text: The oxides of d metals such as , ,  dissolve at high pH to give orthometalates, , , . For  and , the nature of the dissolved species at high pH is less clear, but these oxides also form polyoxometalates. 
  As the pH is lowered, orthometalates protonate to give oxide–hydroxide compounds such as  and . 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  species entails loss of water and the formation of  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 result. For example, the phosphotungstate anion  consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.