Rhenium heptafluoride

Rhenium hexafluoride is readily prepared by the direct interaction of purified elemental fluorine over hydrogen-reduced, 300 mesh (ca 48 pm) rhenium powder at 120°C. The reaction is exothermic and temperature rises rapidly. Failure to control the temperature may result in the formation of rhenium heptafluoride. The latter could be reduced to rhenium hexafluoride by heating with rhenium metal at 400°C. 

Rhenium heptafluoride [17029-21 -9], ReF, is obtained by the direct interaction of elemental fluorine with hydrogen-reduced rhenium powder at 400°C and slightly over atmospheric pressure of fluorine. It is a pale yeUow soHd, mol wt 319.19 mp, 48.3°C and bp, 73.7°C. 

Iodine heptafluoride exhibits a very similar dimorphism (99), with a high-temperature cubic form, unit-cell dimension a = 6.28 A, at -110°C, and a transition below -120°C to an orthorhombic form with a = 8.74, b = 8.87, andc = 6.14 A, measured at - 145°C. A single-crystal study of the low-temperature phase was reported to show a configuration for the heptafluoride distorted from the expected pentagonal bipyramid (100). There has been some controversy over the interpretation of the X-ray results but the experimental work is very difficult and the distorted model appears valid for the data available. The unit-cell similarity for the high-temperature form, and the low-temperature transition, suggests that rhenium heptafluoride is isostructural with the iodine compound. 

Although osmium readily combines with oxygen to form the tetroxide the octafluoride is unknown. The usual product of fluorinatlon of osmium is the hexafluoride, although a thermally unstable heptafluoride has been described. The reported instability of osmium heptafluoride and difficulty of its preparation contrast with the high thermal stability of rhenium heptafluoride and with its ease of formation. ... 

Rhenium heptafluoride was prepared in a similar fashion but with a large F2 excess. When ReOF was formed (from oxide... 

The number of species isoeiectronic with XeF, is quite limited. The anions SbBr, TeCl , and TeBr are octahedral. Both IF and XeF are nonoctahedral." Iodine heptafluoride and rhenium heptafluoride may be considered isoeiectronic with these species if they are all considered to have 14 valence shell electrons of approximately equal steric requirements. Both have a pentagonal bipyramidal structure (see Fig. 6.12). Most interestingly, the XeF anion, formed by the Lewis acid XeF ... 

Vogt T, Fitch AN, Cockcroft JK (1994) Crystal and molecular structures of rhenium heptafluoride. Science 263 1265-1267... 

Except by electrolysis, HF is not easily oxidized. It should be a suitable solvent for strong oxidants. Dove and Clifford (1971) describe some such uses, but report experimental difficulties, involving reaction with reducing impurities, and attack by such very reactive solutes as rhenium heptafluoride and xenon hexafluoride on pctfe containers. Tables of electrochemical potentials are also presented. 

Only two transition metal heptafluorides are known. Rhenium hep-tafluoride is stable, but osmium heptafluoride is so unstable that no structural results for the solid compound have been reported (4). The rhenium compound is cubic (92) at room temperature, with unit-cell dimension a = 6.26(2) A, which is the same as that for rhenium hexafluoride (Table V). This cubic form is stable to -90°C, when a transition occurs to a form of lower symmetry, for which unit-cell dimensions are unknown (92). 

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