Molybdate 2309658 221905048 2008-06-26T16:50:35Z Axiosaurus 3399137 tidied up minus signs and changed to reflist [[Image:Molybdate.png|thumb|150px|Structure of molybdate]] 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 [[polyoxometalate]]s, and because they contain only one type of metal atom are often called isopolymetalates<ref name = greenwood> {{Greenwood&Earnshaw}}</ref>. The discrete molybdenum oxoanions range in size from the simplest MoO<sub>4</sub><sup>2<nowiki>&minus;</nowiki></sup>, found in [[sodium molybdate]] up to extremely large structures found in [[Molybdenum_blue#Isopoly_molybdenum_blues|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 [[chromate]]s, CrO<sub>4</sub><sup>2<nowiki>&minus;</nowiki></sup>, Cr<sub>2</sub>O<sub>7</sub><sup>2<nowiki>&minus;</nowiki></sup>, Cr<sub>3</sub>O<sub>10</sub><sup>2<nowiki>&minus;</nowiki></sup> and Cr<sub>4</sub>O<sub>13</sub><sup>2<nowiki>&minus;</nowiki></sup> ions which are all based on tetrahedral chromium. [[Tungsten]] is similar to molybdenum and forms many [[tungstate]]s containing 6 coordinate tungsten<ref name= cotton>{{Cotton&Wilkinson6th}}</ref>. <br /> ==Examples of molybdate anions== Examples of molybdate oxoanions are: *discrete MoO<sub>4</sub><sup>2<nowiki>&minus;</nowiki></sup>, in e.g. [[sodium molybdate|Na<sub>2</sub>MoO<sub>4</sub>]] and the mineral [[powellite]], CaMoO<sub>4</sub> *discrete Mo<sub>2</sub>O<sub>7</sub><sup>2<nowiki>&minus;</nowiki></sup> in the tetrabutylammonium salt and polymeric Mo<sub>2</sub>O<sub>7</sub><sup>2<nowiki>&minus;</nowiki></sup> in the ammonium salt<ref>''Synthesis and characterization of the dimolybdate ion, Mo<sub>2</sub>O<sub>7</sub><sup>2<nowiki>&minus;</nowiki></sup> V. W. Day, M. F. Fredrich, W. G. Klemperer, and W. Shum Journal of the American Chemical Society 99, 18, (1977), 6146 {{doi|10.1021/ja00460a074}}</ref> *polymeric Mo<sub>3</sub>O<sub>10</sub><sup>2<nowiki>&minus;</nowiki></sup> in the [[ethylenediamine]] salt <ref>''Hydrothermal Synthesis and Crystal Structure of Anhydrous Ethylenediamine Trimolybdate (C2H10N2)[Mo3O10]'' Guillou N.; Ferey G. Journal of Solid State Chemistry, Volume 132, Number 1, August 1997 , pp. 224-227(4) {{doi|10.1006/jssc.1997.7502}}</ref> *polymeric Mo<sub>4</sub>O<sub>13</sub><sup>2<nowiki>&minus;</nowiki></sup> in the potassium salt<ref>''Crystal structure of potassium tetramolybdate, K2Mo4O13, and its relationship to the structures of other univalent metal polymolybdates'' B. M. Gatehouse and P. Leverett J. Chem. Soc. A, 1971, 2107 - 2112, {{doi|10.1039/J19710002107}}</ref> *polymeric Mo<sub>5</sub>O<sub>16</sub><sup>2<nowiki>&minus;</nowiki></sup> in the anilinium, (C<sub>6</sub>H<sub>5</sub>NH<sub>3</sub>)<sup>+ </sup>salt <ref>''Crystal Structure of Anilinium Pentamolybdate from Powder Diffraction Data. The Solution of the Crystal Structure by Direct Methods Package POWSIM'' W. Lasocha and H. Schenk J. Appl. Cryst. (1997). 30, 909-913 {{doi|10.1107/S0021889897003105}} </ref> *discrete Mo<sub>6</sub>O<sub>19</sub><sup>2<nowiki>&minus;</nowiki></sup>(hexa-molybdate) in the tetramethylammonium salt<ref>''The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI)'' S. Ghammami Cryst. Res. Technol. 38, 913 (2003){{doi|10.1002/crat.200310112}}</ref> *discrete Mo<sub>7</sub>O<sub>24</sub> <sup>6<nowiki>&minus;</nowiki></sup> in [[ammonium molybdate]], (NH<sub>4</sub>)<sub>6</sub>Mo<sub>7</sub>O<sub>24</sub>.4H<sub>2</sub>O<ref>''Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo<sub>7</sub>O<sub>24</sub>]<sup>6<nowiki>&minus;</nowiki></sup>, in the ammonium and potassium tetrahydrate salts'' Howard T. Evans jun. , Bryan M. Gatehouse and Peter Leverett J. Chem. Soc., Dalton Trans., 1975, 505 - 514, {{doi|10.1039/DT9750000505}}</ref>. *Mo<sub>8</sub>O<sub>26</sub><sup>4<nowiki>&minus;</nowiki></sup> in [[trimethylammonium]] salt.<ref name = greenwood /> The naming of molybdates generally follows the convention of a prefix to show the number 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, Mo<sub>7</sub>O<sub>24</sub> <sup>6<nowiki>&minus;</nowiki></sup>, is often called "paramolybdate". ==Structure of molybdate anions== The smaller anions, MoO<sub>4</sub><sup>2−</sup> and Mo<sub>2</sub>O<sub>7</sub><sup>2−</sup> contain only 4 coordinate molybdenum, MoO<sub>4</sub><sup>2−</sup> is [[tetrahedral_molecular_geometry|tetrahedral]] and Mo<sub>2</sub>O<sub>7</sub><sup>2−</sup> can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom.<ref name =greenwood />. In the larger anions Molybdenum is generally, but not exclusively, 6 coordinate with edges or vertices of the MoO<sub>6</sub> octahedra being shared. The octahedra are distorted, typical M-O bond lengths are: *in terminal non bridging M-O approximately 1.7A *in bridging M-O-M units approximately 1.9A The Mo<sub>8</sub>O<sub>26</sub><sup>4<nowiki>&minus;</nowiki></sup> anion contains both octahedral and tetrahedral molybdenum and can be isolated in 2 isomeric forms, alpha and beta<ref name = cotton />.<br /> 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 140 [[picometer|pm]], molybdenum(VI) is much smaller, 59 pm<ref name = greenwood />. There are strong similarities between the structures of the molybdates and the molybdenum oxides, ([[molybdenum trioxide|MoO<sub>3</sub>]], [[molybdenum dioxide|MoO<sub>2</sub>]] and the "[[crystallographic shear]]" oxides, Mo<sub>9</sub>O<sub>26</sub> and Mo<sub>10</sub>O<sub>29</sub>) whose structures all contain close packed oxide ions<ref> ''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</ref> . [[Image:hexamolybdate_ion_polyhedral_representation.jpg|300px|hexamolybdate]] [[Image:Heptamolybdate space fill and ball and stick.png|300px|heptamolybdate]] ==Equilibria in aqueous solution== When MoO<sub>3</sub>, molybdenum trioxide is dissolved in alkali solution the simple MoO<sub>4</sub><sup>2<nowiki>&minus;</nowiki></sup> anion is produced. As the pH is reduced the first species to be formed is the heptamolybdate rather than any of the smaller anions: :7 MoO<sub>4</sub><sup>2−</sup> + 8H<sup>+</sup> {{unicode|&#8652;}} Mo<sub>7</sub>O<sub>24</sub><sup>6−</sup> + 4H<sub>2</sub>O <ref name = cotton /> As the pH is increased the octamolybdate is formed further anions with 8 and probably 16–18 Mo atoms are present<ref name = greenwood /> :Mo<sub>7</sub>O<sub>24</sub><sup>6−</sup> + 3H<sup>+</sup> {{unicode|&#8652;}} Mo<sub>8</sub>O<sub>26</sub><sup>4−</sup> + 2H<sub>2</sub>O <ref name = cotton /> 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 not appear to be in solution to be precipitated. <ref name =greenwood />. ==References== {{reflist}} [[Category:molybdates]] [[Category:molybdenum compounds]] [[Category:Oxoanions]] [[bs:Molibdat]] [[nl:Molybdaat]]