Polyoxometalates (POMs) are an inorganic class of compounds constituted only from transition metal(s) (M) and oxygen (O) of the general chemical formula [MmOo]k- with m ≥ 3, o ≥ 9, k > 0, i.e., they are larger, anionic metal-oxygen coordination complexes. The structural connections between the coresponding transition metal oxides can be quite close (as in the case of the polyoxovanadates) or only very loose (as in the case of the polxoxomolybates), depending on the transitions metal's characteristics.
Formation The oxides of d0 metals such as V2O5, MoO3, WO3 dissolve at high pH to give orthometalates, VO43−, MoO42−, WO42−. For Nb2O5 and Ta2O5. The nature of the dissolved species at high pH is less defined, but these oxides also form polyoxometalates. As the pH is lowered, solutions of orthometalates give oxide–hydroxide compounds such as WO3(OH)− and VO3(OH)2−, these species condense via the process of olation. The replacement of terminal M=O bonds, which have triple bond character, is compensated by the increase in coordination number. The nonobservation of polyoxochromate cages is rationalized by the small radius of Cr(VI), which may not accommodate octahedral coordination geometry. Polycondensation of the MO3(OH)n− species entails loss of water and the formation of M−O−M linkages. The stoichiometry for hexamolybdate is shown:
6 MoO42− + 10 HCl → Mo6O192− + 10 Cl− + 5 H2O An abbreviated condensation sequence illustrated with vanadates is:
4 VO43− + 8 H+ → V4O124− + 4 H2O 5 V4O124− + 12 H+ → 2 [V10O26(OH)2]4− + 4 H2O When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate can result. For example, the phosphotungstate anion PW12O403− consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
History
Ammonium phosphomolybdate, PMo12O403− anion, was reported in 1826. The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer. The 1970s witnessed the introduction of quaternary ammonium salts of POMs. This innovation enabled systematic study without the complications of hydrolysis and acid/base reactions. The introduction of 17O NMR spectroscopy allowed the structural characterization of POMs in solution. The first example of a mineral with a polyoxometalate cation, Ramazzoite, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.
Structure and bonding The typical framework building blocks are polyhedral units, with 6-coordinate metal centers. Usually, these units share edges and/or vertices. The coordination number of the oxide ligands varies according to their location in the cage. Surface oxides tend to be terminal or doubly bridging oxo ligands. Interior oxides are typically triply bridging or even octahedral. POMs are sometimes viewed as soluble fragments of metal oxides. Recurring structural motifs allow POMs to be classified. Iso-polyoxometalates (isopolyanions) feature octahedral metal centers. The heteropolymetalates form distinct structures because the main group center is usually tetrahedral. The Lindqvist and Keggin structures are common motifs for iso- and heteropolyanions, respectively. Polyoxometalates typically exhibit coordinate metal-oxo bonds of different multiplicity and strength. In a typical POM such as the Keggin structure PW12O403−, each addenda center connects to single terminal oxo ligand, four bridging μ2-O ligands and one bridging μ3-O deriving from the central heterogroup. Metal–metal bonds in polyoxometalates are normally absent and owing to this property, F. Albert Cotton opposed to consider polyoxometalates as a form of cluster materials. However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.
Three dimensional and rotatable models of these molecular structures can be found on the web-site: "Polyoxometalate structure types: an interactive gallery".
Polymolybdates and tungstates The polymolybdates and polytungstates are derived, formally at least, from the dianionic MO42− precursors. The most common units for polymolybdates and polyoxotungstates are the octahedral {MO6} centers, sometimes slightly distorted. Some polymolybdates contain pentagonal bipyramidal units. These building blocks are found in the molybdenum blues, which are mixed valence compounds.
Polyoxotechnetates and rhenates
Polyoxotechnetates form only in strongly acidic conditions, such as in HTcO4 or trifluoromethanesulfonic acid solutions. The first empirically isolated polyoxotechnetate was the red Tc20O684−. It contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the hydronium salt (H7O3)4Tc20O68·4H2O by concentrating an HTcO4 solution. Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure. The only polyoxorhenate formed in acidic conditions in presence of pyrazolium cation. The first empirically isolated polyoxorhenate was the white Re4O152−. It contains Re(VII) in both octahedral and tetrahedral coordination. Mixed polyoxo(technetate-rhenate) [Tc4O4(H2O)2(ReO4)14]2− polyanion crystals that contain Tc(V) and Re(VII)were also isolated and structurally characterized.
Polyoxotantalates, niobates, and vanadates The polyniobates, polytantalates, and vanadates are derived, formally at least, from highly charged MO43− precursors. For Nb and Ta, most common members are M6O198− (M = Nb, Ta), which adopt the Lindqvist structure. These octaanions form in strongly basic conditions from alkali melts of the extended metal oxides (M2O5), or in the case of Nb even from mixtures of niobic acid and alkali metal hydroxides in aqueous solution. The hexatantalate can also be prepared by condensation of peroxotantalate [Ta(O2)4]3− in alkaline media. These polyoxometalates display an anomalous aqueous solubility trend of their alkali metal salts inasmuch as their Cs+ and Rb+ salts are more soluble than their Na+ and Li+ salts. The opposite trend is observed in group 6 POMs. The decametalates with the formula M10O286− (M = Nb, Ta) are isostructural with decavanadate. They are formed exclusively by edge-sharing {MO6} octahedra (the structure of decatungstate W10O324− comprises edge-sharing and corner-sharing tungstate octahedra).
Heteroatoms
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