Overview of Molybdenum Chemistry
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- Category: Molybdenum knowledge
- Published on 17 September 2014
- Written by jwy
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Molybdenum is a transition metal in Group 6 of the Periodic Table between chromium and tungsten. Although molybdenum is sometimes described as a ‘heavy metal’ its properties are very different from those of the typical heavy metals, mercury, thallium and lead. It is much less toxic than these and other heavy metals. Its low toxicity makes molybdenum an attractive substitute for more toxic materials.
Compounds of molybdenum which are commonly encountered have molybdenum in its highest oxidation state, VI, for example molybdenum trioxide, MoO3, sodium molybdate, Na2MoO4.2H2O, and ammonium di- and heptamolybdate, (NH4)2Mo2O7 and (NH4)6Mo7O24.4H2O. In aqueous solution molybdenum(VI) is present as the simple molybdate, [MoO4] 2- ion which is like sulfate or, depending on the concentration and pH as a polymeric polymolybdate ion. The lower oxidation state, IV, is found in the commonest ore of molybdenum the disulfide, MoS2. Molybdenum(IV) also forms an oxide, MoO2. The redox chemistry of molybdenum-oxygen compounds, as in selective oxidation catalysts and molybdenum oxidase enzymes, has molybdenum cycling between oxidation states (VI) and (IV).
The chemistry of molybdenum is extraordinarily versatile: oxidation states from (-II) to (VI), coordination numbers from 4 to 8 and, accordingly a very varied stereochemistry; the ability to form compounds with most inorganic and organic ligands and bi- and polynuclear compounds containing molybdenum-molybdenum bonds and bridging ligands. It is this versatility which makes the chemistry of molybdenum challenging and exciting and the actual and potential applications of its compounds many and varied. Molybdenum is the first of the transition metals to have an extensive sulfur chemistry shown, for example, having as its principal ore molybdenum disulfide, MoS2, its binding by sulfur ligands in molybdenum containing enzymes, application of MoS2 as an important industrial catalyst, and formation of many sulfur complexes some of which are used as soluble lubricating oil additives.
Molybdenum has an extensive organometallic chemistry in its lower oxidation states. These compounds contain molybdenum-carbon bonds. A well known example is molybdenum hexacarbonyl, Mo(CO)6. These compounds are difficult to prepare and may decompose on exposure to air. They have specialised small volume uses as for example catalysts in fine chemicals synthesis.
Molybdenum-based technical chemicals exploit the versatility of molybdenum chemistry in oxidation states. (VI), (V) and (IV). Materials made from molybdates are oxidation catalysts, are photoactive, and semiconducting. Many of the properties of molybdenum provide development opportunities and new commercial applications through the exploitation of its chemistry.
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