In chemistry a molybdate is a compound containing an oxoanion with molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 in its highest oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 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 polyoxometalate
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 a large, closed 3-dimensional framework....

s, and because they contain only one type of metal atom are often called isopolymetalates. The discrete molybdenum oxoanions 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
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 only forms the chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...

s, CrO42, Cr2O72, Cr3O102 and Cr4O132 ions which are all based on tetrahedral chromium. Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 is similar to molybdenum and forms many tungstate
In chemistry a tungstate is a compound that contains an oxoanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is WO42−, "orthotungstate"...

s containing 6 coordinate tungsten.

Examples of molybdate anions

Examples of molybdate oxoanions are:
  • discrete MoO42−, in e.g. Na2MoO4
    Sodium molybdate
    Sodium molybdate, Na2MoO4, is useful as a source of molybdenum. It is often found as the dihydrate, Na2MoO4·2H2O.The molybdate anion is tetrahedral. Two sodium cations coordinate with every one anion.-History:...

     and the mineral powellite
    Powellite is a calcium molybdate mineral with formula CaMoO4. Powellite crystallizes with tetragonal - dipyramidal crystal structure as transparent adamantine blue, greenish brown, yellow to grey typically anhedral forms. It exhibits distinct cleavage and has a brittle to conchoidal fracture. It...

    , CaMoO4;

  • discrete Mo2O72− in the tetrabutylammonium salt and polymeric Mo2O72− in the ammonium salt;

  • polymeric Mo3O102− in the ethylenediamine salt;

  • polymeric Mo4O132− in the potassium salt;

  • polymeric Mo5O162− in the anilinium, (C6H5NH3)+ salt;

  • discrete Mo6O192−(hexa-molybdate) in the tetramethylammonium salt;

  • discrete Mo7O246− in ammonium molybdate
    Ammonium molybdate
    Ammonium heptamolybdate is an odourless crystalline compound ranging in colour from white to yellow-green. It is usually encountered as the tetrahydrate, whose chemical formula is 6Mo7O24·4H2O...

    , (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, Mo7O24 6−, is often called "paramolybdate".

Structure of molybdate anions

The smaller anions, MoO42− and Mo2O72− contain only 4 coordinate molybdenum, MoO42− is tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 and Mo2O72− 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:
  • in terminal non bridging M-O approximately 1.7 Å
    The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

  • in bridging M-O-M units approximately 1.9 Å

The Mo8O264 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(VI) is much smaller, 0.59 Å. There are strong similarities between the structures of the molybdates and the molybdenum oxides, (MoO3, MoO2
Molybdenum dioxide
Molybdenum dioxide is the chemical compound with the formula MoO2. It is a violet-colored solid and is a metallic conductor. It crystallizes in a monoclinic cell, and has a distorted rutile, crystal structure. In TiO2 the oxide anions are close packed and titanium atoms occupy half of the...

 and the "crystallographic shear" oxides, Mo9O26 and Mo10O29) whose structures all contain close packed oxide ions.

Equilibria in aqueous solution

When MoO3, molybdenum trioxide is dissolved in alkali solution the simple MoO42 anion is produced. As the pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 is reduced the first species to be formed is the heptamolybdate rather than any of the smaller anions:
7 MoO42− + 8H+ Mo7O246− + 4H2O

As the pH is decreased the octamolybdate is formed further anions with 8 and probably 16–18 Mo atoms are present:
Mo7O246− + 3H+ Mo8O264− + 2H2O

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.

Industrial uses

Molybdate has been used in industrial water treatment as a corrosion inhibitor
Corrosion inhibitor
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...

. It was initially thought that it would be a good replacement for chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...

, when chromate was banned for toxicity. However, molybdate displays only moderate corrosion inhibition, and is mainly used in high temperature closed-loop cooling circuits.

Molybdate (usually in the form of potassium molybdate) is also used in the analytical colorimetric
Colorimetry is "the science and technology used to quantify and describe physically the human color perception."It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the CIE 1931 XYZ color space...

 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 dye malachite green.
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