Osmium compounds

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Oxidation states of osmium
−2 Na
2[Os(CO)
4]
−1 Na
2[Os
4(CO)
13]
0 Os
3(CO)
12
+1 OsI
+2 OsI
2
+3 OsBr
3
+4 OsO
2, OsCl
4
+5 OsF
5
+6 OsF
6
+7 OsOF
5
+8 OsO
4, Os(NCH
3)
4

Osmium compounds are compounds containing the element

cluster compounds.[9][10]

Oxides

Structure of OsO3(N-t-Bu) (multiple bonds are not drawn explicitly), illustrating the type of osmium(VIII)-oxo-imide that adds alkenes en route to the amino alcohol.[11]

Osmium tetroxide is the most notable compound of osmium, having many uses. The name "osmium" even derives from Greek "ὀσμή, osme, 'smell'" because of the smell of osmium tetroxide.[12] It also has a number of unusual properties, one being that the solid is volatile. Its volatility, along with its strong oxidizing power, is the origin of its quite serious toxicity - inhalation provides a very effective route for the compound to react with tissue. The compound is colourless, but most samples appear yellow.[13] This is most likely due to the presence of the impurity OsO2, which is yellow-brown in colour.[14] In biology, its property of binding to lipids has made it a widely used stain in electron microscopy. OsO4 is formed slowly when osmium powder reacts with O2 at ambient temperature. Reaction of bulk solid requires heating to 400 °C.[15]

OsO4 is a

imido
derivative is produced:

OsO4 + Me3CNH2 → OsO3(NCMe3) + H2O

Similarly, with

nitrido complex
:

OsO4 + NH3 + KOH → K[Os(N)O3] + 2 H2O

The [Os(N)O3]− anion is isoelectronic and isostructural with OsO4. OsO4 is very soluble in

tert-butyl alcohol. In solution, it is readily reduced by hydrogen to osmium metal. The suspended osmium metal can be used to catalyze hydrogenation
of a wide variety of organic chemicals containing double or triple bonds.

OsO4 + 4 H2 → Os + 4 H2O

OsO4 undergoes "reductive carbonylation" with carbon monoxide in methanol at 400 K and 200 sbar to produce the triangular cluster Os3(CO)12:

3 OsO4 + 24 CO → Os3(CO)12 + 12 CO2[15]
Osmium dioxide has a rutile structure.

Osmium dioxide is another known oxide of osmium, which can be obtained by the reaction of osmium with a variety of oxidizing agents, including, sodium chlorate, osmium tetroxide, and nitric oxide at about 600 °C.[18][19] It does not dissolve in water, but is attacked by dilute hydrochloric acid.[20][21] The crystals have rutile structure.[22] Unlike osmium tetroxide, OsO2 is not toxic.[23]

Halides

Fluorides

Unit cell of osmium hexafluoride.

Osmium hexafluoride is one of the 17 known binary hexafluorides, which can be made by the direct reaction of osmium metal exposed to an excess of elemental fluorine gas at 300 °C. It is a yellow crystalline solid that melts at 33.4 °C and boils at 47.5 °C.[24] The solid structure measured at −140 °C is orthorhombic space group Pnma. Lattice parameters are a = 9.387 Å, b = 8.543 Å, and c = 4.944 Å. There are four formula units (in this case, discrete molecules) per unit cell, giving a density of 5.09 g·cm−3.[25] The OsF6 molecule itself (the form important for the liquid or gas phase) has octahedral molecular geometry, which has point group (Oh). The Os–F bond length is 1.827 Å.[25] Partial hydrolysis of OsF6 produces OsOF4.[26] Osmium pentafluoride is a tetramer in the solid state that can be prepared by reduction of osmium hexafluoride with iodine as a solution in iodine pentafluoride:[27]

10 OsF6 + I2 → 10 OsF5 + 2 IF5

Chlorides

Main article: Osmium(IV) chloride
The structure of osmium tetrachloride (green = chlorine, blue = osmium

Osmium tetrachloride exists in two crystalline forms, and is used to prepare other osmium complexes. It was first reported in 1909 as the product of chlorination of osmium metal.[28]
This route affords the high temperature polymorph:[29]

Os + 2 Cl2 → OsCl4

This reddish-black polymorph is orthorhombic and adopts a structure in which osmium centres are octahedrally coordinated, sharing opposite edges of the OsCl6 octahedra to form a chain.[30] A brown, apparently cubic polymorph forms upon reduction of osmium tetroxide with thionyl chloride:[31]

OsO4 + 4 SOCl2 → OsCl4 + 2 Cl2 + 4 SO2

Osmium tetroxide dissolves in hydrochloric acid to give the hexachloroosmate anion:

OsO4 + 10 HCl → H2OsCl6 + 2 Cl2 + 4 H2O

Bromides

Main article: Osmium tetrabromide
Structure of osmium tetrabromide.

technetium tetrachloride. As such, osmium is in octahedral coordination. Each osmium center bonds to four doubly bridging bromide ligands and two mutually cis terminal bromide ligands.[32] Osmium tribromide, OsBr3, is the only other binary osmium bromide is that has been crystallized.[33]

Iodides

Main article: Osmium iodides

Osmium(I) iodide is a metallic grey solid produced by the reaction of

amorphous compound.[34] Osmium(II) iodide is a black solid[35] produced by the reaction of osmium tetroxide and hydroiodic acid at 250 °C in nitrogen:[34]

OsO4 + HI → OsI2 + H2O

This compound decomposes in contact with water.

osmic acid, H4OsO6, with hydroiodic acid[36]) produced dihydroxonium hexaiodoosmate instead of the tetraiodo compound, and instead contained mono, di and tri-iodo osmium compounds.[34]

Borides

Main article: Osmium borides
The orthorhombic structure of OsB2. Here, the pink atoms denote boron atoms, and the green atoms are osmium atoms.

superhard materials, however this has not been demonstrated yet. For example, OsB2 is hard (hardness comparable to that of sapphire), but not superhard.[37]
These borides are produced in vacuum or inert atmosphere to prevent formation of osmium tetroxide, which is a hazardous compound. Synthesis occurs at high temperatures (~1000 °C) from a mixture of MgB2 and OsCl3.[37] Three osmium borides are known: OsB, Os2B3 and OsB2. The first two have hexagonal structure,[38] similar to that of rhenium diboride. Osmium diboride was first also sought as hexagonal,[39] but one of its phases was later reassigned to orthorhombic.[37][40] In recent methods of synthesis, it has also been found that a hexagonal phase of OsB2 exists with a similar structure to ReB2.[41]

See also

References

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