The Tungstate Complexes

B = Mg, Ca, Sr, Ba) the influence of point charges on the A and B sites around the tungstate complex was also taken into account. 

Polytungsta.tes, An important and characteristic feature of the tungstate ion is its abiUty to form condensed complex ions of isopolytungstates in acid solution (38). As the acidity increases, the molecular weight of the isopolyanions increases until tungstic acid precipitates. However, the extensive investigations on these systems have been hampered by lack of weU-defined soHd derivatives. 

From strongly addic solutions benzoin-a-oxime precipitates molybdate and tungstate ions quantitatively chromate, vanadate, niobate, tantalate, and palladium) II) are partially predpitated. The molybdate complex is best ignited at 500-525 °C to Mo03 before weighing alternatively, the predpitate may be dissolved in ammonia solution and the molybdenum predpitated as lead molybdate, in which form it is conveniently weighed. 

These are complexes formed between tungstates and molybdates, and silicate or phosphate, and have been used to generate hydroxyl radicals photochemically. The tungstates PWj204q and SiWj204o have been used most frequently. 

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

Another route to thioaldehyde and thioketone complexes uses the tungstate [W(CO)5SH] as the precursor. When [W(CO)5SH] was treated with ketones in the presence of trifluoroacetic acid, thioketone complexes [W(CO)5 S = C(R1)R2 ] were formed. Analogously, thiobenzaldehyde complexes were obtained from [W(CO)sSH]- and benzaldehydes bearing electron-releasing para substituents. Benzaldehyde, benzaldehydes with electron-withdrawing substituents, and aliphatic aldehydes did not react. Presumably, the binuclear dianion [p,-S W(CO)5 2]2 is an intermediate in the reaction, as on acidification of a mixture of [ju,-S W(CO)5 2]2 and p-N, A -dimethylaminobenzaldehyde the thioaldehyde complex [W(CO)5 S = C(C6H4NMe2-p)H ] is formed.143... 

The molybdate and tungstate slow band spectra are similar to those obtained for molybdate and tungstate complexes of the fast and intermediate bands, and they are similar to the reported spectra of bispheno-late complexes of molybdate and tungstate (47, 48). 

For the molybdate and tungstate complexes, the conditional stability constants did not change over a pH range of 3.5-6. This suggests a pH-independent overall reaction like that formulated for other bisphenolate complexes, Reaction 3. If this reaction is valid, the stabilities in Table IV represent true formation constants rather than conditional constants. The stability of the Fe(III) complex has been determined only at pH 5. Since the molybdate and tungstate data indicate phenolate-coordinating groups, it is reasonable to assume that Reaction 4 represents the Fe(III) reaction. Hence, the Fe(III) constant is a conditional constant. 

The oxygen atom of acyl metallates may present nucleophilic character (Figures 3.25, 4.16). In 1964, E. O. Fischer showed that protonation of the benzoyl tungstate complex obtained from W(CO)6 and phenyllithium (Figure 3.20) provided the first characterized example of a metal-carbon double bond, viz. the hydroxycarbene complex (CO)5W=C(OH)Ph. This labile species decomposed to provide ben-zaldehyde however, it could be esterified with diazomethane to provide... 

Element 106. The chemical properties of element 106 (eka-tungsten) are predicted to be similar to those of tungsten, molybdenum and to some extent chromium, offering an even richer chemistry of complex ions than these elements. The hexafluoride should be quite volatile and the hexachloride, pentachloride and oxychloride should be moderately volatile. Penneman and Mann predict a -)-4 oxidation state in aqueous solution. Jprgensen s selection of k is for the hydrated cation and is not intended to account for the effects of complex ion formation. However, since tungsten is stabilized in the oxidation state of -t-6 by the tungstate ion, an analogous situation may be expected for element 106. 

The following complex tungstates containing aluminium have been described ... 

X-ray structures have been published of the catalytic subunit of rabbit muscle PP-1 complexed with the polypeptide microcystin131 and of human PP-1 with tungstate.132 The X-ray structures are also available of free human calcineurin (Fig. 23), as well as of human and bovine brain calcineurin complexed with the immunosuppressant complex FKBP12/FK506, which does not contact the active site, but binds adjacent in a manner that hinders access.133,134 Calcineurin is involved in the... 

Bi-capped versions of the Anderson structure are found with [(As C C MogO )]4-307 and [HxXMo6Oi8 RC(CH20)3 2](6 m x) (Xm 1 Fe31, Ni2+, Zn2+).308 The arsenite complex was isolated from a reaction between Co(H20)62+ and [Asin3Mo30i5]3, an anion that was probably first reported in the nineteenth century. The structure of the latter and its tungstate analogue, of... 

A characteristic feature of the tungstate ion which only occurs monomerically in alkaline or neutral solutions is its tendency to form condensed, complexed isopolytungstate ions in slightly acidic medium. The ability of polycondensation is common for group 5 and 6 elements and is more pronounced for elements of higher atomic mass within the same group. The dimeric dichromate ion is a typical example. 

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