Rhenium binary compounds

Rhenium combines with phosphorus, arsenic, silicon, selenium, and tellurium at elevated temperatures forming binary compounds. The metal, however, is stable in hydrogen and nitrogen at high temperatures. 

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. 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Savitskii et al. (1966) reported on the Sc-Re phase diagram. It follows from the figure in their paper that the liquidus curve in the phase diagram and the boundaries of some phase fields need additional refinement. Two binary compounds occur in this system (table 4). pSc dissolves <1 at.% Re, and the solubility of scandium in solid rhenium does not exceed 5-6at.%. Gschneidner (1975) presented the data of Savitskii et al., however he corrected in the figure of the phase diagram the temperatures of some phase transitions with the accepted values. 

At present only two binary sulphides of lechnetium are known, TcS7 and TC2S7, which closely resemble the analogous compounds of rhenium. 

Binary Systems and Related Compounds.—Halides. The vapourization characteristics of ReBr3 have been studied by effusion techniques and the enthalpy and entropy of sublimation determined as 200 kJ mol and 227 e.u. (per mole of trimer), respectively.The X-ray characteristics of rhenium(iii) iodide, prepared from HRe04 and HI in MeOH at 80-90 °C, have been reported. 

---