Rhenium compounds are compounds formed by the transition metalrhenium (Re). Rhenium can form in manyoxidation states, and compounds are known for every oxidation state from −3 to +7 except −2, although the oxidation states +7, +4, and +3 are the most common.^[1] Rhenium is most available commercially as salts ofperrhenate, includingsodium andammonium perrhenates. These are white, water-soluble compounds.^[2] The tetrathioperrhenate anion [ReS₄]⁻ is possible.^[3]

Rhenium(IV) oxide (or rhenium dioxide) is an oxide of rhenium, with theformula ReO₂. This gray to black crystallinesolid is a laboratory reagent that can be used as acatalyst. It adopts therutile structure. It forms viacomproportionation:^[4]

2 Re₂O₇ + 3 Re → 7 ReO₂

Single crystals are obtained bychemical transport, usingiodine as the transporting agent.^[5] At high temperatures it undergoesdisproportionation. It formsperrhenates with alkalinehydrogen peroxide andoxidizing acids.^[6] In molten sodium hydroxide it forms sodium rhenate.^[7]

Rhenium(VI) oxide, or rhenium trioxide, is another oxide of rhenium. It is the only stable group 7 trioxide. It has an appearance somewhat likecopper. It can be formed by reducingrhenium(VII) oxide withcarbon monoxide at 200 °C or elementalrhenium at 400 °C.^[8] Re₂O₇ can also be reduced withdioxane.^[9] Rhenium trioxide crystallizes with aprimitivecubicunit cell, with alattice parameter of 3.742Å (374.2pm). The structure of ReO₃ is similar to that ofperovskite (ABO₃), without the large A cation at the centre of the unit cell. Each rhenium center is surrounded by anoctahedron defined by six oxygen centers. These octahedra share corners to form the 3-dimensional structure. The coordination number of O is 2, because each oxygen atom has 2 neighbouring Re atoms.^[10]

Rhenium(VII) oxide, or rhenium heptoxide, is another oxide of rhenium. It is the anhydride form ofperrhenic acid, and is the raw material for all rhenium compounds.^[11] Solid Re₂O₇ consists of alternating octahedral and tetrahedral Re centres. Upon heating, the polymercracks to give molecular (nonpolymeric) Re₂O₇. This molecular species closely resemblesmanganese heptoxide, consisting of a pair of ReO₄ tetrahedra that share a vertex, i.e., O₃Re–O–ReO₃.^[12]

Rhenium disulfide is asulfide with the formula ReS₂. It has a layered structure where atoms are strongly bonded within each layer. The layers are held together by weakVan der Waals bonds, and can be easily peeled off from the bulk material. It is a two-dimensional (2D) group VIItransition metal dichalcogenide (TMD). ReS₂ was isolated down to monolayers which is only one unit cell in thickness for the first time in 2014.^[13] ReS₂ is found in nature as the mineralrheniite.^[14] It can be synthesized from the reaction between rhenium and sulfur at 1000 °C, or the decomposition ofrhenium(VII) sulfide at 1100 °C:^[15]

Re + 2 S → ReS₂

Re₂S₇ → 2 ReS₂ + 3 S

Rhenium diselenide (ReSe₂) also has a layered structure, although, contrary to the other dichalcogenides, rhenium ditelluride does not.^[16] In addition, rhenium also forms a heptoxide, which can be produced by the direct reaction of those elements, or through the reaction ofReO₄⁻ andH₂S in 4NHCl.^[17]

The perrhenate ion is theanion with the formulaReO⁻
₄, or a compound containing this ion. The perrhenate anion is tetrahedral, being similar in size and shape toperchlorate and the valenceisoelectronicpermanganate. The perrhenate anion is stable over a broad pH range and can be precipitated from solutions with the use of organic cations. At normal pH, perrhenate exists as metaperrhenate (ReO⁻
₄), but at high pH mesoperrhenate (ReO³⁻
₅) forms. Perrhenate, like its conjugate acidperrhenic acid, features rhenium in theoxidation state of +7 with a d⁰ configuration. Solid perrhenate salts takes on the color of the cation.^[18] These salts are prepared by oxidation of rhenium compounds with nitric acid followed by neutralization of the resulting perrhenic acid.^[19][20][21] Addition oftetrabutylammonium chloride to aqueous solutions of sodium perrhenate gives tetrabutylammonium perrhenate, which is soluble in organic solvents.^[22]

Rhenium can form at least four fluorides, of whichrhenium heptafluoride is the most common. This is the only thermally stable metal heptafluoride.^[23] It has apentagonal bipyramidal structure similar toIF₇,^[24] and can be prepared by the direct reaction of the elements at 400 °C.^[25] Combining this with additional rhenium metal at 300 °C in apressure vessel would producerhenium hexafluoride.^[26] It is one of the seventeen known binaryhexafluorides. Both of these fluorides have a very low melting point.^[27] In addition to this, rhenium also forms apentafluoride, which form yellow-green crystals,^[28] and atetrafluoride, which forms blue crystals.^[29]

The most common rhenium chlorides are ReCl₆,ReCl₅, ReCl₄, andReCl₃. Unlike fluorine, chlorine cannot oxidize rhenium past +V; the hexachloride is made from the hexafluoride and the heptachloride is entirely unknown.^[23] Rhenium(III) chloride (ReCl₃ or sometimes written as Re₃Cl₉), is a dark-red hygroscopic solid, prepared from rhenium(V) chloride and insoluble in ordinary solvents. Historically, the trichloride is one of the earliest cluster compounds with recognizable metal-metal multiple bonds. Indeed, all the chlorides feature extensive Re-Re bonding, which appears characteristic of rhenium in oxidation states lower than VII. Salts of [Re₂Cl₈]²⁻ feature aquadruple metal-metal bond. The metal-metal bonds and antibonds lie close to theFermi level in many dinuclear chlororhenate complexes; both oxidized and reduced derivatives with lesser bond order (some of themmixed-valence) are known.^[30]

Rhenium(III) bromide also adopts the same structure, and is a black lustrous crystalline solid.^[31][32] It can be obtained by the direct reaction between rhenium metal andbromine at 500 °C under nitrogen:^[33]

6 Re + 9 Br₂ → 2 Re₃Br₉

Rhenium also forms two iodides,rhenium tetraiodide, which can be reduced fromperrhenic acid withhydrogen iodide, andrhenium triiodide, which forms from the decomposition of this.^[34][35]

Liketungsten andmolybdenum, with which it shares chemical similarities, rhenium forms a variety ofoxyhalides. The oxychlorides are most common, and include ReOCl₄, ReOCl₃.

Dirhenium decacarbonyl is a common entry point to other rhenium carbonyls. The general patterns are similar to the relatedmanganese carbonyls. It is possible to reduce this dimer with sodiumamalgam to Na[Re(CO)₅] with rhenium in the formal oxidation state −1. Bromination of dirhenium decacarbonyl givesbromopentacarbonylrhenium(I),^[36] then reduced withzinc andacetic acid topentacarbonylhydridorhenium:^[37]

Re₂(CO)₁₀ + Br₂ → 2 Re(CO)₅Br

Re(CO)₅Br + Zn + HOAc → Re(CO)₅H + ZnBr(OAc)

Bromopentacarbonylrhenium(I) is readily decarbonylated. In refluxing water, it forms the triaquo cation:^[38]

Re(CO)₅Br + 3 H₂O → [Re(CO)₃(H₂O)₃]Br + 2 CO

Withtetraethylammonium bromide Re(CO)₅Br reacts to give the anionic tribromide:^[39]

Re(CO)₅Br + 2 NEt₄Br → [NEt₄]₂[Re(CO)₃Br₃] + 2 CO

Rhenium forms a variety of alkyl and aryl derivatives, often with pi-donor coligands such as oxo groups. Well known ismethylrhenium trioxide ("MTO"), CH₃ReO₃ a volatile, colourless solid, a rare example of a stable high-oxidation state metal alkyl complex. This compound has been used as acatalyst in some laboratory experiments. It can be prepared by many routes, a typical method is the reaction of Re₂O₇ andtetramethyltin:^[40]

Re₂O₇ + (CH₃)₄Sn → CH₃ReO₃ + (CH₃)₃SnOReO₃

Analogous alkyl and aryl derivatives are known. Although PhReO₃ is unstable and decomposes at –30 °C, the corresponding sterically hindered mesityl and 2,6-xylyl derivatives (MesReO₃ and 2,6-(CH₃)₂C₆H₃ReO₃) are stable at room temperature. The electron poor 4-trifluoromethylphenylrhenium trioxide (4-CF₃C₆H₄ReO₃) is likewise relatively stable.^[41] MTO and other organylrhenium trioxides catalyze oxidation reactions withhydrogen peroxide as well as olefin metathesis in the presence of a Lewis acid activator.^[42] Terminalalkynes yield the corresponding acid or ester, internal alkynes yield diketones, andalkenes give epoxides. MTO also catalyses the conversion ofaldehydes anddiazoalkanes into an alkene.^[43]

Rhenium is also able to make complexes withfullerene ligands such as Re₂(PMe₃)₄H₈(η²:η²C₆₀).

One of the firsttransition metal hydride complexes to be reported was (C₅H₅)₂ReH. A variety ofhalf-sandwich compounds have been prepared from (C₅H₅)Re(CO)₃ and (C₅Me₅)Re(CO)₃. Notable derivatives include the electron-precise oxide (C₅Me₅)ReO₃ and (C₅H₅)₂Re₂(CO)₄.

• Perrhenate
• Perrhenic acid
• Rhenium

• Rhenium compounds
• Chemical compounds by element

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