Molybdenum complexes oxalate

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

Note that these compounds are not enantiomers, but true diastereomers with different properties, and they may be separated by fractional crystallization. The asymmetric carbon atom has an 5 configuration in both diastereomers, but the chirality about the molybdenum atom is different. Thus the asymmetric carbon aids in the resolution of the molybdenum center, but its presence is not necessary for the complex to be chiral. It is merely necessary for the Schiff base to be unsymmetric, i.e., have one pyridine nitrogen and one imino nitrogen. If the bkJentate ligand had been ethylenediamine, bipyridine, or the oxalate ion, there would have been a mirror plane and no duality at the molybdenum. 

There are two principal synthetic routes to dicarboxylate complexes. One of these uses an aqueous solution of the alkali metal dicarboxylate and the corresponding metal halide,93 while the other depends upon the dicarboxylic acid reduction of higher oxidation state metals. This reductive property of oxalic acid results in its ready dissolution of iron oxides and hence a cleaning utility in nuclear power plants.94 Mention must also be made of the successful ligand exchange synthesis of molybdenum dicarboxylates, Mo(dicarboxylate)2 H2 O, from the corresponding acetate complex. Unfortunately the polymeric, amorphous and insoluble nature of these complexes has restricted the study of these systems, which may well provide examples of multiple M—M bonding in dicarboxylate coordination chemistry.95... 

Molybdates.—Normal molybdates of the type R aMoO., exist in solution but are relatively unstable, and readily form acid salts or complex polymolybdates. Thus dimolybdates, R aMogOy, can be obtained by fusion of molybdenum trioxide with sodium or potassium nitrate trimolybdates, R gMojOio, and tetramolybdates, R Mo Oig, by heating molybdenum trioxide with an aqueous solution of sodium or potassium carbonate. Even more highly acid salts—for example, octa- and deca-molybdates—can be obtained. Solutions of normal molybdates, when treated with hj droehlorie add or nitric acid, yield a precipitate of acid molybdate this reaction does not, however, take place with sulphuric, acetic, oxalic, or tartaric acids. 

Vanadium, the first element in Subgroup oA, and neighbour to chromium, is found in association with molybdenum in certain complex compounds. For example, when boiling solutions of molybdo-oxalates react with vanadium pentoxide, crystalline products are obtained which are thought to be substituted vanadates containing the complex anion... 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe(ni), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. 

A very thorough investigation by Kitson and Mellon reveals that hardly any ions cause interference and only chromate and dichromate ions were found to interfere in concentration equivalent to the phosphate concentra-tion.2 Reducing agents will reduce the complex to molybdenum blue, a feature actually used in some determinations. Organic ions such as citrate, tartrate, and oxalate will form a complex with molybdate. It should be kept in mind though that these compounds are reported as potential interferences in quantitative determination. Whether they could actually eliminate a false positive reaction in an identification is questionable. When used for quantitative determinations the color is reported to be stable for 3 h, while otiiers report a stability of 24 h. ... 

Bayot et al. [70] prepared and characterized ammonium derivatives of molybdenum(VI) complexes of general formula (NH4)2[MoO(02)2(HxL)] nH20 and (NH4)2[Mo02(C>2)(L)] widi L=oxalate, citrate, tartrate, glycolate and malate on the basis of elemental and thermal analysis as well as by IR and NMR spectroscopy. 

Molybdenum-based high-temperature syntheses from the starting elements, with high yields, resulted [81] in chemically inert coordination polymers in which the desired cluster units are connected by bridging ligands into 1-D, 2-D, and 3-D frameworks. Sokolov and coworkers introduced an alternative method of cluster excision from the solids based on the mechanochemical reaction of cluster coordination polymers with an appropriate ligand. TG helped the characterization of these cluster oxalate complexes. 

An example of the model complexes that point the way to potentially fascinating polymers in the future is given by the tetranuclear species 6.21 [49]. Remarkable square arrays of molybdenum-molybdenum quadruple bonds with oxalate spacers have also been prepared by simple assembly procedures, as illustrated by the formation of the complex Mog(oxalate)4(N-N)g (6.22) (N-N = di(p-methoxyphenyl)forma-midinate) (Fig. 6.11) from [Mo2(N-N)2(NCMe)2] and [oxalate] " [52]. 

Transitory outbreaks of gushing have been observed due to the precipitation of microcrystals of calcium oxalate while other outbreaks have been associated with the presence of heavy metals, in particular, iron, nickel, tin, and molybdenum. Such outbreaks can often be cured by the addition of EDTA to the beer which forms complexes with calcium and heavy metals. The balance of heavy metals in beer is obviously critical for it was observed that iron and nickel only caused gushing when complexed with isohumulone. Cobalt, on the other hand, shows little tendency to cause gushing and, indeed it was found during one severe epidemic of gushing that the addition of 0-2-1 0 ppm of cobalt dramatically reduced the incidence of the complaint. However, in another case it was without effect. 

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