Molybdate

Structure of molybdate

3D model of the molybdate ion

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.^[1] 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 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.^[2]

Examples of molybdate anions

Examples of molybdate oxoanions are:

• discrete MoO₄²⁻, in e.g. Na₂MoO₄ and the mineral powellite, CaMoO₄;

• discrete Mo₂O₇²⁻ in the tetrabutylammonium salt and polymeric Mo₂O₇²⁻ in the ammonium salt;^[3]

• polymeric Mo₃O₁₀²⁻ in the ethylenediamine salt;^[4]

• polymeric Mo₄O₁₃²⁻ in the potassium salt;^[5]

• polymeric Mo₅O₁₆²⁻ in the anilinium, (C₆H₅NH₃)⁺ salt;^[6]

• discrete Mo₆O₁₉²⁻(hexa-molybdate) in the tetramethylammonium salt;^[7]

• discrete Mo₇O₂₄⁶⁻ in ammonium molybdate, (NH₄)₆Mo₇O₂₄.4H₂O;^[8]

• Mo₈O₂₆⁴⁻ in trimethylammonium salt.^[1]

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 and Mo₂O₇²⁻ can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom.^[1] 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
• 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.^[2]

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.^[1] There are strong similarities between the structures of the molybdates and the molybdenum oxides, (MoO₃, MoO₂ and the "crystallographic shear" oxides, Mo₉O₂₆ and Mo₁₀O₂₉) whose structures all contain close packed oxide ions.^[9]

Equilibria in aqueous solution

When MoO₃, molybdenum trioxide is dissolved in alkali solution the simple MoO₄²⁻ anion is produced. As the pH is reduced the first species to be formed is the heptamolybdate rather than any of the smaller anions:

7 MoO₄²⁻ + 8H⁺ Mo₇O₂₄⁶⁻ + 4H₂O ^[2]

As the pH is decreased the octamolybdate is formed further anions with 8 and probably 16–18 Mo atoms are present:^[1]

Mo₇O₂₄⁶⁻ + 3H⁺ Mo₈O₂₆⁴⁻ + 2H₂O ^[2]

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.^[1]

Industrial uses

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 displays only moderate corrosion inhibition,^[10] and is mainly used in high temperature closed-loop cooling circuits.^[11]

Molybdate (usually in the form of potassium molybdate) is also used in the analytical colorimetric testing for the concentration of silica in solution, called the molybdenum blue method.^[12] Additionally, it is used in the colorimetric determination of phosphate quantity in association with the dye malachite green.

References

1. ^ ^{a b c d e f} Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 
2. ^ ^{a b c d} Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
3. ^ V. W. Day; M. F. Fredrich; W. G. Klemperer; W. Shum (1977). "Synthesis and characterization of the dimolybdate ion, Mo₂O₇²⁻". Journal of the American Chemical Society 99 (18): 6146. doi:10.1021/ja00460a074. 
4. ^ Guillou N.; Ferey G. (August 1997). "Hydrothermal Synthesis and Crystal Structure of Anhydrous Ethylenediamine Trimolybdate (C₂H₁₀N₂)[Mo₃O₁₀]". Journal of Solid State Chemistry 132 (1): 224–227(4). doi:10.1006/jssc.1997.7502. 
5. ^ B. M. Gatehouse; P. Leverett (1971). "Crystal structure of potassium tetramolybdate, K₂Mo₄O₁₃, and its relationship to the structures of other univalent metal polymolybdates". J. Chem. Soc. A: 2107–2112. doi:10.1039/J19710002107. 
6. ^ W. Lasocha; H. Schenk (1997). "Crystal Structure of Anilinium Pentamolybdate from Powder Diffraction Data. The Solution of the Crystal Structure by Direct Methods Package POWSIM". J. Appl. Cryst. 30 (6): 909–913. doi:10.1107/S0021889897003105. 
7. ^ S. Ghammami (2003). "The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI)". Cryst. Res. Technol. 38 (913): 913. doi:10.1002/crat.200310112. 
8. ^ Howard T. Evans jr.; Bryan M. Gatehouse; Peter Leverett (1975). "Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo₇O₂₄]⁶⁻, in the ammonium and potassium tetrahydrate salts". J. Chem. Soc., Dalton Trans. (6): 505–514. doi:10.1039/DT9750000505. 
9. ^ Oxides: solid state chemistry W.H. McCarroll Encyclopedia of Inorganic Chemistry Ed. R. Bruce King, John Wiley and sons (1994)ISBN 0-471-93620-0
10. ^ http://www.gewater.com/handbook/cooling_water_systems/ch_31_open.jsp
11. ^ http://www.gewater.com/handbook/cooling_water_systems/ch_32_closed.jsp
12. ^ http://www.astm.org/Standards/D7126.htm