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Polyoxometalate
Polyoxometalates (POMs) are a group of inorganic anionic molecular metal oxides.
The oxides of d0 metals such as V2O5, MoO3, WO3 dissolve at high pH to give orthometalates, VO3−4, MoO2−4, WO2−4. 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 called olation. The replacement of terminal M=O bonds, which in fact 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.
Condensation of the MO3(OH)n− species entails loss of water and the formation of M−O−M linkages. The stoichiometry for hexamolybdate is shown:
An abbreviated condensation sequence illustrated with vanadates is:
When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate can result. For example, the phosphotungstate anion [PW12O40]3− consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
Ammonium phosphomolybdate, [PMo12O40]3− 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.
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Polyoxometalate
Polyoxometalates (POMs) are a group of inorganic anionic molecular metal oxides.
The oxides of d0 metals such as V2O5, MoO3, WO3 dissolve at high pH to give orthometalates, VO3−4, MoO2−4, WO2−4. 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 called olation. The replacement of terminal M=O bonds, which in fact 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.
Condensation of the MO3(OH)n− species entails loss of water and the formation of M−O−M linkages. The stoichiometry for hexamolybdate is shown:
An abbreviated condensation sequence illustrated with vanadates is:
When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate can result. For example, the phosphotungstate anion [PW12O40]3− consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
Ammonium phosphomolybdate, [PMo12O40]3− 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.