Osmium tetroxide Totally Explained

Everything about Osmium Tetroxide totally explained

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Osmium tetroxide is the chemical compound with the formula OsO₄. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile.

Physical properties

Osmium tetroxide exists as a pale yellow-brown crystalline solid with a characteristic acrid odor similar to ozone. In fact, 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, and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). Pure osmium tetraoxide is probably colourless and it has been suggested that its yellow hue is due to osmium dioxide (OsO₂)impurities although osmium (IV) oxide normally exists as a black powder so this may not be true.

Structure and electron configuration

With a d⁰ configuration, Os(VIII) is expected to form tetrahedral complexes when bound to four ligands. Tetrahedral structures are seen for the electronically related oxides MnO₄⁻ and CrO₄²⁻.
The osmium of OsO₄ has a formal oxidation state of 8+, the highest oxidation state known for a transition metal. The osmium atom has eight valence electrons. If one assumes that two electrons are donated by each of the four oxide ligands, the total electron count for the complex is 16, as also seen for the isoelectronic species permanganate and chromate.

Synthesis

OsO₄ is formed slowly when osmium powder reacts with O₂ at 298 K. Reaction of bulk solid requires heating to 670 K. ť Os + 2 O₂ → OsO₄

Reactions

Oxofluorides

Osmium forms several oxofluorides, all of which are very sensitive to moisture. Purple cis-OsO₂F₄ forms at 77 K in an aqueous solution of HF: ť OsO₄ + 2 KrF₂ → cis-OsO₂F₄ + 2 Kr + O₂

OsO₄ also reacts with F₂ to form yellow OsO₃F₂:
ť 2 OsO₄ + 2 F₂ → 2 OsO₃F₂ + O₂

OsO₄ reacts with one equivalent of [Me₄N]F at 298 K and 2 equivalents at 253 K:

Miscellaneous reactions

OsO₄ dissolves in alkaline aqueous solution to give the osmate anion: ť OsO₄ + 2 NaOH → Na₂[cis-OsO₄(OH)₂] + O₂

OsO₄ is a Lewis acid, and when the Lewis bases are amines, the oxides can undergo substitution. Thus with NH₃ one obtains the nitrido-oxide: ť OsO₄ + NH₃ + KOH → K[Os(N)O₃] + 2 H₂O

The [Os(N)O₃]^- anion is isoelectronic and isostructural with OsO₄. Using primary amine tert-BuNH₂ one obtains the corresponding imido derivative: ť OsO₄ + 4 Me₃CNH₂ → Os(NCMe₃)₄ + 4 H₂O

OsO₄ undergoes "reductive carbonylation" in methanol at 400 K and 200 bar of pressure to produce the triangular cluster Os₃(CO)₁₂: ť 3 OsO₄ + 24 CO → Os₃(CO)₁₂ + 12 CO₂ Osmium tetroxide is also used in catalytic amount in the Sharpless oxyamination to give vicinal amino-alcohols.
In combination with sodium periodate, OsO₄ is used for the oxidative cleavage of alkenes (the Lemieux-Johnson oxidation). Here the periodate serves both to cleave the diol formed by dihydroxylation, and to reoxidize the OsO₃ back to OsO₄. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.

Biological staining

OsO₄ is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image. As a lipid stain, it's also useful in scanning electron microscopy (SEM) as an alternative to sputter coating. It embeds a heavy metal directly into cell membranes, creating a high secondary electron emission without the need for coating the membrane with a layer of metal, which can obscure details of the cell membrane. Additionally, Osmium tetroxide is also used for fixing biological samples in conjunction with HgCl₂. Its rapid killing abilities are used to quickly kill specimen like protozoa. Osmium tetroxide is also used as a stain for lipids in optical microscopy. OsO₄ also stains the human cornea (see safety considerations).

Polymer Staining

It is also used to stain copolymers preferentially, the best known example being block copolymers where one phase can be stained so as to show the microstructure of the material. For example, styrene-butadiene block copolymers have a central polybutadiene chain with polystyrene end caps. When treated with OsO₄, the butadiene matrix reacts preferentially and so absorbs the oxide. The presence of a heavy metal is sufficient to block the electron beam, so the polystyrene domains are seen clearly in thin films in TEM.

Osmeth

OsO₄ can be recycled and stored in the form of osmeth, a golden crystalline solid. Osmeth is OsO₄ complexed with hexamine and doesn't emit toxic fumes as opposed to pure OsO₄. It can be dissolved in tetrahydrofuran (THF) and diluted in an aqueous buffer solution to make a dilute (0.25%) working solution of OsO₄.

Osmium ore refining

OsO₄ is an intermediate in osmium ore refining. Osmium residues are reacted with Na₂O₂ forming [OsO₄(OH)₂]^2- anions, which, when reacted with chlorine (Cl₂) gas and heated, form OsO₄. The oxide is dissolved in alcoholic NaOH forming [OsO₂(OH)₄]^2- anions, which, when reacted with NH₄Cl, forms OsO₂Cl₂(NH₄)₄. This is ignited under hydrogen (H₂) gas leaving behind pure osmium (Os).

Safety considerations

OsO₄ is highly poisonous, even at low exposure levels, and must be handled with appropriate precautions. In particular, inhalation at concentrations well below those at which a smell can be perceived can lead to pulmonary edema, and subsequent death. Noticeable symptoms can take hours to appear after exposure. OsO₄ also stains the human cornea, which can lead to blindness if proper safety precautions are not observed.
On the 6th of April 2004 British intelligence sources believed they'd foiled a plot to detonate a bomb involving OsO₄. Experts interviewed by New Scientist affirmed osmium tetroxide's toxicity, though some highlighted the difficulties of using it in a weapon: osmium tetroxide is very expensive. The osmium tetroxide may be destroyed by the blast; what remaining toxic fumes may also be dispersed by the blast as well.

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