Molybdenum in solution

Recent results from the authors laboratory69 on the x-ray emission spectrography of tungsten or molybdenum in solution illustrate some of the points made in Section 7.13. The also show the usefulness of internal standards (7.12). Finally, the work on tungsten is closely related to the experiments on the absorption effect in sodium tungstate solutions, the results of which are summarized in Table 7-2. 

The availability of Mo to plants depends on the amounts of Mo present in three forms (1) soluble Mo, (2) adsorbed Mo, and (3) firmly held Mo (Barrow, 1977). Molybdenum in solution is in equilibrium with adsorbed Mo. Firmly held Mo is not in direct equilibrium with the molybdate in solution it can undergo a change to the adsorbed form... 

Molybdenum(V) compounds generally occur as mononuclear or dinuclear species. Molybdenum pentachloride [10241-05-1] MoCl, formed by combination of the elements, serves as a usebil and reactive starting material (Fig. 1). MoCl has a dinuclear stmcture (Fig. 3) in the soHd state but is mononuclear in the gas phase. In solution or in the soHd state the compound, actually Mo2C1 q (Fig- 3a), is readily hydroly2ed in air to form MoOCl ... 

Molecular examples of trivalent molybdenum are known in mononuclear, dinuclear, and tetranuclear complexes, as illustrated in Figure 5. The hexachloride ion, MoCk (Fig- 5a) is generated by the electrolysis of Mo(VI) in concentrated HCl. Hydrolysis of MoCP in acid gives the hexaaquamolybdenum(III) ion, Mo(H20) g, which is obtainable in solution of poorly coordinating acids, such as triflic acid (17). Several molybdenum(III) organometaUic compounds are known. These contain a single cyclopentadienyl ligand (Cp) attached to Mo (Fig. 5d) (27). 

Table 2. Aqueous Ions of Molybdenum in Acid Solutions...

This reaction is cataly2ed in solution by complexes of tungsten, molybdenum, or rhenium in high oxidation states, eg, Re ". Examples of active catalysts are... 

The selective oxidation is catalyzed by silver, which is the only good catalyst. Other olefins are not converted selectively to the epoxides in the presence of silver. However, propylene epoxidation is appHed commercially the catalysts are either molybdenum complexes in solution or soHd Ti02—Si02 (see... 

As the result of the performed investigations was offered to make direct photometric determination of Nd microgram quantities in the presence of 500-fold and 1100-fold quantities of Mo and Pb correspondingly. The rare earth determination procedure involves sample dissolution in HCI, molybdenum reduction to Mo (V) by hydrazine and lead and Mo (V) masking by EDTA. The maximal colour development of Nd-arsenazo III complex was obtained at pH 2,7-2,8. The optimal condition of Nd determination that was established permit to estimate Nd without separation in solution after sample decomposition. Relative standard deviations at determination of 5-20 p.g of Nd from 0,1 g PbMoO are 0,1-0,03. The received data allow to use the offered procedure for solving of wide circle of analytical problems. 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

The above procedure may be adapted to the determination of molybdenum in steel. Dissolve a 1.00 g sample of the steel (accurately weighed) in 5 mL of 1 1 hydrochloric acid and 15 mL of 70 per cent perchloric acid. Heat the solution until dense fumes are evolved and then for 6-7 minutes longer. Cool, add 20 mL of water, and warm to dissolve all salts. Dilute the resulting cooled solution to volume in a 1 L flask. Pipette 10.0 mL of the diluted solution into a 50 mL separatory funnel, add 3 mL of the tin(II) chloride solution, and continue as detailed above. Measure the absorbance of the extract at 465 rnn with a spectrophotometer, and compare this value with that obtained with known amounts of molybdenum. Use the calibration curve prepared with equal amounts of iron and varying quantities of molybdenum. If preferred, a mixture of 3-methylbutanol and carbon tetrachloride, which is heavier than water, can be used as extractant. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Complexes of molybdenum in the lower valence-states of -t 2 and + 3 have been produced only in the past two years. For the Mo(II) species, the usual starting-material is Mo2(acetate>4. Reaction of this with KS2COEt in THF gives two products, a green complex tentatively assigned as [Mo2(Etxant>4], which solvates to form the red complex [Mo2(Etxant)4(THF)2]. The structure of the latter complex was elucidated by X-ray analysis 169). Steele and Stephenson 170) were also able to synthesize a red, crystalline solid (methanol solution), which they formulated as [Mo(Etxant)2]2 (XI), and reacted this with Lewis bases, e.g., pyridine, to form [Mo(Etxant)2L]2- Thus, there appears to be a difference between the two compounds formulated as [Mo2(Et-xant)2]2 that... 

FeMoco can be extracted from the MoFe protein into A(-methylfor-mamide (NMF) solution 32) and has been analyzed extensively using a wide range of spectroscopic techniques both bound to the protein and in solution after extraction from it (33). The extracted FeMoco can be combined with the MoFe protein polypeptides, isolated from strains unable to synthesize the cofactor, to generate active protein. The structure of the FeMoco is now agreed 4, 5, 7) as MoFeTSg homocitrate as in Fig. 4. FeMoco is bound to the a subunit through residues Cys 275, to the terminal tetrahedral iron atom, and His 442 to the molybdenum atom (residue numbers refer to A. vinelandii). A number of other residues in its environment are hydrogen bonded to FeMoco and are essential to its activity (see Section V,E,2). The metal... 

In the molybdenum flotation circuit, the use of sodium sulfide as a depressant is quite important in order to depress sulfides of copper which are invariably associated. In solution, sodium sulfide hydrolyzes and then dissociates ... 

Section 2.17). We shall find that it dominates much of life s organic chemistry too. There is also the rather unusual insolubility of VS4. We need to explain the presence of some V, Co, Ni and Zn in early life, but note that their sulfides are relatively soluble in somewhat acid solutions, pH < 7, which may have prevailed at least locally some 4 x 109 years ago. Molybdenum and heavy metals of Groups 13 to 16 are also insoluble as sulfides but tungsten does not precipitate so easily and is known to be more available than molybdenum in sulfide-rich seas. 

Because of its resistance to attack by most reagents in solution, MoBrs is best analyzed by hydrogen reduction at 650°, whereby molybdenum is determined as the metal and bromine as hydrogen bromide. 

In a related study, the same group investigated molybdenum-catalyzed alkylations in solution and on a solid phase [35], demonstrating that microwave irradiation could also be applied to highly enantioselective reactions (Scheme 7.15). For these examples, commercially available and stable molybdenum hexacarbonyl [Mo(CO)6] was used to generate the catalytic system in situ. The reactions in solution provided good yields (see Scheme 6.50). In contrast, the conversion rates for the solid-phase examples were rather poor. However, the enantioselectivity was excellent (>99% ee) for both the solution and solid-phase reactions. 

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