Definition
Rhenium(VII) oxide is an inorganic chemical compound with the formula Re₂O₇. It is the anhydride of perrhenic acid (HOReO₃) and is commonly referred to as rhenium heptoxide or dirhenium heptoxide.
Overview
Re₂O₇ is the highest oxidation‑state oxide of rhenium and serves as the primary commercial source for all rhenium compounds. The compound is obtained as the volatile fraction when rhenium‑bearing ores are roasted in air at temperatures of 500–700 °C. In its solid state, Re₂O₇ forms a polymeric structure composed of alternating octahedral and tetrahedral rhenium centers. Upon heating, the polymer breaks down to a molecular species consisting of two ReO₄ tetrahedra sharing a common oxygen atom (O₃Re–O–ReO₃), a motif similar to that of manganese(VII) oxide.
Etymology/Origin
The name “rhenium(VII) oxide” follows standard IUPAC nomenclature, indicating an oxide of rhenium in the +7 oxidation state. The alternative name “rhenium heptoxide” derives from the stoichiometry of the compound (seven oxygen atoms per two rhenium atoms). The element rhenium itself is named after the river Rhine (German: Rhein), where the metal was first discovered in the early 20th century.
Characteristics
- Chemical formula: Re₂O₇
- Molar mass: 484.40 g mol⁻¹
- Appearance: Yellow crystalline powder or solid; sublimates on heating.
- Density: 6.10 g cm⁻³ (solid).
- Melting point: ~360 °C (sublimes rather than melting).
- Solubility: Reacts violently with water, hydrolyzing to perrhenic acid (HReO₄).
- Structure: Solid polymeric network; molecular Re₂O₇ consists of two tetrahedral ReO₄ units linked by a shared oxygen atom.
- Reactivity:
- Oxidizes to give perrhenic acid on contact with water.
- Decomposes on further heating to rhenium dioxide (ReO₂) with release of oxygen: 2 Re₂O₇ → 4 ReO₂ + 3 O₂.
- Reacts with organotin reagents (e.g., tetramethyltin) to form methylrhenium trioxide (CH₃ReO₃), a useful oxidation catalyst.
- Forms organorhenium siloxides upon reaction with hexamethyldisiloxane.
Related Topics
- Perrhenic acid (HReO₄): The aqueous acid formed by hydrolysis of Re₂O₇.
- Methylrhenium trioxide (MTO): An organorhenium oxide derived from Re₂O₇, employed as a homogeneous catalyst in oxidation and metathesis reactions.
- Rhenium compounds: A broader class of inorganic and organometallic species containing rhenium in oxidation states ranging from –3 to +7.
- Transition‑metal heptoxides: Analogous compounds such as manganese(VII) oxide (Mn₂O₇) and technetium(VII) oxide (Tc₂O₇).
- Catalysis in organic synthesis: Re₂O₇ and its derivatives are used as catalysts for ethenolysis, carbonyl reductions, amide reductions, and hydrogenation processes.
References: Wikipedia article “Rhenium(VII) oxide”, PubChem, Sigma‑Aldrich product data, Ullmann’s Encyclopedia of Industrial Chemistry.