Molybdenum and Rhenium

Before scmbbing procedures were estabUshed for copper ore, most of the rhenium was lost as the volatile (Re202). A small portion, perhaps 10%, was retained in flue dust, which was processed to give the metal. A commercial flotation (qv) process for the recovery of the molybdenite by-product is available that permits a high recovery of molybdenum and rhenium. This process is used at the Caridad copper mine in Mexico. 

Solid-state cluster chemistry is dominated by octahedral (M 5L8)L6 and (MsLi2)L units which are the focus of this paper. These two cluster types are different in the way the metal octahedral core is surrounded by the ligands. In (MsLg)L6-type clusters (Fig. 6.1a), typical for molybdenum and rhenium halides, chalcogenides, and chalcohalides, eight innei hgands (L ) cap the octahedron faces and six outer ligands (L ) are located in the apical positions [9]. For metals with a smaller number of valence electrons, the (M6L i2)L -type clusters... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

A partial separation of technetium from molybdenum and rhenium may be obtained at a controlled cathode potential of — 1.1 V vs. SCE. However, deposition only takes place at technetium concentrations of lO M or greater . 

The optical emission spectrum of technetium is uniquely characteristic of the element " with a few strong lines relatively widely spaced as in the spectra of manganese, molybdenum and rhenium. Twenty-five lines are observed in the arc and spark spectra between 2200 and 9000 A. Many of these lines are free from ruthenium or rhenium interferences and are therefore useful analytically. Using the resonance lines of Tc-I at 4297.06, 4262.26, 4238.19, and 4031.63 A as little as 0.1 ng of technetium can be reliably determined. 

Kopf-Maier P, Kopf H (1994) Organometallic titanium, vanadium, niobium, molybdenum and rhenium complexes - early transition metal antitumor drugs. In Fricker SP (ed) Metal Compounds in Cancer Therapy. Chapman Hall, London, pp 109-146... 

The current understanding for molybdenum and rhenium is that the involved species are bisperoxo- and hydroperoxo complexes, and that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen is significantly preferred over the two-step insertion mechanism proposed by Mimoun. 

Figure 16 Comparison of observed (open) and calculated (solid) depletions of phosphorus, tungsten, cobalt, nickel, molybdenum, and rhenium (circles) together with those for gallium, tin, and copper (inverted triangles) (sources Righter and Drake, 1997, 1999, 2000). The calculated depletions utilize the partitioning expressions of Righter and Drake (1999) for conditions of 2,250 ( 300) K (1,973 °C), 27 ( 6) GPa, AIW = — 0.4 ( 0.3) between a hydrous peridotite (NBO/t = 2.65) magma ocean and metallic liquid. The observed depletions are those of McDonough and Sun (1995), but volatility corrected as described by Newsom and Sims (1991), where the correction is made based on comparisons to trends of lithophile volatile element depletions.

A large number of tungsten organometallic compounds plays an important role, or can be regarded as prominent candidates, as catalyst in organic syntheses. Tungsten as well as molybdenum and rhenium catalysts are closely associated with metathesis polymerization. 

P. Gisdakis, 1. V. Yudanov, N. Rdsch, Olefin epoxidation by molybdenum and rhenium peroxo and hydroperoxo compounds A density functional study of energetics and mechanisms, Inorg. Chem. 40 (2001) 3755. 

Induced Codeposition of Alloys of Tungsten, Molybdenum and Rhenium with Transition Metals... 

Molybdenum and rhenium complexes have also been reported to promote the Baeyer-Villiger oxidation using O2. Although limited to cyclobutanones, Sharpless asymmetric epoxidation conditions have provided optically active lactones from hydroxymethylbutanone.35... 

Niobium metal is typically gray or dull silver in color. It is one of the refractory metals along with tantalum, tungsten, molybdenum, and rhenium, due to its very high melting point. It is estimated that niobium has a natural occurrence in Earth s crust of approximately 20 parts per milhon (ppm). The largest niobium-containing mineral reserves are located in Brazil and Canada. 

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