Molybdate

In chemistry, a molybdate is a compound containing an oxyanion with molybdenum in its highest oxidation state of 6: . Molybdenum can form a very large range of such oxyanions, which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state. The larger oxyanions are members of group of compounds termed polyoxometalates, and because they contain only one type of metal atom are often called isopolymetalates. The discrete molybdenum oxyanions range in size from the simplest MoO42−, found in potassium molybdate up to extremely large structures found in isopoly-molybdenum blues that contain for example 154 Mo atoms. The behaviour of molybdenum is different from the other elements in group 6. Chromium only forms the chromates, CrO42−, Cr2O72−, Cr3O102− and Cr4O132− ions which are all based on tetrahedral chromium. Tungsten is similar to molybdenum and forms many tungstates containing 6 coordinate tungsten. Examples of molybdate oxyanions are: MoO42−, in e.g. Na2MoO4 and the mineral powellite, CaMoO4; Mo2O72−, as hydrated ammonium dimolybdate. The anhydrous tetrabutylammonium salt of Mo2O72− is also known; Mo3O102− in the ethylenediamine salt; Mo4O132− in the potassium salt; Mo5O162− in the anilinium (C6H5NH3+) salt; Mo6O192−(hexa-molybdate) in the tetramethylammonium salt; Mo7O246− in ammonium heptamolybdate, (NH4)6Mo7O24·4H2O; Mo8O264− in trimethylammonium salt. The naming of molybdates generally follows the convention of a prefix to show the number of Mo atoms present. For example, dimolybdate for 2 molybdenum atoms; trimolybdate for 3 molybdenum atoms, etc.. Sometimes the oxidation state is added as a suffix, such as in pentamolybdate(VI). The heptamolybdate ion, Mo7O246−, is often called "paramolybdate". The smaller anions, MoO42− and Mo2O72− feature tetrahedral centres. In MoO42− the four oxygens are equivalent as in sulfate and chromate, with equal bond lengths and angles. Mo2O72− can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom.