Molybdenum tetravalent

Representative compounds for the +4 oxidation state are shown in Figure 4. The violet tetravalent molybdenum dioxide [18868-43 ] M0O2, is formed by the reduction of M0O3 with H2 at temperatures below which Mo metal is formed or M0O3 is volatile (ca 450°C). MoCl [13320-71 -3] is formed upon treatment of M0O2 at 250°C with CCl (see Fig. 1). 

In the preparation of Mo/KL, the addition of 1 was stopped when pH of the solution was lowered to about 3. The resulting Mo/KL contained only 2.1 wt% (74% of added molybdenum clusters) of molybdenum. Chlorine was absent, which indicates that the cluster 1 acted as a tetravalent cation. The LTL structure is characterized by a monodimensional system of channels, whose diameter (0.70 nm) [14] is close to the size of the cluster 1. It is conceivable that, once the cluster 1 was incorporated into an LTL main channel and present at a site near the external surface, the sites deep in the channel are no more accessible to another cluster. 

In keeping with its 4d%5s electron configuration, molybdenum forms many compounds in which its oxidation state is 6+. to an even greater extent than chromium. Also, like chromium, it forms compounds in which II is divalent and those in which it is trivalenl unlike chromium, il forms a number of pentavalenl compounds, and a few more tetravalent compounds, especially complexes. 

In addition to the dioxide MoO and the disulfide. MoS . example of leLravalem molybdenum compounds include the tetrachloride. MoClj. and teurabromide. MoBr. both of which are hydrolyzed in hot l-l-O. Other tetravalent compounds include a few of the complexes. 

As staled above, hexavalem compounds of molybdenum constitute ihe most numerous group. The hexavalem molybdenum oxyhalides include M0OF4. MoOjFj. M0OCI4. MoO-Cl . and MuO-Br-.Mo-OiClj may con-lain tetravalent and hexavalem molybdenum. Hexavalem molybdenum also exists in Ihe sulfide. M0S1. and in various oxvacid salts, and molybdates. 

Molybdenum forms many other complexes. Of particular interest are the octacyano complexes, containing eight cyanide ions. CN. coordinated to a single tetravalent of pentavalenl molybdenum ion. MolCN)) and Mo(CN)MOg . the latter being exceptionally stable, and both form oclacyanomolybdic acids H, Mo(CN)R 3H-0 and R((Mo[Pg.1039]

In keeping witii its 5d 6s2 electron configuration, tungsten forms many compounds in which its oxidation state is 6+, just as molybdenum does. It forms divalent and tetravalent compounds to about the same extent as molybdenum but its bivalent and pentavalent compounds are somewhat fewer. Its anion chemistry is closely akin to that of molybdenum. 

Molybdenum trioxide, intercalation into, 12, 823 Molybdocenes, as anticancer agents, 1, 892 MOMNs, see Metal-organometallic coordination networks Monisocyanides, with silver(I) complexes, 2, 223 Monitoring methods, kinetic studies, 1, 513 Mono(acetylacetonate) complexes, with Ru and Os halfsandwich rf-arenes, 6, 523 tj2-Monoalkene monodentate ligands, with platinum divalent derivatives, 8, 617 tetravalent derivatives, 8, 625 theoretical studies, 8, 625 zerovalent derivatives, 8, 612... 

Of the rather limited number of tetravalent molybdenum and tungsten derivatives known, wre shall mention here only molybdenum disulfide (M0S2), which occurs as the ore molybdenite and the octacyano complexes Mo(CN)i 4 and W(CN) 4 which, along with their pentavalent counterparts (p. 333), represent two of the very few examples of octa-covalency. 

Firsova et al. (122) reported that the room temperature Mossbauer spectrum of supported tin molybdate, which had been aged in vacuo at 723 K, showed the presence of tetravalent tin. Only after exposure to oxygen at 473 K did the sample act as an adsorbent for propylene. It then gave a Mossbauer spectrum that showed the reduction of the tetravalent tin to the divalent state. Reduction without exposure to oxygen was achieved at 673 K but supported tin in the absence of molybdenum was not reduced. The results were interpreted in terms of the proposals (123) for the synergistic oxidation-reduction during catalysis. 

Few derivatives of molybdenum dioxide, MoOj, have been prepared, and it is doubtful whether simple salts containing tetravalent molybdenum can be formed in solution. By the electrolytic reduction of acid molybdate solutions, brownish-coloured liquids apparently containing the metal in this stage of oxidation have been obtained, but the evidence is insufficient to determine whether Mo ions are actually present, or whether the liquids merely contain mixed Mo and Mo ions. Potential measurements indicate the presence of mixed ions. The only simple substances containing tetravalent molybdenum, in addition to the oxide, are the sulphide, MoSj, the tetrachloride, M0CI4, and the tetra-bromide, MoBr. There are, however, two series of complex molybdo-eyanides, of the types R 4[Mo(OH)4(CN)J.aq. and R 4[Mo(CN)8].aq. respectively, which contain tetravalent molybdenum and yield well-cry stallised salts. Their existence is probably due to the resistance of the stable complex to hydrolytic decomposition. 

Two definite series of complex molybdoeyanides, containing tetravalent molybdenum, are now recognisable (1) the reddish-rnolet salts, containing the complex anion... 

Properties Dense, silvery solid. D 19.0, mp 1132C, bp3818C, heat of fusion 4.7 kcal/mole, heat capacity 6.6 cal/mole/C. Strongly electropositive, ductile and malleable, poor conductor of electricity. Forms solid solutions (for nuclear reactors) with molybdenum, niobium, titanium, and zirconium. The metal reacts with nearly all nonmetals. It is attacked by water, acids, and peroxides, but is inert toward alkalies. Green tetravalent uranium and yellow uranyl ion (U()2") are the only species that are stable in solution. 

Unless otherwise noted, catalysts were prepared by coprecipitating the hydrous oxides of uranium, antimony, and a tetravalent metal from a hydrocholoric acid solution of their salts by the addition of ammonium hydroxide. The precipitates were washed, oven dried, then calcined at 910 C overnight or at 930 C for two hours to form crystalline phases. Attrition resistant catalysts, containing 50% by weight silica binder, were prepared by slurrying the washed precipitate with silica-sol prior to drying. In some cases, small amounts of molybdenum or vanadium were added by impregnating the oven dried material with ammonium paramolybdate or ammonium metavanadate solution. The details of these preparations may be found elsewhere (5-8). 

Trivalent uranium ion reduces water to hydrogen. Hence, stable aqueous solutions of trivalent uranium compounds cannot be prepared. Compounds of tetravalent uranium are generally similar to those of zirconium or thorium, except that some uranium compounds can be oxidized to the hexavalent form. Compounds of pentavalent uranium are of little importance because they disproportionate readily into tetravalent and hexavalent forms. The properties of hexavalent uranium are generally similar to those of hexavalent molybdenum or tungsten. In aqueous solution hexavalent uranium forms the uranyl ion UO2 ... 

From the above discussion it follows that tetravalent and hexavalent thorium, uranium, and plutonium can be separated from the trivalent rare-earth fission products by taking advantage of differences in complexing properties. More highly charged cation fission products, such as tetravalent cerium and the fifth-period transition elements zirconium, niobium, molybdenum, technetium, and ruthenium, complex more easily than the trivalent rare-earths and are more difficult to separate from uranium and plutonium by processes involving complex formation. 

Other tetravalent metal species were examined and found to have no activity. These were vanadyl (VO4+), molybdenum, and mixed zirconium tungsten phosphates. 

This compound is measured photometrically at 623 nm. Its blue colour can probably be traced to the mixed oxide formation of the tetravalent and hexavalent molybdenum (Holleman-Wiberg). 

Henry and Van Lierde developed a three-compartment cell equipped with an reticulated vitreous carbon cathode mixed with ion-exchange resin and two Ti—Pt anodes separated by a Daramic diaphragm to selectively separate the vanadium from molybdenum (Henry Van Lierde, 1998). In this designed membrane reactor, the pentavalent vanadyl anion was selectively desorbed with respect to molybdenum after its reduction to a tetravalent vanadyl cation. Consequently, 93% vanadium recovery yields were obtained. 

Molybdenum(V) chloride, tungsten(VI) chloride, tungsten(V) chloride and tung-sten(V) bromide are reduced by acetonitrile to the tetravalent compounds MX4(AN)2, which are characterized as six coordinate complexes. Likewise vana-dium(IV) chloride is reduced to the solvate of the trichloride VCl3(AN)3 58. 

With potassium ethylxanthate in strong acid solution, technetium in the tetravalent or heptavalent state forms a purple complex which is readily extractable with carbon tetrachloride. Molybdenum behaves similarly (see page 320). 

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