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| Names | |
|---|---|
| IUPAC name Tetraoxorhenic(VII) acid | |
| Other names Hydrated rhenium(VII) oxide | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.033.968 |
| EC Number |
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| RTECS number |
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| Properties | |
| H4Re2O9 (solid) HReO4 (gas) | |
| Molar mass | 251.2055 g/mol |
| Appearance | Pale yellow solid |
| Boiling point | sublimes |
| Soluble | |
| Acidity (pKa) | −1.25[1] |
| Conjugate base | Perrhenate |
| Structure | |
| octahedral-tetrahedral (solid) tetrahedral (gas) | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Corrosive |
| GHS labelling: | |
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| Danger | |
| H302,H314,H332 | |
| P260,P261,P264,P270,P271,P280,P301+P312,P301+P330+P331,P303+P361+P353,P304+P312,P304+P340,P305+P351+P338,P310,P312,P321,P330,P363,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Related compounds | |
Related compounds | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Perrhenic acid is thechemical compound with the formulaRe2O7(H2O)2. It is obtained by evaporating aqueous solutions ofRe2O7. Conventionally, perrhenic acid is considered to have the formulaHReO4, and a species of this formula forms when rhenium(VII) oxide sublimes in the presence of water or steam.[2] When a solution ofRe2O7 is kept for a period of months, it breaks down and crystals ofHReO4·H2O are formed, which contain tetrahedralReO−4.[3] For most purposes, perrhenic acid andrhenium(VII) oxide are used interchangeably.Rhenium can be dissolved innitric or concentratedsulfuric acid to produce perrhenic acid.
The structure of solid perrhenic acid is[O3Re−O−ReO3(H2O)2].[4] This species is a rare example of a metaloxide coordinated towater; most often metal–oxo–aquo species are unstable with respect to their correspondinghydroxides:
The two rhenium atoms have different bonding geometries, with one beingtetrahedral and the otheroctahedral, and with the water ligands coordinated to the latter.
Gaseous perrhenic acid is tetrahedral, as suggested by its formulaHReO4.
Perrhenic acid or the related anhydrous oxideRe2O7 converts todirhenium heptasulfide upon treatment withhydrogen sulfide:
The heptasulfide catalyzes variousreductions.[5]
Perrhenic acid in the presence ofhydrochloric acid undergoes reduction in the presence ofthioethers andtertiary phosphines to give rhenium(V) complexes with the formulaReOCl3L2.[6]
Perrhenic acid combined withplatinum on a support gives rise to a usefulhydrogenation andhydrocracking catalyst for thepetroleum industry.[7] For example,silica impregnated with a solution of perrhenic acid is reduced withhydrogen at 500 °C.[citation needed] This catalyst is used in thedehydrogenation ofalcohols and also promotes the decomposition ofammonia.
Perrhenic acid is a precursor to a variety ofhomogeneous catalysts, some of which are promising in niche applications that can justify the high cost of rhenium. In combination withtertiary arsines, perrhenic acid gives acatalyst for theepoxidation ofalkenes withhydrogen peroxide.[8] Perrhenic acid catalyses the dehydration ofoximes tonitriles.[9]
Perrhenic acid is also used in the manufacture ofx-ray targets.[10][11]
