Rhenium, fluoride

Tantalum is a gray, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below ISOoC, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. The element has a melting point exceeded only by tungsten and rhenium. Tantalum is used to make a variety... 

Fluorination of tungsten and rhenium produces tungsten hexafluoride, WF, and rhenium hexafluoride [10049-17-9J, ReF, respectively. These volatile metal fluorides are used in the chemical vapor deposition industry to produce metal coatings and intricately shaped components (see Thin films,... 

Molybdenum hexafluoride is used in the manufacture of thin films (qv) for large-scale integrated circuits (qv) commonly known as LSIC systems (3,4), in the manufacture of metallised ceramics (see MetaL-MATRIX COMPOSITES) (5), and chemical vapor deposition of molybdenum and molybdenum—tungsten alloys (see Molybdenumand molybdenum alloys) (6,7). The latter process involves the reduction of gaseous metal fluorides by hydrogen at elevated temperatures to produce metals or their alloys such as molybdenum—tungsten, molybdenum—tungsten—rhenium, or molybdenum—rhenium alloys. 

Rhenium also forms several important oxyfluorides rhenium oxytetrafluoride [17026-29-8], ReOF rhenium oxypentafluoride [23377-53-9], ReOF rhenium dioxytrifluoride [57246-89-6], Re02F2 and perrhenyl fluoride [25813-73-4], ReO F. AH are soHds at room temperature. Properties are summari2ed in Table 1. 

Attempts to prepare fluoro derivatives of (102) failed and resulted in the formation of the unusual /i-pyrazolyl fi-oxo dimer [ ReO HBF(pz)2-A,A 2(/i-pz)2(M-0)] (105)." An X-ray structural analysis shows that one pyrazolyl residue of each tridentate ligand has been substituted by fluoride. With the analogous complex containing hydridotris(3,5-dimethyl-l-pyrazolylborate), however, mixed Cl/F, I/F, or OSO2CF3/F derivatives of (102) are readily formed. The different reactivity of the hydridotris(pyrazolyl)borato rhenium complexes has been attributed to the greater steric protection of the boron atom afforded by the dimethyl-substituted ligand. 

Most chemical properties of technetium are similar to those of rhenium. The metal exhibits several oxidation states, the most stable being the hep-tavalent, Tc +. The metal forms two oxides the black dioxide Tc02 and the heptoxide TC2O7. At ambient temperature in the presence of moisture, a thin layer of dioxide, Tc02, covers the metal surface. The metal burns in fluorine to form two fluorides, the penta- and hexafluorides, TcFs and TcFe. Binary compounds also are obtained with other nonmetaUic elements. It combines with sulfur and carbon at high temperatures forming technetium disulfide and carbide, TcS2 and TcC, respectively. 

One of the simplest oxides is the rhenium trioxide (ReOs) structure shown in figure lA(b). It consists of an incomplete fee host lattice of with Re in one-quarter of the octahedral sites. (Crystallographic shear (CS) phases (discussed in 1.10.5) based on ReOs may be considered as consisting of the cubic MO2 structure.) Many oxides and fluorides adopt the ReOs structure and are used in catalysis. 

It has been suggested (79) that polymorphism may occur for transition metal pentafluorides as it does for oxide tetrafluorides (91). X-Ray powder photographs of products from the reduction of hexafluorides in anhydrous hydrogen fluoride (79) showed that the samples of rhenium and osmium pentafluorides had different structures from those previously reported, but no unit-cell dimensions could be derived. 

There are five oxide fluorides of rhenium, ReOF4, ReOF3, ReOF5, ReC>2F3, and Re03F. Early reports of Re02F2 and ReOF2 (162, 163) have not been substantiated and must be considered doubtful. 

Rhenium oxide fluorides have not escaped activity concerned with... 

Fig. 5.34. GC separation of metal fluorides. Peaks 1 = fluorine plus reaction products 2 = impurity in rhenium 3 = WF6 4 = ReF6 5 = OsF6 6 = ReOFj. Conditions PTFE column, 22 ft. X 1/4 in. O.D., 15% Kel-F No. 10 oil on Chromosorb T (40-60 mesh) helium flow-rate, 28 ml/min temperature, 75°C. (Reproduced from Anal. Chem., 38 (1966) 1860, by courtesy of R.S. Juvet and the American Chemical Society.)...

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

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