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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. 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.
Additional recommended knowledge
Examples of molybdate anions
Examples of molybdate oxoanions are:
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, MoO42- and Mo2O72- contain only 4 coordinate molybdenum, MoO42- is tetrahedral and Mo2O72- 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:
The Mo8O264− anion contains both octahedral and tetrahedral molybdenum and can be isolated in 2 isomeric forms, alpha and beta.
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:
As the pH is increased the octamolybdate is formed further anions with 8 and probably 16–18 Mo atoms are present
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. .
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Molybdate". A list of authors is available in Wikipedia.|