What are the most common oxidation states of rhenium?

Rhenium most commonly exists in the +7 and +4 oxidation states. The +7 state appears in dirhenium heptoxide (Re2O7) and the perrhenate ion (ReO4-), which are the primary commercial forms. The +4 state is found in rhenium tetrachloride (ReCl4) and rhenium dioxide (ReO2). The +3, +5, and +6 states also appear in well-characterized compounds, while negative states (-1, -3) occur in specialized carbonyl and organometallic chemistry.

Why does rhenium have such a wide range of oxidation states?

Rhenium has the electron configuration [Xe] 4f14 5d5 6s2, giving it seven valence electrons available for bonding. The 5d orbitals in third-row transition metals are more spatially extended and energetically accessible than in lighter congeners, allowing rhenium to form stable compounds across the full range from -3 to +7. The similar energies of the 5d and 6s orbitals, combined with strong metal-ligand bonding, stabilize high oxidation states like +7 that would be unstable for manganese.

How does rhenium compare to manganese in oxidation state chemistry?

Both are Group 7 elements, but rhenium's oxidation state chemistry differs significantly. Manganese's +7 state (in permanganate, MnO4-) is a powerful oxidizer that readily accepts electrons and reverts to lower states. Rhenium's +7 state (in perrhenate, ReO4-) is far more stable and a much weaker oxidizer due to stronger Re-O bonds from the more diffuse 5d orbitals. Rhenium also more readily forms stable low and negative oxidation states in carbonyl and organometallic complexes.

What is the perrhenate ion and where is it used?

The perrhenate ion (ReO4-) contains rhenium in the +7 oxidation state, analogous to permanganate but chemically very different. It is tetrahedral, colorless, and a far weaker oxidizer than permanganate. Ammonium perrhenate (NH4ReO4) is the primary commercial form of rhenium and is the starting material for producing rhenium metal and most rhenium catalysts. Perrhenate is used in catalyst preparation (Pt-Re reforming catalysts), as a precursor for CVD-deposited rhenium films, and in medical imaging (186Re and 188Re are therapeutic radioisotopes).

Does rhenium have negative oxidation states?

Yes. Rhenium can achieve -1 in rhenium nonacarbonyl anion [Re(CO)5]- and related carbonyl complexes. The -3 state is found in the remarkable nonahydridorhenate anion [ReH9]2-, one of the highest-coordinate hydride complexes known. Negative oxidation states require strong electron-donating ligands (CO, H-) to stabilize the electron-rich metal center; they are restricted to organometallic and coordination chemistry under inert conditions.

What makes rhenium valuable in superalloys?

Rhenium is added (typically 3-6 wt%) to nickel-based superalloys used in jet engine turbine blades. It increases creep resistance — the ability to withstand deformation at high temperatures — by segregating to grain boundaries and impeding dislocation motion. Rhenium-containing single-crystal superalloys (like CMSX-4 with ~3% Re) can operate closer to their melting point, improving turbine efficiency. This application consumes roughly 80% of world rhenium production.

Rhenium Oxidation States

Name: Rhenium Oxidation States
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Author: Nham Vu

Reference for rhenium (Re, Z=75) oxidation states: +7 and +4 are most common; the full range -3 to +7 makes Re one of the most versatile transition metals in chemistry.

Atomic # 75 Re Rhenium

Atomic Mass

186.207 u

Group

7 (VIIB)

Period

Block

d-block

Electronegativity

1.90 (Pauling)

Oxidation Range

-3 to +7

Interactive Oxidation State Explorer

Click a state to see details. Rhenium spans one of the widest oxidation state ranges of any element.

Rhenium has the ground-state configuration [Xe] 4f¹⁴ 5d⁵ 6s², providing seven valence electrons and one of the widest oxidation state ranges known: -3 to +7. The +7 state dominates in oxygen-rich environments; +4 is common in halide chemistry. The 5d orbitals of third-row metals form stronger bonds than 3d/4d analogs, stabilizing both high and low oxidation states.

State	Stability	Example Compounds	Notes
+7	Most stable	Re₂O₇, ReO₄^-, NH₄ReO₄	The dominant commercial state. Perrhenate (ReO4-) is tetrahedral, colorless, and far less oxidizing than permanganate. Re2O7 is the anhydride of perrhenic acid.
+6	Moderate	ReO₃, ReF₆	Metallic-looking ReO3 has the highest room-temperature conductivity of any oxide. ReF6 is a volatile solid used as a fluorinating agent.
+5	Moderate	ReCl₅, ReF₅, Re₂Cl₁₀	Occurs mainly in halide chemistry. Re2Cl10 has a face-sharing bioctahedral structure. Easily oxidized to Re(VII) or reduced to Re(IV).
+4	Common	ReCl₄, ReO₂, ReBr₄	The second most important oxidation state. ReCl4 and ReO2 are stable solids. ReO2 has a distorted rutile structure and is used as a catalyst precursor.
+3	Moderate	ReCl₃, Re₃Cl₉, Re₂(CO)₁₀(Cl)₂	Often forms metal-metal bonded clusters. Re3Cl9 is a triangular cluster with Re-Re bonds. Stable in appropriate ligand environments.
+2	Rare	Re₂(CO)₈Br₂	Rare in simple compounds; occurs primarily in metal-metal bonded species and carbonyl halide complexes. Strongly reducing.
+1	Very rare	Re(CO)₅Br	Found only in carbonyl halide and organometallic complexes. Low-valent state stabilized by strong pi-accepting CO ligands.
0	Elemental	Re metal	Assigned to pure rhenium metal by convention. Also the formal state in Re2(CO)10 (actually treated as Re(0) in the 18-electron count).
-1	Rare	[Re(CO)₅]^-	Found in pentacarbonylrhenate(−1) anion. Requires strongly electron-donating CO ligands; synthesized by reduction of Re(CO)5Br with alkali metals.
-3	Rare	[ReH₉]^2-	Nonahydridorhenate is one of the highest-coordinate hydride complexes known. Re is formally Re(-3) with nine H ligands in a tricapped trigonal prismatic geometry.

Why is +7 most stable for Re but not Mn?
Both Re and Mn are Group 7, but permanganate (MnO₄^-) is a strong oxidizer while perrhenate (ReO₄^-) is not. The 5d orbitals of Re form stronger bonds with oxygen than Mn's 3d orbitals, making Re(VII) compounds thermodynamically stabler and kinetically inert toward reduction. This is a general trend: heavier d-block metals favor higher oxidation states.

Rhenium forms compounds across its full oxidation range. Commercial chemistry centers on the +7 perrhenate salts and +4 halides/oxides. Organometallic chemistry extends to low and negative states via carbonyl and hydride ligands.

Compound	Formula	Re State	Notes
Dirhenium Heptoxide	Re₂O₇	+7	Yellow solid; anhydride of perrhenic acid (HReO4). Dissolves in water to give perrhenate. Used as an olefin metathesis catalyst and Re source for CVD.
Ammonium Perrhenate	NH₄ReO₄	+7	Primary commercial form of rhenium. Colorless crystalline solid; precursor to Re metal powder and Pt-Re reforming catalysts.
Perrhenic Acid	HReO₄	+7	Strong acid in aqueous solution. Aqueous perrhenate (ReO4-) is the dominant Re species in water; tetrahedral, colorless, weakly oxidizing.
Rhenium Trioxide	ReO₃	+6	Metallic red solid with extremely high electrical conductivity (comparable to Cu at RT). ReO3-type structure is a reference for many mixed oxides.
Rhenium Hexafluoride	ReF₆	+6	Volatile yellow solid; octahedral geometry. Used as a fluorinating agent and Re(VI) source in synthesis.
Rhenium Pentachloride	ReCl₅	+5	Dark brown-red solid; dimeric Re2Cl10 structure with face-sharing octahedra. Intermediate in Re halide chemistry.
Rhenium Tetrachloride	ReCl₄	+4	Dark solid; paramagnetic (d3 configuration). Precursor to other Re(IV) compounds and reduced Re species.
Rhenium Dioxide	ReO₂	+4	Dark blue-black solid with distorted rutile structure. Used as a catalyst and as a precursor to other rhenium oxides.
Rhenium Trichloride	Re₃Cl₉	+3	Triangular Re3 cluster with Re-Re bonds (~2.48 Å). A classic example of metal-metal bonded rhenium halide cluster chemistry.
Dirhenium Decacarbonyl	Re₂(CO)₁₀	0	Colorless solid; two octahedral Re(CO)5 fragments connected by a Re-Re bond. Standard starting material for Re organometallic synthesis.
Nonahydridorhenate	[ReH₉]^2-	-3	One of the highest-coordinate hydride complexes ever isolated. Tricapped trigonal prismatic geometry. Synthesized from Re2O7 and NaBH4.
Rhenium(I) Tricarbonyl Diimine	fac-[Re(CO)₃(bpy)Cl]	+1	Extensively studied luminescent complex; used in photocatalysis, CO2 reduction, and as a luminescent label in bioimaging.

Ground State Configuration

Notation

Full: [Xe] 4f¹⁴ 5d⁵ 6s²

Noble gas: [Xe] 4f¹⁴ 5d⁵ 6s²

Re⁷⁺ ion: [Xe] 4f¹⁴ (all 5d+6s removed)

Seven valence electrons: 5d⁵ 6s².
All seven can formally be removed to reach Re⁷⁺, explaining the +7 maximum state. Negative states arise when electron-donating ligands push electron density onto the metal.

Half-filled 5d subshell:
With five 5d electrons in Re, each of the five 5d orbitals holds one electron in the free atom (half-filled, high-spin arrangement). This does not produce as strong an extra stability as the 3d half-filled shell in Mn, but it does influence the chemistry of intermediate oxidation states.

Ionization Sequence to Re⁷⁺

[Xe] 4f¹⁴ 5d⁵ 6s²

Neutral atom — 7 valence electrons

Re⁺

[Xe] 4f¹⁴ 5d⁵ 6s¹

Remove one 6s electron (IE₁ ≈ 760 kJ/mol)

Re²⁺

[Xe] 4f¹⁴ 5d⁵

Remove second 6s electron (IE₂ ≈ 1260 kJ/mol)

Re³⁺

[Xe] 4f¹⁴ 5d⁴

Remove 5d electron (IE₃ ≈ 2510 kJ/mol)

Re⁴⁺

[Xe] 4f¹⁴ 5d³

Remove 5d electron (IE₄)

Re⁵⁺

[Xe] 4f¹⁴ 5d²

Remove 5d electron (IE₅)

Re⁶⁺

[Xe] 4f¹⁴ 5d¹

Remove 5d electron (IE₆)

Re⁷⁺

[Xe] 4f¹⁴

All valence electrons removed — maximum state

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