Reference for all known osmium oxidation states (-2 through +8) with electron-configuration context, example compounds, and a compound-to-oxidation-state lookup.
Use the Osmium Oxidation States
Atomic #76OsOsmium
Atomic Mass
190.23 u
Group
8
Period
6
Block
d-block
Electronegativity
2.20 (Pauling)
Oxidation States
-2 to +8
Osmium spans oxidation states from -2 to +8 — the widest confirmed range in the periodic table
for a stable compound. Click a state card to explore its compounds and chemistry.
Oxidation State −2
Osmium reaches its lowest common oxidation state of -2 in carbonyl anion clusters, where strong pi-backdonation from the metal to CO ligands stabilizes the formally negative metal center. These compounds contain Os-Os bonds and require strongly reducing conditions or reaction with alkali metals to form. The -2 state reflects the ability of the 5d electrons to back-donate extensively into CO antibonding orbitals, pushing electron density onto the metal.
How to Identify This State
Look for osmium carbonyl anion complexes such as [Os(CO)4]2-. With no other ligands, use the carbonyl rule: CO is neutral, overall charge is 2-, so Os = -2. These require alkali-metal reduction of Os3(CO)12 or similar neutral precursors.
Example Compounds
Formula
Name
Assignment
Notes
[Os(CO)4]2-
Tetracarbonylosmate(2-)
CO neutral (×4 = 0); charge = 2- → Os = -2
Formed by reduction of Os3(CO)12 with alkali metals. Prototype of the -2 state; high reactivity.
[Os(CO)4]2-
Tetracarbonylosmate(−II)
CO is neutral (×4 = 0); the ion charge is 2−, so Os + 0 = −2 → Os = −2.
A highly reduced carbonyl complex formed under strongly reducing, oxygen-free conditions.
Common Uses
Precursors in organoosmium cluster synthesis
Research into metal-metal bonding in anionic cluster frameworks
Probes of d-block redox chemistry under strongly reducing conditions
Oxidation State 0
Osmium(0) is best represented by Os3(CO)12, a triangular cluster of three Os atoms bridged by twelve CO ligands. All three osmium centers are formally Os(0), assigned by the 18-electron rule and the neutral-ligand model. The compound is a major entry point for organoosmium chemistry and a standard precursor for higher-oxidation-state Os species.
How to Identify This State
In Os3(CO)12: CO is a neutral ligand. The cluster is electrically neutral. No charge, no ionic ligands → Os = 0 by convention. Elemental osmium metal also carries Os = 0.
Example Compounds
Formula
Name
Assignment
Notes
Os3(CO)12
Triosmium Dodecacarbonyl
CO neutral ×12 = 0; cluster neutral → Os = 0
Yellow solid; triangular Os3 framework. Major organoosmium precursor; undergoes photochemical and thermal substitution.
Os (metal)
Elemental Osmium
Elemental → Os = 0
Densest naturally occurring element (22.59 g/cm3). Blue-white, brittle metal. Used in fountain pen tips and electrical contacts.
Common Uses
Os3(CO)12 as a synthetic entry point for organoosmium clusters
Os metal as a hardening agent for other platinum-group metals
Research into metal-carbonyl photochemistry
Oxidation State +2
Osmium(II) (d6 configuration) is stable in octahedral coordination environments, particularly with strong-field ligands such as bipyridine, phenanthroline, and CO. Os(II) polypyridyl complexes are extensively studied for their photophysical properties — long-lived MLCT excited states and spin-orbit coupling from the heavy osmium center make them attractive for photocatalysis, solar energy, and bioimaging.
How to Identify This State
In [Os(bpy)3]2+: bpy (2,2′-bipyridine) is a neutral ligand. Ion charge = 2+. So Os + 0 = 2+ → Os = +2. In OsCl2(CO)2(PR3)2: each Cl is -1 (2 Cl = -2), CO and PR3 neutral, neutral complex → Os + (-2) = 0 → Os = +2.
Example Compounds
Formula
Name
Assignment
Notes
[Os(bpy)3]2+
Tris(bipyridyl)osmium(II)
bpy neutral ×3; charge = 2+ → Os = +2
Photoluminescent complex; long-lived MLCT emission (~μs). Studied for solar cells and photocatalysis.
Key synthesis intermediate; trans isomer is common.
K2[OsCl4]
Potassium tetrachloroosmate(II)
4(-1) + charge = -4 + (-2)? No — K2 gives +2; OsCl4 has charge -2: Os + 4(-1) = -2 → Os = +2
Ionic salt; Os(II) in square-planar-like environment.
Common Uses
Photocatalysts for solar-energy conversion (light-driven H2 generation)
Bioimaging agents (luminescent Os(II) complexes in cell staining)
Reference photophysical standards in ultrafast spectroscopy
Oxidation State +3
Osmium(III) is a d5 state. It is paramagnetic in many environments (low-spin or high-spin depending on the ligand field). Os(III) halide complexes such as OsCl3 are important synthetic precursors. The +3 state is less common than +4 and +8 but appears in many coordination compounds. Strong-field ligands like cyanide stabilize low-spin Os(III), which is isoelectronic with low-spin Fe(III) and Ru(III).
How to Identify This State
In OsCl3: Cl = -1, 3 Cl = -3, neutral compound → Os + (-3) = 0 → Os = +3. In [OsCl6]3-: 6(-1) = -6; ion charge = -3; Os + (-6) = -3 → Os = +3.
Example Compounds
Formula
Name
Assignment
Notes
OsCl3
Osmium(III) Chloride
3(-1) + Os = 0 → Os = +3
Dark brown solid; common laboratory precursor for Os(II) and Os(IV) synthesis. Reacts with CO, phosphines, and bipyridyl ligands.
[OsCl6]3-
Hexachloroosmate(III)
6(-1) + Os = -3 → Os = +3
Paramagnetic d5 complex; formed in concentrated HCl/oxidant mixtures.
Os(acac)3
Tris(acetylacetonate)osmium(III)
acac is -1 ×3 = -3; neutral complex → Os = +3
Volatile precursor used in CVD and ALD thin-film deposition of osmium metal.
Common Uses
OsCl3 as starting material in coordination and organometallic synthesis
Thin-film deposition precursors (CVD/ALD)
Electrochemical and spectroelectrochemical studies of Os(III/II) and Os(III/IV) redox couples
Oxidation State +4
Osmium(IV) is one of the two most common and commercially relevant oxidation states (the other being +8). It is a d4 configuration, typically octahedral. OsO2 is a black, insoluble solid that forms a protective film on osmium metal and is used as a catalyst. Os(IV) halide complexes (OsCl4, OsBr4) are accessible from Os metal or OsO4 reduction. The Os(IV)/Os(VIII) couple is central to the catalytic cycle when OsO4 is used in ligand-accelerated dihydroxylation (Sharpless AD reaction).
How to Identify This State
In OsO2: O = -2 (2 O × -2 = -4); neutral solid → Os = +4. In OsCl4: Cl = -1 (4 Cl = -4); neutral solid → Os = +4. In [OsCl6]2-: 6(-1) = -6; charge = 2-; Os + (-6) = -2 → Os = +4.
Example Compounds
Formula
Name
Assignment
Notes
OsO2
Osmium(IV) Oxide
2(-2) + Os = 0 → Os = +4
Black, insoluble solid. Forms as a passivating oxide layer on Os metal. Catalyst for some oxidation reactions; also a reduction product of OsO4.
OsCl4
Osmium(IV) Chloride
4(-1) + Os = 0 → Os = +4
Red-brown solid; moisture-sensitive. Important precursor in coordination chemistry and thin-film synthesis.
[OsCl6]2-
Hexachloroosmate(IV)
6(-1) + Os = -2 → Os = +4
Diamagnetic d4 low-spin octahedral complex. Formed by treating OsO4 with concentrated HCl.
OsBr4
Osmium(IV) Bromide
4(-1) + Os = 0 → Os = +4
A binary osmium(IV) halide.
Common Uses
OsO2 as an oxidation catalyst in organic synthesis
Os(IV) as the re-oxidized product in catalytic dihydroxylation cycles
OsCl4 as a synthetic precursor for Os(II) and Os(VI) compounds
Electrochemistry: Os(IV/II) couples in modified electrodes
Oxidation State +6
Osmium(VI) is a d2 state, accessible with strongly oxidizing ligands. Osmium hexafluoride (OsF6) and osmyl compounds (containing the [OsO2]2+ osmyl dication) are the main representatives. OsO2F4 and related osmyl fluorides are strong fluorinating and oxidizing agents. This state bridges the more stable +4 and the highly oxidized +8.
How to Identify This State
In OsF6: F = -1 (6 F × -1 = -6); neutral molecule → Os = +6. In OsO2Cl2: O = -2 (2 × -2 = -4), Cl = -1 (2 × -1 = -2); neutral → Os + (-4) + (-2) = 0 → Os = +6.
Example Compounds
Formula
Name
Assignment
Notes
OsF6
Osmium Hexafluoride
6(-1) + Os = 0 → Os = +6
Yellow solid at room temperature. Strong fluorinating agent; reacts violently with water. One of only a few stable MF6 compounds.
OsO2Cl2
Osmium Dioxodichloride
2(-2) + 2(-1) + Os = 0 → Os = +6
Contains the osmyl [OsO2]2+ unit. Reacts with donor ligands to give six-coordinate osmyl complexes.
OsO2F2
Osmium Dioxydifluoride
2(-2) + 2(-1) + Os = 0 → Os = +6
An osmium(VI) oxyfluoride.
Common Uses
OsF6 as a fluorinating reagent in inorganic synthesis
Osmyl complexes as enantioselective catalysts in some reactions
Research into high-oxidation-state d2 metal spectroscopy (spin-orbit effects)
Oxidation State +7
Osmium(VII) is rare and strongly oxidizing. OsO3F (osmium trioxofluoride) and the peroxo-osmium species [OsO4(OH)]- have been characterized spectroscopically, but Os(VII) compounds have no commercial use. This state is interesting primarily as a one-electron-oxidized intermediate between the stable +6 and +8 states, relevant in electrochemical studies.
How to Identify This State
In OsO3F: O = -2 (3 × -2 = -6), F = -1; neutral → Os + (-6) + (-1) = 0 → Os = +7.
Example Compounds
Formula
Name
Assignment
Notes
OsO3F
Osmium Trioxofluoride
3(-2) + (-1) + Os = 0 → Os = +7
Rare; characterized by spectroscopy. Thermally unstable; disproportionates to Os(VI) and Os(VIII) species.
[OsO4]-
Perosmate Ion
4(-2) + Os = -1 → Os = +7
A rare Os(VII) oxoanion; Os(VII) species are strongly oxidizing and much less stable than Os(VIII).
Common Uses
Electrochemical research (Os(VII/VIII) and Os(VI/VII) redox couples)
Theoretical studies of relativistic effects at very high oxidation states
Oxidation State +8
Osmium(VIII) is the highest confirmed oxidation state of any element in a stable molecular compound. All eight valence electrons (5d6 6s2) of osmium participate in bonding with four oxygen atoms. OsO4 (osmium tetroxide) is a pale yellow, volatile solid with a tetrahedral structure. Despite being isoelectronic with permanganate (in a sense), OsO4 is a covalent, neutral molecule rather than an ionic species. It is indispensable in organic synthesis for the Upjohn and Sharpless asymmetric dihydroxylation of alkenes, producing cis-diols with high stereoselectivity.
How to Identify This State
In OsO4: O = -2 (4 O × -2 = -8); neutral molecule → Os + (-8) = 0 → Os = +8. In [OsO4(OH)2]2-: 4(-2) + 2(-2+1) = -8 + (-2) = -10; charge = 2-; Os + (-10) = -2 → Os = +8.
Example Compounds
Formula
Name
Assignment
Notes
OsO4
Osmium Tetroxide
4(-2) + Os = 0 → Os = +8
Pale yellow volatile solid (mp 40 °C); highly toxic. Reagent of choice for cis-dihydroxylation of alkenes; TEM staining agent for lipids.
OsO4·py
Osmium Tetroxide–Pyridine Adduct
Pyridine is a neutral donor; the OsO4 unit retains Os = +8
A neutral Lewis-base adduct of osmium tetroxide used in osmylation chemistry.
OsO4·4-methylmorpholine-N-oxide
OsO4·NMO Complex
OsO4 component: Os = +8
In situ catalytic system (Upjohn conditions): OsO4 is the catalyst; NMO re-oxidizes Os(VI) back to Os(VIII). Very small amounts of OsO4 needed.
Common Uses
Cis-dihydroxylation of alkenes in organic total synthesis
Asymmetric dihydroxylation (Sharpless AD) with chiral amine ligands (DHQ, DHQD)
Electron microscopy: staining lipid membranes in biological TEM samples
Fingerprint visualization in forensic science (reacts with organic residues)
Select a compound from the list to see the oxidation state of osmium
with a step-by-step calculation.
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Oxidation state of Os:
Step-by-step
Oxidation State Summary
State
Frequency
Key Example
Notes
-2
Rare
[Os(CO)4]2-
Carbonyl anion clusters; formed by reduction with alkali metals.
0
Known
Os3(CO)12
Trinuclear cluster; major precursor for organoosmium chemistry.
Key synthetic precursor; d5 configuration; paramagnetic in many ligand fields.
+4
Very Common
OsO2
Stable d4 state; product of OsO4 reduction in catalytic dihydroxylation.
+6
Moderate
OsF6
Strong oxidant; osmyl [OsO2]2+ unit common in Os(VI) coordination chemistry.
+7
Rare
OsO3F
Thermally unstable; confirmed spectroscopically; no commercial use.
+8
Very Common
OsO4
Highest confirmed oxidation state of any element; all 8 valence electrons bonded.
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Summary
Osmium (Os, atomic number 76) is a platinum-group metal in Period 6, Group 8, with ground-state electron configuration [Xe] 4f14 5d6 6s2. It holds the record for the highest known oxidation state of any element in a stable compound: +8 in osmium tetroxide (OsO4). The extensive range of oxidation states (-2 through +8) arises from the availability of both 5d and 6s electrons for bonding and the relativistic stabilization of the 6s orbital, which makes very high oxidation states accessible. The most commercially important states are +4 (found in osmium dioxide and many coordination compounds) and +8 (OsO4, a vital reagent in organic synthesis for cis-dihydroxylation).
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Use cases
Students learning about d-block chemistry and high oxidation states in Period 6 transition metals.
Organic chemists checking the oxidation state of Os in OsO4 before using it for dihydroxylation reactions.
Inorganic chemistry courses covering platinum-group metal compounds and coordination chemistry.
Researchers distinguishing Os(IV) from Os(VIII) species in catalytic or analytical contexts.
Teachers preparing reference materials on the widest oxidation-state range in the periodic table.
Frequently Asked Questions
Osmium reaches +8 in osmium tetroxide (OsO4), which is the highest confirmed oxidation state of any element in a stable molecular compound. All eight valence electrons (5d6 6s2) are used in bonding. OsO4 is a volatile, yellow solid used in organic synthesis for the cis-dihydroxylation of alkenes.
Osmium exhibits oxidation states from -2 to +8. Negative states (-2, -1) occur in osmium carbonyl anions and cluster compounds. Zero appears in Os3(CO)12. The positive states +2, +3, +4, +6, and +8 are the most chemically significant; +4 and +8 are the most common in ordinary compounds. The +1, +5, and +7 states exist but are rare.
In OsO4, each oxygen is -2 (four O atoms × -2 = -8). The molecule is neutral, so Os + (-8) = 0, giving Os = +8. This is the maximum oxidation state and the highest achievable in any stable binary compound.
Osmium is a 5d transition metal with the configuration [Xe] 4f14 5d6 6s2. The five 5d orbitals and the 6s orbital all participate in bonding. Relativistic effects in Period 6 stabilize the 6s electrons enough to hold high-oxidation-state compounds together (especially with strongly electronegative ligands like O or F). The result is an unusually wide range from -2 to +8.
Yes. Osmium tetroxide is highly toxic and volatile (vapor pressure ~7 mmHg at 25 °C). It reacts with lipid membranes and corneal tissue, causing permanent eye damage, and is a strong oxidizer. Despite its hazards it is indispensable in organic synthesis and electron microscopy (as a staining agent for lipids).
Chlorine is -1 (more electronegative than Os). Three Cl atoms total -3. The compound is neutral: Os + 3(-1) = 0, so Os = +3.
