page_title: Polyoxometalate
section_title: Polyoxotantalates, niobates, and vanadates
hierarchical_section_title: Polyoxometalate / Structure / Polyoxotantalates, niobates, and vanadates
caption_attribution_description: English: Molecular strucutures of polyoxometalates
context_page_description: In chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 or less commonly group 5 transition metals in their high oxidation states. They are usually colorless or orange, diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion. Many exceptions to these general statements exist.
context_section_description: The properties of polyniobates and polytantalates are similar, but substantially different from the polyoxovanadates. In fact, polyvanadates are more similar to the oxomolybdates and tungstates.
  The similarities between the polyniobates and polytantalates arise primarily from the equivalence in the charge of their stable 5+ ion and their size (64 pm) due to the lanthanide contraction. The most common members are M
  ₆O⁸⁻
  ₁₉ (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(O
  ₂)³⁻
  ₄ 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 M
  ₁₀O⁶⁻
  ₂₈ (M = Nb, Ta) are isostructural with decavanadate and are formed exclusively by edge-sharing {MO₆} octahedra whereas the structure of decatungstate W
  ₁₀O⁴⁻
  ₃₂ comprises edge-sharing and corner-sharing tungstate octahedra.
caption: Molecular structures of polyoxometalates.
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