Rhenium oxide,0-, single crystals

The crystal structure of cadmium rhenium(V) oxide, as determined by single-crystal technique,1 is of the face-centered cubic pyrochlore type (a = 10.219 A.). The only positional parameter for the 48 (/) oxygens is x = 0.309 0.007 when rhenium is at the origin. The density, determined pycnometrically, is 8.82 0.03 g./cc., compared with the theoretical value of 8.83 g./cc. for Z = 8. The resistivity between 4.2 K and room temperature is very low (10-3-10-4 J2-cm.) and has a positive temperature coefficient. Over the same temperature range the magnetic susceptibility is low and temperature-independent. These properties indicate that cadmium rhenium(V) oxide exhibits metallic conductivity. 

The observed E0 values in this case indicate the highest stability of rhenium (V) derivatives in this series. The comparison of the preparative data published indicates that these are the +5 and +6 oxidation states that appear to be most stable for the alkoxocomplexes of rhenium. The low-valent (+1 — +3) complexes should either be stabilized by JT-acceptor ligands (CO, PR3, NO, unsaturated hydrocarbons) or contain multiple M M bonds [321, 586, 729, 762,]. The compounds of rhenium (VII) are very unstable and decompose at room temperature in several minutes when isolated. They can be isolated and kept for several days as the complexes with N-donor ligands such as Tmeda or Py [533, 519, 1358]. The decomposition products of rhenium (VI) and (VIT) alkoxides are often described in literature as a black tar. The compound with this kind of appearence turned to be the major product of the anodic oxidation of rhenium in methanol (at high current density) and was shown by the X-ray single crystal study to be Re402(0Me)16 [906]. 

Rhenium is a rare, silvery white metal. It tarnishes slowly in moist air but resists corrosion and oxidation fairly well. Rhenium is used as an important component in superalloys for blades in turbine engines and this is the major use today. In particular, alloys with 3-6% Re, produced as single crystals, have very good properties. 

One problem encountered in all experiments on fluid metals is the difficulty of confining samples at high temperatures without contamination from reactions with the cell material. Only a few materials are suitable. For studies of mercury and the alkali metals near their criticeil points, acceptable materials include pure tungsten, molybdenum, and the alloys tungsten-rhenium and tungsten-molybdenum. Sintered aluminum oxide or single crystal sapphire may be used to contain alkali metals up to about 1500 °C and to somewhat higher temperatures for mercury. 

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