Heteropoly-molybdates and tungstates

Heteropoly Molybdates and Tungstates. The u.v. absorption spectra and half-wave reduction potentials of the heteropoly anions of formula [X M, M Mj2, O40 ] (M = Si, M = Mo or W n = 0 q = 0 M = Si, M = Mo, M = W, n = 0, q = 1 M = P,M = V,M = MoorW,n = 0,q = 1 M = P,X = Mn +orCo, M = W, n = 1, = 1) and P2WigOgJ have been measured in H2S04. A linear correlation was found between the energy of the first charge transfer band and E. Recording the optical spectra of the reduction products of several heteropolyanions, which cannot be obtained either chemically or electrochemically, has been achieved by photochemically reducing the heteropolyanion in polyvinylalcohol films.  

Trioctylamine has been used to extract quantitatively H4SiMOj204 , and H4PVMoii04q from acidic media and the conditions leading to the formation of a- and P-H4SiMo 2 4o H4PVM011O40 have been investigated. The 

Launay, M. Boyer, and F. Chauveau, J. Inorg. Nuclear Chem., 1976,38,243. 

Chaki Won and Park Kee Chae, Taehan Hwahak Hoechi, 1975,19,225 (Chem. Abs. 1975,83,209971). 

The complexes [SiCrWij04o] ,18H20 have been prepared and characterized and the dimetalloheteropolytungstates K AsZ2WjgOg7(OHj25 where Z is usually a first row transition metal ion, have been isolated and their stability and structures discussed. The neutral heteropoly complex [Co2(en)4(H20)2P2Mo5 0 23],8H20 has been prepared and from solution studies and analytical data the complex is best represented as a neutral zwitterionic species.  

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In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

Mild and reversible reduction of 1 12 and 2 18 heteropoly-molybdates and -tungstates produces characteristic and very intense blue colours ( heteropoly blues ) which find application in the quantitative determinations of Si, Ge, P and As, and commercially as dyes and pigments. The reductions are most commonly of 2 electron equivalents but may be of 1 and up to 6 electron equivalents. Many of the reduced anions can be isolated as solid salts in which the unreduced structure remains essentially unchanged and... 

These are dark blue species obtained by reduction of both isopoly- and heteropoly molybdates and tungstates, and thus the class is broader than the name implies. It appears that in general, these reductions are reversible and major structural features are retained. In the case of the [M6Oi9]2 ions, reduction by one electron gives a 3-ion in which the added electron is weakly trapped at one metal atom but is thermally mobile. 

Weakley, T.J.R. Some Aspects of the Heteropoly molybdates and Heteropoly tungstates. Vol. 18, pp. 131-176. 

Salts of Molybdic and Tungstic Acids. What types of salts of molybdic and tungstic acids exist Give examples of isopoly- and heteropoly molybdenum and tungsten compounds. What is the composition of para- and metamolybdates and of para- and metatungstates Which molybdates and tungstates dissolve poorly in water ... 

Heteropoly acids and their salts are formed when molybdate and tungstate solutions containing other oxoanions (e.g., P043- and Si044-) or metal ions are acidified. At least 35 elements are known to be capable of functioning as the hetero atoms. 

In the spectrophotometric determination of Si, Ge, P(V), As(V), and V(V) the yellow heteropoly acids occurring in acid solutions in the presence of an excess of molybdate or tungstate are important. The yellow heteropoly acids are the basis of less sensitive spectrophotometric methods, but the blue reduction products (e.g., phosphomolybdenum blue) are the basis of very sensitive spectrophotometric methods for determining these elements. The conditions for formation and extraction of these compounds have been investigated [133-135]. 

Rosenheim, A. Jaenicke, J. Z. (1917). Iso- and Heteropoly Acids. XV. Heteropoly tungstates and Some Heteropoly Molybdates. Anorg. Allg. Chem., Vol.lOl, pp. 235-275... 

Association of molybdates (tungstates) with phosphate-like structures leads to a class of compounds called heteropoly compounds or heteropoly acids of Mo or W, hereafter schematically written as Mo —P or W—P heteropoly compounds. The heteropoly anions, which may contain Mo, W, P, and other elements, are paired, both in the solid state and in solution, with cations such as H+, NH4, and Na+. Here, only some Mo —P heteropoly compounds playing an important role in hydrotreating catalysts during the catalyst preparation (impregnation) or as supported oxidic phases are discussed. The Mo — P heteropoly compounds have the following properties 18,19) ... 

Ions besides (P +) which may act as the central coordinating atom to form 12-fold heteropoly acids with molybdate include arsenic (As +), silicon (Si +), germanium (Ge +) and under some conditions molybdenum (Mo +) and boron (B +). Tungstate can also be coordinated about P as central atoms but with less avidity. The heteropolycomplexes, before reduction give a yellow hue to their water solution. With high P concentrations, a yellow precipitate is formed. In solution of low enough concentration to be suitable for determination by reduction to form the blue colour, the yellow colour is so faint that, it is not noticed and spectrophotometric measurements is done without any problem. The molybdenum blue colour is produced when either molybdate or its heteropolycomplexes are partially reduced. Some of the molybdenum ions are reduced from h-6 to a low valence state, probably h-3 and/or h-5, involving unpaired electrons due to which spectrophotometric resonance (blue colouration) would be expected. 

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