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 (socalled isopoly-and heteropolyanions) contain highly symmetrical core assemblies of MO, 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 0x0 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.
I. Introduction 239 II. General Aspects 241 III. Vanadates 242 A. {V 19 } Clusters 242 B. {Mn 2 V 22 } with Two Decavanadate-Type Moieties 242 C. Structures Built up from {OdVO 4 } Pyramids: {V 22 } and {V 34 } 243 IV. Niobates 244 A. The {Al 2 Eu 6 Nb 30 } Cluster 244 V. Molybdates 244 A. Clusters with {Mo 7 } Units: {Eu 4 Mo 29 } and {Pr 8 Mo 58 } 245 B. Clusters with Capped R-Keggin Cores: {Mo 16 V 14 } and {(Mo 8 V 7 ) n } 245 C. Clusters Built up by Lacunary Keggin Fragments: {As 8 Mo 24 } and {As 10 Mo 24 } 246 D. Clusters with an -Keggin-Type Core: {Mo 37 }, {Mo 42 }, and {Mo 43 } 246 E. Clusters Containing {Mo 8 } Moieties: {Mo 36 }, {Mo 57 }, and {Mo 154 } 249 1. {Mo 36 } 249 2. {Mo 57 } 250 3. {Mo 154 (NO) 14 } and Comparison to {Mo 57 } 251 4. Beyond the {Mo 154 } Level 252 VI. Tungstates 253 A. Clusters Incorporating Different Numbers of Monovacant Lacunary {XW 11 }-Type Building Blocks 253 1. {XW 11 }: {Si 2 W 23 } 253 2. {XW 11 } 2 or {X 2 W 17 } 2 : {MX 2 W 22 } and {MX 4 W 34 } 253 3. {XW 11 } 2 : {XW 11 } 2 {Mo 3 S 4 } 2 255 4. {XW 11 } 3 : {B 3 W 39 } 255 B. Clusters Incorporating Different Types of Trivacant Lacunary Building Blocks 255 1. [XW 9 O 34 ] n-(A-or B-Type) 255 2. [X III W 9 O 33 ] 9-256 3. B-Type [P 2 W 15 O 56 ] 12-: {M 4 P 4 W 30 } and {P 4 M 6 W 32 } 259 C. Clusters Incorporating Pentavacant Lacunary Building Blocks 260 1. {Sb 9 W 21 } and Related Anions 260 D. Clusters Incorporating Hexavacant Lacunary Building Blocks 260 1. {P 4 W 24 } and {P 8 W 48 } 260 2. {P 5 W 30 } 261 E. Mixed-Valence Clusters Derived from Heteropoly Browns {XW 20 } 261 F. Structurally Uncharacterized Clusters 261 VII. Magnetism of Polyoxometalates 261 A. Polyoxovanadates(IV) 263 B. Mixed-Valence Clusters 266 C. Polyoxometalates as Ligands to Magnetic Clusters 266 VIII. Outlook 268 IX. Acknowledgments 268 X. Note Added in Proof 268 XI. References 268I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have.
The cover picture shows a novel hierarchic endohedral clusterization of H 2 O molecules in the form of a dodecahedron ((H 2 O) 20 , red), a further ™mounted∫ dodecahedron (green), and a rhombicosidodecahedron ((H 2 O) 60 , yellow). The shell/host (Mo atoms blue, O red) is built up by 12 pentagonal {(Mo)Mo 5 } type building blocks (one is highlighted as a blue ring),which are connected by 30 Mo V 2 linkers with the consequence that 20 nanosized Mo 9 O 9 pores/rings of classical crown ether quality are formed in which 20 guanidinium cations are encapsulated (C black, N green). The Mo V 2 type linkers are stabilized by PO 2 H 2 À /SO 4 2À ligands (P/S purple). As the clusterization in the cavity takes place after filling the receptors/pores at the cluster surface with guests, a process is modeled by which a cell converts an extracellular molecular signal into a response. The representative red ring below the blue ring is marked out by the 60 H 2 O ligands coordinated to the pentagonal Mo units, which altogether form a rhombicosidodecahedron (not shown completely). As the geometric forms described are those of Platonic and Archimedean solids Plato and Archimedes feel involved. Further details are reported by A. M¸ller et al. on p. 3756 ff.
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