Reference for all known osmium oxidation states (-2 through +8) with electron-configuration context, example compounds, and a compound-to-oxidation-state lookup.
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.
[HOs(CO)4]-
Hydridotetracarbonylosmate
H is -1 here (metal hydride); CO neutral (×4 = 0); charge = 1- → Os + (-1) + 0 = -1 → consider H as -1: Os = -2 if ion charge is attributed correctly (Os−1 + H−1 = −2 overall)
Formed by protonation of [Os(CO)4]2-. Illustrates the subtlety of assigning formal states in metal hydride carbonyls.
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.
OsO2(OH)2
Osmium(IV) Oxide Dihydrate (approx.)
2(-2) + 2(-2+1) + Os = 0 → Os = +4
Formed from OsO4 reduction in aqueous media; relevant in analytical osmium chemistry.
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.
OsO3F2
Osmium Trioxodifluoride
3(-2) + 2(-1) + Os = 0 → Os = +6+2? No: 3(-2)=-6, 2(-1)=-2, sum=-8; neutral → Os = +8... actually OsO3 would give +6 with O; this compound is Os +6 only if it is OsO2F2 with one extra O — for clarity use OsF6.
Illustrative of the Os+6 fluoride family.
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]+
Osmium Tetroxide Cation
4(-2) + Os = +1... → Os = +7+2=+9? No: 4O at -2 = -8; cation +1 → Os = +9?
This formulation is hypothetical; included to illustrate that Os(IX) has not been confirmed stable — Os(VII) remains the border for confirmed compounds.
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.
K2[OsO4(OH)2]
Potassium Osmate(VI) Dihydrate
4(-2) + 2(-1) = -10; K2 gives +2; ion charge = -2 → Os = +8 (osmate ester intermediate)
Stable, easily handled form of osmium used as a safer alternative to OsO4 in dihydroxylation reactions.
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.
Select a compound to see the oxidation state calculation.
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
Reference for all known osmium oxidation states (-2 through +8) with electron-configuration context, example compounds, and a compound-to-oxidation-state lookup.
How it works
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Use the Compound Lookup tab to select a known osmium compound and see the oxidation state of Os explained step by step.
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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.
