Molybdenum aqueous solution chemistry

Molybdenum has an extensive aqueous solution chemistry for oxidation states II through VI. It is unique in having aqua or aqua/oxo ions for all five states in acidic solution (pH < 2). These are well defined in all but the Mg 1 case, the study of which is complicated by the existence of rapid equilibria involving protonated/deprotonated monomer/dimer (and higher) forms. The VI state is without question the most stable and in contrast to Crvi is only the mildest of oxidants. Compounds which have contributed to the development of the aqueous solution chemistry, including the aqua ions themselves, are considered under Section 36.1.2. It is only since 1971 that the aqua forms of oxidation state II-V ions have been identified, and... 

Molybdenum is at present unique in having aqua ions in five oxidation states. Whereas the complexities of Mo(VI) aqueous solution chemistry have been understood in general terms for some time, it is only in the last 15 years that the aqua ions of the lower oxidation states II through V have been identified, and their structures clearly established. Metal aqua ions are notoriously difficult to crystallize for X-ray diffraction studies, and structures of derivative complexes... 

Molybdenum The Element and Aqueous Solution Chemistry A. G. Sykes, University of Newcastle upon Tyne, UK 1229... 

Molybdates, heptacyano-, 1235 Molybdates, hexaaqua-, 1235 oxidation, 1236,1237 substitution reactions, 1235 Molybdates, hexachloro-, 1234 Molybdates, hexaisocyanato-, 1235 Molybdates, hexaoxo-reduction, 1242 Molybdates, octachlorodi-aqueous solution chemistry, 1232 Molybdates, pentachlorooxo- 0-exchange, 1254 Molybdates, tetraoxo- 0-exchange, 1259 Molybdates, tetrathio-iron complexes, 1426 Molybdenite, 1229,1431 Molybdenum, 1229 aqueous solution chemistry, 1229 biological relevance, 1229 occurrence, 1229 uses, 1229... 

Arsonium salts have found considerable use in analytical chemistry. One such use involves the extraction of a metal complex in aqueous solution with tetraphenyiarsonium chloride in an organic solvent. Titanium(IV) thiocyanate [35787-79-2] (157) and copper(II) thiocyanate [15192-76-4] (158) in hydrochloric acid solution have been extracted using tetraphenyiarsonium chloride in chloroform solution in this manner, and the Ti(IV) and Cu(II) thiocyanates deterrnined spectrophotometricaHy. Cobalt, palladium, tungsten, niobium, and molybdenum have been deterrnined in a similar manner. In addition to their use for the deterrnination of metals, anions such as perchlorate and perrhenate have been deterrnined as arsonium salts. Tetraphenyiarsonium permanganate is the only known insoluble salt of this anion. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The aqueous chemistry of molybdenum and tungsten is complicated by polymer formation in acid solution and reduction potential data are not known with certainty. The acid-solution chemistry of molybdenum is summarized in Table 7.17. 

The redox chemistries of di-, tri and tetra-nuclear Mo—S cyanide complexes have been discussed in relation to their electronic structures.135 Passage of oxygen into an aqueous solution of [Mo2S2(CN)s]6 leads to the formation of a dark violet mixed-crystal compound of the composition K4+J,[Mo2(S02)(S2)(CN)8] [Mo2(S02)(S2)(CN)8]1 -4H20 (x = 0.3). In the crystal the two anions, whilst structurally similar, are located at crystallographically independent positions each involves both molybdenum atoms surrounded by an approximately... 

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... 

It is interesting to speculate on the chemical properties of molybdenum which make it suitable for its biological function. Obvious features in the chemistry of molybdenum are (a) a range of oxidation states which can be stabilized in aqueous solution by the common ligands of biology (b) the formation of oxo compounds and the sulfur analogue (c) the ability to participate in atom-transfer reactions and (d) the possibility of higher coordination numbers. 

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. 

Haight, G. P., and Boston, D. R. (1973). Molybdenum species in aqueous solution - a brief summary. In First International Conference on the Chemistry and Uses of Molybdenum, Proceedings of a Conference Held at the University of Reading, England, September 17-21, 1973, ed. P. C. H. Mitchell, pp. 48-51. London Climax Molybdenum Company, Ltd. (AMAX). 

The octacyatiomolybdate(IV) anion [Mo(CN)g] " has long been known and characterized and many aspects of its chemistry have been reviewed.This complex is readily obtained from the addition of cyanide ions to aqueous solutions of Mo or Mo compounds. A simple preparation is by the reaction between molybdate(VI), K[BH4] and CN in the presence of acetic acid, followed by precipitation with ethanol. llie crystal structure of several complexes containing the [Mo(CN)g] anion have been reported. In K4[Mo(CN)g]-2H20, [C6H6N02]4[Mo(CN)g] and Rb4[Mo(CN)g] 3H20 dodecahedral coordination has been found, but all the bond lengths are equal, at 2.163(7) However, the molybdenum... 

There is a rich chemistry of heteropolyoxomolybdates and polyoxomolybdates, and it has been shown that the chemistry of these is extraordinarily versatile, which is directly related to the fact that it is very easy to reduce Mo to Mo in aqueous solution. The generation of such molybdenum blue species adds a new dimension to polyoxometalate chemistry. 

The solution chemistry of the oxyanion of tungsten and molybdenum is complex. For tungsten, a variety of species containing 1,6, and 12 tungsten atoms can be present in aqueous solution, which, at present, can be controlled to some extent by the selection of the solution pH (28,33) ... 

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