• Tungsten;
  • Molybdenum;
  • Vanadium;
  • Polyoxometalates;
  • Oxometalates


Inorganic metal–oxygen cluster anions form a class of compounds that is unique in its topological and electronic versatility and is important in several disciplines. Names such as Berzelius, Werner, and Pauling appear in the early literature of the field. These clusters (so-called isopoly- and heteropolyanions) contain highly symmetrical core assemblies of MOx units (M = V, Mo, W) and often adopt quasi-spherical structures based on Archimedean and Platonic solids of considerable topological interest. Understanding the driving force for the formation of high-nuclearity clusters is still a formidable challenge. Polyoxoanions are important models for elucidating the biological and catalytic action of metal–chalcogenide clusters, since metal–metal interactions in the oxo clusters range from very weak (virtually none) to strong (metal–metal bonding) and can be controlled by choice of metal (3d, 4d, 5d), electron population (degree of reduction), and extent of protonation. Mixed-valence vanadates, in particular, show novel capacities for unpaired electrons, and the magnetic properties of these complexes may be tuned in a stepwise manner. Many vanadates also act as cryptands and clathrate hosts not only for neutral molecules and cations but also for anions, whereby a remarkable “induced self-assembly process” often occurs. Polyoxometalates have found applications in analytical and clinical chemistry, catalysis (including photocatalysis), biochemistry (electron transport inhibition), medicine (antitumoral, antiviral, and even anti-HIV activity), and solid-state devices. These fields are the focus of much current research. Metal–oxygen clusters are also present in the geosphere and possibly in the biosphere. The mixed–valence vanadates contribute to an understanding of the extremely versatile geochemistry of the metal. The significant differences between the chemistry of the polyoxoanions and that of the thioanions of the same elements is of relevance to heterogeneous catalysis, bioinorganic chemistry, and veterinary medicine.