In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The larger oxoanions 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 oxoanions range in size from the simplest , 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,,, and ions which are all based on tetrahedral chromium. Tungsten is similar to molybdenum and forms many tungstates containing 6 coordinate tungsten.
Examples of molybdate anions
Examples of molybdate oxoanions are:
, in e.g. Na2MoO4 and the mineral powellite, CaMoO4;
, as hydrated diammonium dimolybdate. The anhydrous tetrabutylammonium salt of is also known;
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. The heptamolybdate ion,, is often called "paramolybdate".
Structure of molybdate anions
The smaller anions, and feature tetrahedral centres. In the four oxygens are equivalent as in sulfate and chromate, with equal bond lengths and angles. can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom. In the larger anions molybdenum is generally, but not exclusively, 6 coordinate with edges or vertices of the MoO6 octahedra being shared. The octahedra are distorted, typical M-O bond lengths are:
The anion contains both octahedral and tetrahedral molybdenum and can be isolated in 2 isomeric forms, alpha and beta. The hexamolybdate image below shows the coordination polyhedra. The heptamolybdate image shows the close packed nature of the oxygen atoms in the structure. The oxide ion has an ionic radius of 1.40 Å, molybdenum is much smaller, 0.59 Å. There are strong similarities between the structures of the molybdates and the molybdenum oxides, whose structures all contain close packed oxide ions.
When MoO3, molybdenum trioxide is dissolved in alkali solution the simple anion is produced. As the pH is reduced the first species to be formed is the heptamolybdate rather than any of the smaller anions: As the pH is decreased the octamolybdate is formed further anions with 8 and probably 16–18 Mo atoms are present: A further decrease leads to anions with probably 16–18 Mo atoms. However careful manipulation of the pH and temperature coupled with very long precipitation times can cause compounds with ions that do not appear to be in solution to be precipitated.
Peroxomolybdates
Many peroxomolybdates are known. They tend to form upon treatment of molybdate salts with hydrogen peroxide. Notable is the monomer–dimer equilibrium 2−2−. Also known but unstable is 2−. Some related compounds find use as oxidants in organic synthesis.
Tetrathiomolybdate
The red tetrathiomolybdate anion results when molybdate solutions are treated with hydrogen sulfide: Like molybdate itself, undergoes condensation in the presence of acids, but these condensations are accompanied by redox processes.
Industrial uses
Catalysis
Molybdates are widely used in catalysis. In terms of scale, the largest consumer of molybdate is as a precursor to catalysts for hydrodesulfurization, the process by which sulfur is removed from petroleum. Bismuth molybdates, nominally of the composition Bi9PMo12O52, catalyzes ammoxidation of propylene to acrylonitrile. Ferric molybdates are used industrially to catalyze the oxidation of methanol to formaldehyde.
Corrosion inhibitors
Sodium molybdate has been used in industrial water treatment as a corrosion inhibitor. It was initially thought that it would be a good replacement for chromate, when chromate was banned for toxicity. However, molybdate requires high concentrations when used alone, therefore complementary corrosion inhibitors are generally added, and is mainly used in high temperature closed-loop cooling circuits.. According to an experimental study, Molybdate has been reported as an efficient biocide against microbiologically induced corrosion, where adding 1.5 mM of Molybdate/day resulted in a 50% decrease in the corrosion rate.
Supercapacitors
Molybdates have been used as anode or cathode materials in aqueous capacitors. Due to pseudocapacitive charge storage, specific capacitance up to 1500 F g−1 has been observed.
requires molybdoenzymes in legumes. For this reason, fertilizers often contain small amounts of molybdate salts. Coverage is typically less than a kilogram per acre. Molybdate chrome pigments are speciality but commercially available pigments. Molybdate is also used in the analytical colorimetric testing for the concentration of silica in solution, called the molybdenum blue method. Additionally, it is used in the colorimetric determination of phosphate quantity in association with the dyemalachite green.
