# Molybdate

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In [chemistry](/source/Chemistry), a **molybdate** is a compound containing an [oxyanion](/source/Oxyanion) with [molybdenum](/source/Molybdenum) in its highest [oxidation state](/source/Oxidation_state) of +6: O-\sMo(\dO)2\sO-. Molybdenum can form a very large range of such [oxyanions](/source/Oxyanion), which can be discrete structures or [polymeric](/source/Polymer) extended structures, although the latter are only found in the [solid](/source/Solid) state. The larger oxyanions are members of group of compounds termed *[polyoxometalates](/source/Polyoxometalate)*, and because they contain only one type of metal atom are often called *isopolymetalates*.[1] 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](/source/Molybdenum_blue#Isopoly_molybdenum_blues) that contain for example 154 Mo atoms. The behaviour of molybdenum is different from the other elements in [group 6](/source/Group_6_element). [Chromium](/source/Chromium) only forms the [chromates](/source/Chromate_ion), CrO42−, Cr2O72−, Cr3O102− and Cr4O132− ions which are all based on tetrahedral chromium. [Tungsten](/source/Tungsten) is similar to molybdenum and forms many [tungstates](/source/Tungstate) containing 6 coordinate tungsten.[2]

## Examples of molybdate anions

Examples of molybdate oxyanions are:

- MoO42−, in e.g. [Na2MoO4](/source/Sodium_molybdate) and the mineral [powellite](/source/Powellite), CaMoO4;
- Mo2O72−, as hydrated [ammonium dimolybdate](/source/Ammonium_dimolybdate). The anhydrous tetrabutylammonium salt of Mo2O72− is also known;[3]
- Mo3O102− in the [ethylenediamine](/source/Ethylenediamine) salt;[4]
- Mo4O132− in the potassium salt;[5]
- Mo5O162− in the [anilinium](/source/Aniline) (C6H5NH3+) salt;[6]
- Mo6O192− (hexa-molybdate) in the tetramethylammonium salt;[7]
- Mo7O246− in [ammonium heptamolybdate](/source/Ammonium_heptamolybdate), (NH4)6Mo7O24·4H2O;[8]
- Mo8O264− in [trimethylammonium](/source/Trimethylammonium) salt;[1]
- Mo10O336− in [silver decamolybdate](/source/Silver_decamolybdate).[9]

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](/source/Oxidation_state) is added as a suffix, such as in *pentamolybdate(VI)*. The heptamolybdate ion, Mo7O246−, is often called "paramolybdate".

## Structure of molybdate anions

The smaller anions, [MoO4](2−) and [Mo2O7](2−) feature tetrahedral centres. In [MoO4](2−) the four oxygens are equivalent as in [sulfate](/source/Sulfate) and [chromate](/source/Monochromate), with equal bond lengths and angles. [Mo2O7](2−)) can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom, as in [dichromate](/source/Dichromate).[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](/source/%C3%85ngstrom)
- 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](/source/Isomer) forms, alpha and beta.[2]

The hexamolybdate image below shows the coordination polyhedra. The space filling model of 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](/source/Molybdenum_trioxide), [MoO2](/source/Molybdenum_dioxide) and the "[crystallographic shear](/source/Crystallographic_shear)" oxides, Mo9O26 and Mo10O29) whose structures all contain close packed oxide ions.[10]

## Equilibrium in aqueous solution

When MoO3, molybdenum trioxide is dissolved in [alkali](/source/Alkali) solution the simple MoO4(2-) [anion](/source/Anion) is produced:

- MoO3 + 2 NaOH → Na2MoO4 + H2O

As the [pH](/source/PH) is lowered, condensations ensue, with loss of water and the formation of Mo–O–Mo linkages. The [stoichiometry](/source/Stoichiometry) leading to hexa-, hepta-, and octamolybdates are shown:[1][11]

- 6 [MoO4](2-) + 10 HCl → [Mo6O19](2-) + 10 Cl- + 5 H2O
- 7 [MoO4](2-) + 8 H+ → [Mo7O24](6-) + 4 H2O[2]
- [Mo7O24](6-) + [HMoO4]- + 3 H+ → [Mo8O26](4-) + 2 H2O[2]

### Peroxomolybdates

Many peroxomolybdates are known, e.g. [Mo2O3(O2)2(H2O)2](2-) and the [tetraperoxide complex](/source/Transition_metal_tetraperoxide_complexes) [Mo(O2)4](2-).[12] They form upon treatment of molybdate salts with hydrogen peroxide.

### Tetrathiomolybdate

The red [tetrathiomolybdate](/source/Tetrathiomolybdate) anion results when molybdate solutions are treated with [hydrogen sulfide](/source/Hydrogen_sulfide):

- [NH4]2[MoO4] + 4 H2S → [NH4]2[MoS4] + 4 H2O

Like molybdate itself, MoS4(2-) undergoes condensation in the presence of acids, but these condensations are accompanied by [redox](/source/Redox) processes.

## Industrial uses

### Catalysis

Molybdates are widely used in [catalysis](/source/Catalysis). In terms of scale, the largest consumer of molybdate is as a precursor to catalysts for [hydrodesulfurization](/source/Hydrodesulfurization), the process by which sulfur is removed from petroleum. Bismuth molybdates, nominally of the composition Bi9PMo12O52, catalyzes [ammoxidation](/source/Ammoxidation) of [propylene](/source/Propylene) to [acrylonitrile](/source/Acrylonitrile). Ferric molybdates are used industrially to [catalyze](/source/Catalysis) the [oxidation](/source/Redox) of [methanol](/source/Methanol) to [formaldehyde](/source/Formaldehyde).[13]

### Corrosion inhibitors

Sodium molybdate has been used in industrial water treatment as a [corrosion inhibitor](/source/Corrosion_inhibitor). It was initially thought that it would be a good replacement for [chromate](/source/Chromate_and_dichromate), when chromate was banned for toxicity. However, molybdate requires high concentrations when used alone, therefore complementary [corrosion inhibitors](/source/Corrosion_inhibitor) are generally added,[14] and is mainly used in high temperature closed-loop cooling circuits.[15] According to an experimental study, molybdate has been reported as an efficient [biocide](/source/Biocide) against microbiologically induced [corrosion](/source/Corrosion) (MIC), where adding 1.5 mM MoO42−/day resulted in a 50 % decrease in the corrosion [rate](/source/Reaction_rate).[16]

### Supercapacitors

Molybdates (especially FeMoO4, Fe2(MoO4)3, NiMoO4, CoMoO4 and MnMoO4) have been used as [anode](/source/Anode) or [cathode](/source/Cathode) materials in aqueous capacitors.[17][18][19][20] Due to [pseudocapacitive](/source/Pseudocapacitance) charge storage, specific [capacitance](/source/Capacitance) up to 1500 F g−1 has been observed.[18]

### Medicine

Radioactive [molybdenum-99](/source/Isotopes_of_molybdenum) in the form of molybdate is used as the parent isotope in [technetium-99m generators](/source/Technetium-99m_generator) for [nuclear medicine](/source/Nuclear_medicine) imaging.[21]

### Other

[Nitrogen fixation](/source/Nitrogen_fixation) requires [molybdoenzymes](/source/Molybdenum_in_biology) in legumes (e.g., soybeans, acacia, etc.). For this reason, [fertilizers](/source/Fertilizer) often contain small amounts of molybdate salts. Coverage is typically less than a kilogram per acre.[13]

Molybdate chrome [pigments](/source/Pigment) are speciality but commercially available pigments.[13] Molybdate (usually in the form of potassium molybdate) is also used in the analytical [colorimetric](/source/Colorimetry) testing for the concentration of [silica](/source/Silica) in solution, called the [molybdenum blue](/source/Molybdenum_blue) method.[22] Additionally, it is used in the [colorimetric analysis](/source/Colorimetric_analysis) of [phosphate](/source/Phosphate) concentration in association with the [dye](/source/Dye) [malachite green](/source/Malachite_green).

[Molybdovanadate reagents](/source/Molybdovanadate_reagent) contain both molybdate and [vanadate](/source/Vanadate) ions. They are used in the determination of [phosphate ion](/source/Phosphate_ion) content in the analysis of [wine](/source/Wine) and other fruit based products.[23]

## Natural gems

Molybdate crystals as collected by gem enthusiasts with the world's best samples of crystalized molybdate coming from [Madawaska Mine](/source/Madawaska_Mine) in [Ontario](/source/Ontario) (Canada).[24]

## References

1. 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](https://doi.org/10.1021/ja00460a074)

1. 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). [Bibcode:1997JSSCh.132..224G](https://ui.adsabs.harvard.edu/abs/1997JSSCh.132..224G). [doi:10.1006/jssc.1997.7502](https://doi.org/10.1006/jssc.1997.7502)

1. 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*. [doi:10.1039/J19710002107](https://doi.org/10.1039/J19710002107)

1. 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. Crystallogr.*. **30** (6): 909–913. [doi:10.1107/S0021889897003105](https://doi.org/10.1107/S0021889897003105)

1. S. Ghammami (2003). "The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI)". *Crystal Research and Technology*. **38** (913): 913–917. [doi:10.1002/crat.200310112](https://doi.org/10.1002/crat.200310112). [S2CID 95078211](https://api.semanticscholar.org/CorpusID:95078211)

1. 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](https://doi.org/10.1039/DT9750000505)***

1. Gatehouse, B. M. & Leverett, P. (1970-04-01). ["The crystal structure of silver decamolybdate, Ag6Mo10O33"](https://www.sciencedirect.com/science/article/pii/0022459670901313). *Journal of Solid State Chemistry*. **1** (3): 484–496. [doi:10.1016/0022-4596(70)90131-3](https://doi.org/10.1016/0022-4596(70)90131-3). [ISSN 0022-4596](https://www.worldcat.org/issn/0022-4596)

1. "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

1. Klemperer, W. G. (1990). "Tetrabutylammonium Isopolyoxometalates". *Inorganic Syntheses*. Vol. 27. Inorganic Syntheses. pp. 74–85. [doi:10.1002/9780470132586.ch15](https://doi.org/10.1002/9780470132586.ch15). ISBN 9780470132586.

1. Grzywa, M.; Łasocha, W.; Rutkowska-Żbik, D. (2009). ["Structural investigation of tetraperoxo complexes of Mo(VI) and W(VI)"](https://ruj.uj.edu.pl/xmlui/handle/item/76786). *Journal of Solid State Chemistry*. **182** (4): 973–982. [doi:10.1016/j.jssc.2009.01.009](https://doi.org/10.1016/j.jssc.2009.01.009)

1. Roger F. Sebenik et al. "Molybdenum and Molybdenum Compounds" in *Ullmann's Encyclopedia of Chemical Technology* 2005; Wiley-VCH, Weinheim. [doi:10.1002/14356007.a16_655](https://doi.org/10.1002/14356007.a16_655)

1. ["Open Recirculating Cooling Systems - GE Water"](http://www.gewater.com/handbook/cooling_water_systems/ch_31_open.jsp). *gewater.com*

1. ["Closed Recirculating Cooling Systems - GE Water"](http://www.gewater.com/handbook/cooling_water_systems/ch_32_closed.jsp). *gewater.com*

1. ["Microbiologically Influenced Corrosion in the Upstream Oil and Gas Industry"](http://www.crcpress.com/Microbiologically-Influenced-Corrosion-in-the-Upstream-Oil-and-Gas-Industry/Skovhus-Enning-Lee/p/book/9781498726566)

1. Purushothaman, K. K.; Cuba, M.; Muralidharan, G. (2012-11-01). "Supercapacitor behavior of α-MnMoO4 nanorods on different electrolytes". *Materials Research Bulletin*. **47** (11): 3348–3351. [doi:10.1016/j.materresbull.2012.07.027](https://doi.org/10.1016/j.materresbull.2012.07.027)

1. Senthilkumar, Baskar; Sankar, Kalimuthu Vijaya; Selvan, Ramakrishnan Kalai; Danielle, Meyrick; Manickam, Minakshi (2012-12-05). "Nano α-NiMoO4 as a new electrode for electrochemical supercapacitors". *RSC Adv.*. **3** (2): 352–357. [doi:10.1039/c2ra22743f](https://doi.org/10.1039/c2ra22743f). [ISSN 2046-2069](https://www.worldcat.org/issn/2046-2069)

1. Cai, Daoping; Wang, Dandan; Liu, Bin; Wang, Yanrong; Liu, Yuan; Wang, Lingling; Li, Han; Huang, Hui; Li, Qiuhong (2013-12-26). "Comparison of the Electrochemical Performance of NiMoO4 Nanorods and Hierarchical Nanospheres for Supercapacitor Applications". *ACS Applied Materials & Interfaces*. **5** (24): 12905–12910. [doi:10.1021/am403444v](https://doi.org/10.1021/am403444v). [ISSN 1944-8244](https://www.worldcat.org/issn/1944-8244). [PMID 24274769](https://pubmed.ncbi.nlm.nih.gov/24274769)

1. Xia, Xifeng; Lei, Wu; Hao, Qingli; Wang, Wenjuan; Wang, Xin (2013-06-01). "One-step synthesis of CoMoO4/graphene composites with enhanced electrochemical properties for supercapacitors". *Electrochimica Acta*. **99**: 253–261. [doi:10.1016/j.electacta.2013.03.131](https://doi.org/10.1016/j.electacta.2013.03.131)

1. National Research Council (US) Committee on Medical Isotope Production Without Highly Enriched Uranium. (2009). ["Molybdenum-99/Technetium-99m Production and Use"](https://www.ncbi.nlm.nih.gov/books/NBK215133/). *Medical Isotope Production without Highly Enriched Uranium.*. Washington DC: National Academies Press.

1. ["ASTM D7126 – 15 Standard Test Method for On-Line Colorimetric Measurement of Silica"](http://www.astm.org/Standards/D7126.htm). *astm.org*

1. Amerine, Maynard A.; Amerine, Maynard Andrew; Joslyn, Maynard Alexander (1970-01-01). [*Table Wines: The Technology of Their Production*](https://books.google.com/books?id=FUKIfyhCsCYC&pg=PA760). University of California Press. pp. 760–1. ISBN 978-0-520-01657-6.

1. McDougall, Raymond (2019-09-03). ["Mineral Highlights from the Bancroft Area, Ontario, Canada"](https://doi.org/10.1080/00357529.2019.1619134). *Rocks & Minerals*. **94** (5): 408–419. [doi:10.1080/00357529.2019.1619134](https://doi.org/10.1080/00357529.2019.1619134). [ISSN 0035-7529](https://www.worldcat.org/issn/0035-7529). [S2CID 201298402](https://api.semanticscholar.org/CorpusID:201298402)

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