallah for fruitful discussions on the magnetic properties of compound 1. We also thank Dr. J. Cano and Dr. L. Catala for their help.