text: The polyniobates, polytantalates, and vanadates are derived, formally at least, from highly charged [MO] precursors. For Nb and Ta, most common members are  (M = Nb, Ta), which adopt the Lindqvist structure. These octaanions form in strongly basic conditions from alkali melts of the extended metal oxides (MO), 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  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 = Nb, Ta) are isostructural with decavanadate. They are formed exclusively by edge-sharing {MO} octahedra (the structure of decatungstate  comprises edge-sharing and corner-sharing tungstate octahedra).