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Subclass: Molybdates

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 potassium 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 grou 6. Chromium only forms the chromates, CrO42−, Cr2O72−, Cr3O102− and Cr4O132− ions whic are all based on tetrahedral chromium. Tungsten similar to molybdenum and forms many tungstates containing 6 coordinate tungsten.

Structure:

The smaller anions, MoO42− and Mo2O72− feature tetrahedral centres. Mo2O72− can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom.[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

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 forms, alpha and beta.[2]

The hexamolybdate image below shows the coordination polyhedra. 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, Mo9O26and Mo10O29) whose structures all contain close packed oxide ions.

Name Wulfenite   Pb(MoO4)
Colour Orange-yellow, yellow
Lustre Adamantine, Sub-Adamantine, Resinous
Crystal system Tetragonal
Hardness 2½ - 3 g/cm3
Name Szenicsite Cu3(MoO4)(OH)4
Colour Dark green
Lustre Adamantine, Pearly
Crystal system Orthorhombic
Hardness 3½ - 4 g/cm3
Name Ferrimolybdite Fe2(MoO4)3 nH2O
Colour Yellow, canary yellow
Lustre Adamantine, Silky, Earthy
Crystal system Orthorhombic
Hardness 1 2 g/cm3
Name Powellite Ca(MoO4)
Colour Yellow, straw-yellow
Lustre Sub-Adamantine, Resinous, Pearly
Crystal system Tetragonal
Hardness 3½ - 4 g/cm3

Date: 2015-12-24; view: 164


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