Hydrated rhenium(VII) oxide
3D model (JSmol)
|Molar mass||251.2055 g/mol|
|Appearance||Pale yellow solid|
|S-phrases (outdated)||S26, S36/37, S39, S45|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Perrhenic acid is the chemical compound with the formula Re
2. It is obtained by evaporating aqueous solutions of Re
7. Conventionally, perrhenic acid is considered to have the formula HReO
4, and a species of this formula forms when rhenium(VII) oxide sublimes in the presence of water or steam. When a solution of Re
7 is kept for a period of months, it breaks down and crystals of HReO
2O are formed, which contain tetrahedral ReO−
4 For most purposes, perrhenic acid and rhenium(VII) oxide are used interchangeably. Rhenium can be dissolved in nitric or concentrated sulfuric acid to produce perrhenic acid.
The structure of solid perrhenic acid is [O
2]. This species is a rare example of a metal oxide coordinated to water—most often metal-oxo-aquo species are unstable with respect to the corresponding hydroxides:
2O) → M(OH)
The two rhenium atoms have different bonding geometries, with one being tetrahedral and the other octahedral, and with the water ligands coordinated to the latter.
Gaseous perrhenic acid is tetrahedral, as suggested by its formula HReO
Perrhenic acid or the related anhydrous oxide Re
7 converts to dirhenium heptasulfide upon treatment with hydrogen sulfide:
7 + 7 H
2S → Re
7 + 7 H
The heptasulfide, which has a complex structure, catalyses the hydrogenation of double bonds and is useful because it tolerates sulfur compounds, which poison noble metal catalysts. Re
7 also catalyses the reduction of nitric oxide to N
Perrhenic acid combined with platinum on a support gives rise to a useful hydrogenation and hydrocracking catalyst for the petroleum industry. For example, silica impregnated with a solution of perrhenic acid is reduced with hydrogen at 500 °C. This catalyst is used in the dehydrogenation of alcohols and also promotes the decomposition of ammonia.
Perrhenic acid is a precursor to a variety of homogeneous catalysts, some of which are promising in niche applications that can justify the high cost of rhenium. In combination with tertiary arsines, perrhenic acid gives a catalyst for the epoxidation of alkenes with hydrogen peroxide. Perrhenic acid catalyses the dehydration of oximes to nitriles.
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- http://www.gehealthcare.com/usen/service/time_material_support/docs/Radplus2100.pdf[permanent dead link]