Molybdate

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 sodium 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].

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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 Mo₇O₂₄ ⁶⁻ 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 numer of Mo atoms present e.g. 2 molybdenum atoms, dimolybdate; 3 molybdenum atoms, trimolybdate etc. Sometimes the oxidation state is added as a suffix e.g. pentamolybdate(VI). The heptamolybdate ion, Mo₇O₂₄ ⁶⁻, is often called "paramolybdate".

Structure of molybdate anions

The smaller anions, MoO₄^2- and Mo₂O₇^2- contain only 4 coordinate molybdenum, MoO₄^2- is tetrahedral and Mo₂O₇^2- 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 1.7A
• in bridging M-O-M units approximately 1.9A

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 140 pm, molybdenum(VI) is much smaller, 59 pm^[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⁺ ⇌ Mo₇O₂₄ ⁶⁻ +4H₂0 ^[2]

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

Mo₇O₂₄ ⁶⁻ +3H⁺ ⇌ Mo₈O₂₆ ⁴⁻ + 2H₂0 ^[2]

A further increase 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 no appear to be in solution to be precipitated. ^[1].

References

1. ^ ^{a b c d e f} Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
2. ^ ^{a b c d} Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
3. ^ Synthesis and characterization of the dimolybdate ion, Mo₂O₇²⁻ V. W. Day, M. F. Fredrich, W. G. Klemperer, and W. Shum Journal of the american Chemical Society 99, 18, (1977), 6146 doi:10.1021/ja00460a074
4. ^ Hydrothermal Synthesis and Crystal Structure of Anhydrous Ethylenediamine Trimolybdate (C2H10N2)[Mo3O10] Guillou N.; Ferey G. Journal of Solid State Chemistry, Volume 132, Number 1, August 1997 , pp. 224-227(4) doi:10.1006/jssc.1997.7502
5. ^ Crystal structure of potassium tetramolybdate, K2Mo4O13, and its relationship to the structures of other univalent metal polymolybdates B. M. Gatehouse and P. Leverett J. Chem. Soc. A, 1971, 2107 - 2112, doi:10.1039/J19710002107
6. ^ Crystal Structure of Anilinium Pentamolybdate from Powder Diffraction Data. The Solution of the Crystal Structure by Direct Methods Package POWSIM W. Lasocha and H. Schenk J. Appl. Cryst. (1997). 30, 909-913 doi:10.1107/S0021889897003105
7. ^ The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI) S. Ghammami Cryst. Res. Technol. 38, 913 (2003)doi:10.1002/crat.200310112
8. ^ Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo₇O₂₄]⁶⁻, in the ammonium and potassium tetrahydrate salts Howard T. Evans jun. , Bryan M. Gatehouse and Peter Leverett J. Chem. Soc., Dalton Trans., 1975, 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