Molybdate

In

• MoO²⁻
  ₄, in e.g. Na₂MoO₄ and the mineral powellite, CaMoO₄;
• Mo
  ₂O²⁻
  ₇, as hydrated ammonium dimolybdate. The anhydrous tetrabutylammonium salt of Mo
  ₂O²⁻
  ₇ is also known;^[3]
• Mo
  ₃O²⁻
  ₁₀ in the ethylenediamine salt;^[4]
• Mo
  ₄O²⁻
  ₁₃ in the potassium salt;^[5]
• Mo
  ₅O²⁻
  ₁₆ in the anilinium (C
  ₆H
  ₅NH⁺
  ₃) salt;^[6]
• Mo
  ₆O²⁻
  ₁₉(hexa-molybdate) in the tetramethylammonium salt;^[7]
• Mo
  ₇O⁶⁻
  ₂₄ in ammonium heptamolybdate, (NH₄)₆Mo₇O₂₄·4H₂O;^[8]
• Mo
  ₈O⁴⁻
  ₂₆ in
  trimethylammonium salt.^[1]

The Mo
₈O⁴⁻
₂₆ 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 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, MoO₂ and the "crystallographic shear" oxides, Mo₉O₂₆ and Mo₁₀O₂₉) whose structures all contain close packed oxide ions.^[9]

• 
  (a) Hexamolybdate [Mo₆O₁₉]²⁻ (b) Heptamolybdate [Mo₇O₂₄]⁶⁻
• 
  Ball and stick model of heptamolybdate
• 
  Heptamolybdate with space filling oxygen atoms

Equilibria in aqueous solution

When MoO₃, molybdenum trioxide is dissolved in alkali solution the simple MoO2−4 anion is produced:

${\displaystyle {\ce {MoO3 + 2 NaOH -> Na2MoO4 + H2O}}}$

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:^[1][10]

${\displaystyle {\ce {6 [MoO4]^2- + 10 HCl -> [Mo6O19]^2- + 10 Cl- + 5 H2O}}}$

${\displaystyle {\ce {7 MoO4^2- + 8 H+ -> Mo7O24^6- + 4 H2O}}}$^[2]

${\displaystyle {\ce {Mo7O24^6- + HMoO4^- + 3 H+ -> Mo8O26^4- + 2 H2O}}}$^[2]

Peroxomolybdates

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

${\displaystyle {\ce {[Mo2O3(O2)2(H2O)2]^2- <=> [Mo2O3(O2)4(H2O)2]^2-}}}$

Also known but unstable is [Mo(O₂)₄]²⁻ (see potassium tetraperoxochromate(V)). Some related compounds find use as oxidants in organic synthesis.^[11]

Tetrathiomolybdate

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

${\displaystyle {\ce {[NH4]2[MoO4] + 4 H2S -> [NH4]2[MoS4] + 4 H2O}}}$

Like molybdate itself, MoS2−4 undergoes condensation in the presence of acids, but these condensations are accompanied by redox processes.

Industrial uses

Catalysis

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.^[12]

Corrosion inhibitors

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,^[13] and is mainly used in high temperature closed-loop cooling circuits.^[14] According to an experimental study, Molybdate has been reported as an efficient biocide against microbiologically induced corrosion (MIC), where adding 1.5 mM of Molybdate/day resulted in a 50% decrease in the corrosion rate.^[15]

Supercapacitors

Molybdates (especially FeMoO₄, Fe₂(MoO₄)₃, NiMoO₄, CoMoO₄ and MnMoO₄) have been used as anode or cathode materials in aqueous capacitors.^{[16][17][18][19]} Due to pseudocapacitive charge storage, specific capacitance up to 1500 F g⁻¹ has been observed.^[17]

Medicine

Radioactive molybdenum-99 in the form of molybdate is used as the parent isotope in technetium-99m generators for nuclear medicine imaging.^[20]

Other

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.^[12]

Molybdate chrome pigments are speciality but commercially available pigments.

Molybdate crystals as collected by gem enthusiasts with the world's best samples of crystalized Molybdate coming from Madawaska Mine.^[22]

References