Molybdenum occurrence

The clearest manifestation of molybdenum in biology is its presence in over 20 enzymes which participate in a wide variety of redox processes (44—46). Some of the Mo enzymes and their occurrence are as follows ... 

Occurrence of rhenium and molybdenum together in nature is a consequence of the similarities of these elements. Both elements have a high affinity for sulfide ion. Moreover, the radii of Re" " and Mo" ", 0.74 nm and 0.70 nm, respectively, ate almost identical, so that ReS2 [12038-63-0] and M0S2 have similar crystal stmctures with almost identical dimensions (see Molybdenumcompounds). 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

Feldmann J, WR Cullen (1997) Occurrence of volatile transition metal compounds in landfill has synthesis of molybdenum and tungsten carbonyls in the environment. Environ Sci Technol 31 2125-2129. 

Dilworth GL. 1983. Occurrence of molybdenum in the nicotinic acid hydroxylase from Clostridium barkeri. Arch Biochem Biophys 221 565-9. 

There has been considerable interest in the chemistry of molybdenum in recent years, in part due to its occurrence in nitrogenase which catalyzes the reaction ... 

An odd occurrence takes place when molybdenum oxide (MoO ) is heated... 

Rhenium was the last naturally occurring chemical element to be discovered in 1925 by Noddack, Tacke, and Berg in the mineral gadolinite. The name of this extremely rare element (the estimated occurrence in the earth s crust is about 0.7 ppb ) is derived from the Rhine river. Residues from the processing of molybdenum ores represent the main source of the metal. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

The occurrence of structure changes as a result of electron transfer steps is well-documented [14-16]. Such a reaction has been observed for a molybdenum complex of the macrocyclic tetrathioether Meg[16]aneS4 (Meg[16]aneS4 = 3,3,7,7,11, 11,15,15-octamethyl-l,5,9,13-tetrathiacycl-ohexadecane) [17-20], which is among the... 

Jhe distribution of beryllium, boron, titanium, vanadium, chromium, cobalt, nickel, copper, zinc, gallium, germanium, tin, molybdenum, yttrium, and lanthanum in the principal coal-producing beds of the Interior Province has been studied by the U. S. Geological Survey. Data, methods of sampling, and analyses are discussed by Zubovic and others (II, 12). This chapter discusses the occurrence of 13 of these elements with respect to geological and geochemical environments of coal deposition and chemical properties of the elements. Zinc and tin are not included in this study because they were detected in only a few samples. 

Similarly, metathetical exchange of the labile acetonitrile ligands in the octahedral molybdenum complex (CO)2(MeCN)2BrMo( 3-C3Ph3) with a variety of bidentate amines led to the corresponding amino complexes (equation 267)311. These form crystalline acetonitrile solvates suitable for X-ray determination, which confirm the octahedral structure of these complexes and the occurrence of the cyclopropenyl and bromide groups in a tram-relationship. 

The interaction of molybdenum and bismuth ethoxides is also accompanied by ether elimination and the crystallization of a crystalline cubic phase of approximate composition Bi203 3Mo03. The mixed solutions of bismuth and tungsten ethoxides and those of molybdenum and bismuth 2-methoxyethox-ides turned out to be more stable. The occurrence of the interaction between the components was revealed by their tremendously increased solubility in the presence of each other (see Fig. 8.1 h). 

Figure 4.2 shows the SIMS spectrum of a promoted iron-antimony oxide catalyst used in selective oxidation reactions. Note the simultaneous occurrence of single ions (Si+, Fe+, Cu+, etc.) and molecular ions (SiO+, SiOH+, FeO+, SbO+, SbOSi+). Also clearly visible are the isotope patterns of copper (two isotopes at 63 and 65 amu), molybdenum (seven isotopes between 92 and 100 amu), and antimony (121 and 123 amu). Isotopic ratios play an important role in the identification of peaks, because all peak intensities must agree with natural abundances. Figure 4.2 also illustrates the differences in SIMS yields of the different elements although iron and antimony are present in comparable quantities in the catalyst, the iron intensity in the spectrum is about 25 times as high as that of antimony ... 

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