Osmium tetroxide

Osmium tetroxide

Names
Preferred IUPAC name Osmium tetraoxide
Systematic IUPAC name Tetraoxoosmium
Other names Osmium(VIII) oxide
Identifiers
CAS Number	20816-12-0 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:88215 N
ChemSpider	28158 Y
ECHA InfoCard	100.040.038
EC Number	244-058-7
MeSH	Osmium+tetroxide
PubChem	30318 56370778 (monopotassiate) 75811001 (monoquinuclidiniate) 53113021 (monotemediate)
RTECS number	RN1140000
UN number	UN 2471
InChI InChI=1S/4O.Os Y Key: VUVGYHUDAICLFK-UHFFFAOYSA-N Y InChI=1S/4O.Os Key: VUVGYHUDAICLFK-UHFFFAOYSA-N InChI=1/4O.Os/rO4Os/c1-5(2,3)4 Key: VUVGYHUDAICLFK-TYHKRQCIAE
SMILES O=[Os](=O)(=O)=O
Properties
Chemical formula	OsO4
Molar mass	254.23 g/mol
Appearance	white volatile solid
Odor	acrid, chlorine-like
Density	4.9 g/cm3
Melting point	40.25 °C (104.45 °F; 313.40 K)
Boiling point	129.7 °C (265.5 °F; 402.8 K)
Solubility in water	5.70 g/100 mL (10 °C) 6.23 g/100 mL (25 °C)
Solubility	375 g/100 mL (CCl4) soluble in most organic solvents, ammonium hydroxide, phosphorus oxychloride
Vapor pressure	7 mmHg (20 °C)
Structure
Crystal structure	Monoclinic, mS20
Space group	C2/c; a = 0.4515 nm, b = 0.52046 nm, c = 0.80838 nm, α = 77.677°, β = 73.784°, γ = 64.294°
Hazards
Safety data sheet	ICSC 0528
EU classification (DSD)	T+ (T+) C (C)
R-phrases	R26/27/28, R34
S-phrases	(S1/2), S7/9, S26, S45
NFPA 704	0 3 1 OX
Lethal dose or concentration (LD, LC):
LCLo (lowest published)	1316 mg/m3 (rabbit, 30 min) 423 mg/m3 (rat, 4 hr) 423 mg/m3 (mouse, 4 hr)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 0.002 mg/m3
REL (Recommended)	TWA 0.002 mg/m3 (0.0002 ppm) ST 0.006 mg/m3 (0.0006 ppm)
IDLH (Immediate danger)	1 mg/m3
Related compounds
Other cations	Ruthenium tetroxide
Related osmium oxides	Osmium(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO₄. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity OsO₂, which is yellow-brown in colour.

Osmium(VIII) oxide forms monoclinic crystals. It has a characteristic acrid chlorine-like odor. The element name osmium is derived from osme, Greek for odor. OsO₄ is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents. It is also moderately soluble in water, with which it reacts reversibly to form osmic acid (see below).Pure osmium(VIII) oxide is probably colourless and it has been suggested that its yellow hue is due to osmium dioxide (OsO₂) impurities. The osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO₄ penetrate charged cell membranes. OsO₄ is 518 times more soluble in carbon tetrachloride than in water.

The osmium of OsO₄ has an oxidation number of VIII, however the metal does not possess a corresponding 8+ charge as the bonding in the compound is largely covalent in character (the ionization energy required to produce a formal 8+ charge also far exceeds the energies available in normal chemical reactions). The osmium atom has eight valence electrons (6s², 5d⁶) with double bonds to the four oxide ligands resulting in a 16 electron complex. This is isoelectronic with permanganate and chromate ions.