:Molybdate

Examples of molybdate oxyanions are:

• {{chem|MoO|4|2−}}, in e.g. Na₂MoO₄ and the mineral powellite, CaMoO₄;
• {{chem|Mo|2|O|7|2−}}, as hydrated ammonium dimolybdate. The anhydrous tetrabutylammonium salt of {{chem|Mo|2|O|7|2−}} is also known;{{cite journal | title = Synthesis and characterization of the dimolybdate ion, {{chem|Mo|2|O|7|2−}} | author1 = V. W. Day | author2 = M. F. Fredrich |author3-link=Walter G. Klemperer| author3 = W. G. Klemperer | author4 = W. Shum | journal = Journal of the American Chemical Society | volume = 99 | issue = 18 | year = 1977 | page = 6146 | doi = 10.1021/ja00460a074 }}
• {{chem|Mo|3|O|10|2−}} in the ethylenediamine salt;{{cite journal | title = Hydrothermal Synthesis and Crystal Structure of Anhydrous Ethylenediamine Trimolybdate {{chem|(C|2|H|10|N|2|)[Mo|3|O|10|]}} | author1 = Guillou N. | author2 = Ferey G. | journal = Journal of Solid State Chemistry | volume = 132 | issue = 1 |date=August 1997 | pages = 224–227(4) | doi = 10.1006/jssc.1997.7502 |bibcode = 1997JSSCh.132..224G }}
• {{chem|Mo|4|O|13|2−}} in the potassium salt;{{cite journal | title = Crystal structure of potassium tetramolybdate, {{chem|K|2|Mo|4|O|13}}, and its relationship to the structures of other univalent metal polymolybdates | author1 = B. M. Gatehouse | author2 = P. Leverett | journal = J. Chem. Soc. A | year = 1971 | pages = 2107–2112 | doi = 10.1039/J19710002107 }}
• {{chem|Mo|5|O|16|2−}} in the anilinium ({{chem|C|6|H|5|NH|3|+}}) salt;{{cite journal | title = Crystal Structure of Anilinium Pentamolybdate from Powder Diffraction Data. The Solution of the Crystal Structure by Direct Methods Package POWSIM | author1 = W. Lasocha | author2 = H. Schenk | journal = J. Appl. Crystallogr. | year = 1997 | volume = 30 | pages = 909–913 | doi = 10.1107/S0021889897003105 | issue = 6 }}
• {{chem|Mo|6|O|19|2−}} (hexa-molybdate) in the tetramethylammonium salt;{{cite journal | title = The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI) | author = S. Ghammami | journal = Crystal Research and Technology | volume = 38 | issue = 913 | pages = 913–917 | year = 2003 | doi = 10.1002/crat.200310112 | s2cid = 95078211 }}
• {{chem|Mo|7|O|24|6−}} in ammonium heptamolybdate, (NH₄)₆Mo₇O₂₄·4H₂O;{{cite journal | title = Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo₇O₂₄]⁶⁻, in the ammonium and potassium tetrahydrate salts | author1 = Howard T. Evans jr. | author2 = Bryan M. Gatehouse | author3 = Peter Leverett | journal = J. Chem. Soc., Dalton Trans. | year = 1975 | pages = 505–514 | doi = 10.1039/DT9750000505 | issue = 6 }}
• {{chem|Mo|8|O|26|4−}} 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, {{chem|Mo|7|O|24|6−}}, is often called "paramolybdate".

The smaller anions, {{chem|MoO|4|2−}} and {{chem|Mo|2|O|7|2−}} feature tetrahedral centres. In {{chem|MoO|4|2−}} the four oxygens are equivalent as in sulfate and chromate, with equal bond lengths and angles. {{chem|Mo|2|O|7|2−}} 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
• in bridging M–O–M units approximately 1.9 Å

The {{chem|Mo|8|O|26|4−}} 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 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. 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."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}}

File:Polyederstrukturen Molybdän.png|(a) Hexamolybdate [Mo₆O₁₉]²⁻ (b) Heptamolybdate [Mo₇O₂₄]⁶⁻

File:Heptamolybdate B&S.png|Ball and stick model of heptamolybdate

File:Heptamolybdate space fill.png|Heptamolybdate with space filling oxygen atoms

When {{chem2|MoO3}}, molybdenum trioxide is dissolved in alkali solution the simple {{chem2|MoO4(2-)}} anion is produced:

: MoO3 + 2 NaOH -> Na2MoO4 + H2O

As the pH is lowered, condensations ensue, with loss of water and the formation of Mo–O–Mo linkages. The stoichiometry leading to hexa-, hepta-, and octamolybdates are shown:{{cite book|first1=W. G.|last1=Klemperer|title=Inorganic Syntheses|series=Inorganic Syntheses|chapter=Tetrabutylammonium Isopolyoxometalates|year=1990|volume=27|pages=74–85 |doi=10.1002/9780470132586.ch15|isbn=9780470132586}}

: 6 [MoO4]^2- + 10 HCl -> [Mo6O19]^2- + 10 Cl- + 5 H2O

: 7 MoO4^2- + 8 H+ -> Mo7O24^6- + 4 H2O

: Mo7O24^6- + HMoO4^- + 3 H+ -> Mo8O26^4- + 2 H2O

Many peroxomolybdates are known. They tend to form upon treatment of molybdate salts with hydrogen peroxide. Notable is the monomer–dimer equilibrium:

: [Mo2O3(O2)2(H2O)2]^2- <=> [Mo2O3(O2)4(H2O)2]^2-

Also known but unstable is {{chem2|[Mo(O2)4](2-)}} (see potassium tetraperoxochromate(V)). Some related compounds find use as oxidants in organic synthesis.{{cite journal | last1 = Dickman | first1 = Michael H. | last2 = Pope | first2 = Michael T. | year = 1994 | title = Peroxo and Superoxo Complexes of Chromium, Molybdenum, and Tungsten | journal = Chem. Rev. | volume = 94 | issue = 3| pages = 569–584 | doi = 10.1021/cr00027a002}}

The red tetrathiomolybdate anion results when molybdate solutions are treated with hydrogen sulfide:

: [NH4]2[MoO4] + 4 H2S -> [NH4]2[MoS4] + 4 H2O

Like molybdate itself, {{chem2|MoS4(2-)}} undergoes condensation in the presence of acids, but these condensations are accompanied by redox processes.

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.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}}

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,{{cite web|url=http://www.gewater.com/handbook/cooling_water_systems/ch_31_open.jsp|title=Open Recirculating Cooling Systems - GE Water|work=gewater.com}} and is mainly used in high temperature closed-loop cooling circuits.{{cite web|url=http://www.gewater.com/handbook/cooling_water_systems/ch_32_closed.jsp|title=Closed Recirculating Cooling Systems - GE Water|work=gewater.com}} According to an experimental study, molybdate has been reported as an efficient biocide against microbiologically induced corrosion (MIC), where adding {{Nowrap|1.5 mM {{chem|MoO|4|2−}}/day}} resulted in a 50 % decrease in the corrosion rate.{{cite web|url=http://www.crcpress.com/Microbiologically-Influenced-Corrosion-in-the-Upstream-Oil-and-Gas-Industry/Skovhus-Enning-Lee/p/book/9781498726566|title=Microbiologically Influenced Corrosion in the Upstream Oil and Gas Industry}}

Molybdates (especially FeMoO₄, Fe₂(MoO₄)₃, NiMoO₄, CoMoO₄ and MnMoO₄) have been used as anode or cathode materials in aqueous capacitors.{{Cite journal|last1=Purushothaman|first1=K. K.|last2=Cuba|first2=M.|last3=Muralidharan|first3=G.|date=2012-11-01|title=Supercapacitor behavior of α-MnMoO₄ nanorods on different electrolytes|journal=Materials Research Bulletin|volume=47|issue=11|pages=3348–3351|doi=10.1016/j.materresbull.2012.07.027}}{{Cite journal|last1=Senthilkumar|first1=Baskar|last2=Sankar|first2=Kalimuthu Vijaya|last3=Selvan|first3=Ramakrishnan Kalai|last4=Danielle|first4=Meyrick|last5=Manickam|first5=Minakshi|date=2012-12-05|title=Nano α-NiMoO₄ as a new electrode for electrochemical supercapacitors|journal=RSC Adv.|language=en|volume=3|issue=2|pages=352–357|doi=10.1039/c2ra22743f|issn=2046-2069}}{{Cite journal|last1=Cai|first1=Daoping|last2=Wang|first2=Dandan|last3=Liu|first3=Bin|last4=Wang|first4=Yanrong|last5=Liu|first5=Yuan|last6=Wang|first6=Lingling|last7=Li|first7=Han|last8=Huang|first8=Hui|last9=Li|first9=Qiuhong|date=2013-12-26|title=Comparison of the Electrochemical Performance of NiMoO₄ Nanorods and Hierarchical Nanospheres for Supercapacitor Applications|journal=ACS Applied Materials & Interfaces|volume=5|issue=24|pages=12905–12910|doi=10.1021/am403444v|pmid=24274769|issn=1944-8244}}{{Cite journal|last1=Xia|first1=Xifeng|last2=Lei|first2=Wu|last3=Hao|first3=Qingli|last4=Wang|first4=Wenjuan|last5=Wang|first5=Xin|date=2013-06-01|title=One-step synthesis of CoMoO₄/graphene composites with enhanced electrochemical properties for supercapacitors|journal=Electrochimica Acta|volume=99|pages=253–261|doi=10.1016/j.electacta.2013.03.131}} Due to pseudocapacitive charge storage, specific capacitance up to 1500 F g⁻¹ has been observed.

Radioactive molybdenum-99 in the form of molybdate is used as the parent isotope in technetium-99m generators for nuclear medicine imaging.{{cite book|last1=National Research Council (US) Committee on Medical Isotope Production Without Highly Enriched Uranium.|title=Medical Isotope Production without Highly Enriched Uranium.|date=2009|publisher=National Academies Press|location=Washington DC|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK215133/|chapter=Molybdenum-99/Technetium-99m Production and Use}}

Nitrogen fixation requires molybdoenzymes in legumes (e.g., soybeans, acacia, etc.). 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 (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.{{cite web|url=http://www.astm.org/Standards/D7126.htm|title=ASTM D7126 – 15 Standard Test Method for On-Line Colorimetric Measurement of Silica|work=astm.org}} Additionally, it is used in the colorimetric analysis of phosphate concentration in association with the dye malachite green.

Molybdovanadate reagents contain both molybdate and vanadate ions. They are used in the determination of phosphate ion content in the analysis of wine and other fruit based products.{{Cite book |last=Amerine |first=Maynard A. |url=https://books.google.com/books?id=FUKIfyhCsCYC&pg=PA760 |title=Table Wines: The Technology of Their Production |last2=Amerine |first2=Maynard Andrew |last3=Joslyn |first3=Maynard Alexander |date=1970-01-01 |publisher=University of California Press |isbn=978-0-520-01657-6 |pages=760-1 |language=en}}

Molybdate crystals as collected by gem enthusiasts with the world's best samples of crystalized molybdate coming from Madawaska Mine in Ontario (Canada).{{Cite journal|last=McDougall|first=Raymond|date=2019-09-03|title=Mineral Highlights from the Bancroft Area, Ontario, Canada|url=https://doi.org/10.1080/00357529.2019.1619134|journal=Rocks & Minerals|volume=94|issue=5|pages=408–419|doi=10.1080/00357529.2019.1619134|s2cid=201298402 |issn=0035-7529|url-access=subscription}}

{{Reflist|30em}}

{{Molybdates}}

Category:Transition metal oxyanions

Category:Oxometallates