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Potassium osmate
Chemical compound From Wikipedia, the free encyclopedia
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Potassium osmate is the inorganic compound with the formula K2[OsO2(OH)4]. This diamagnetic purple salt contains osmium in the VI (6+) oxidation state.[4] When dissolved in water a red solution is formed. When dissolved in dilute alcohols, the salt gives a pink solution, and it gives a blue solution when dissolved in methanol.[1][4][5] The salt gained attention as a catalyst for the asymmetric dihydroxylation of olefins.[6]
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Structure
The complex anion is octahedral. Like related d2 dioxo complexes, the oxo ligands are trans.[7] The Os=O and Os-OH distances are 1.75(2) and 1.99(2) Å, respectively. It is a relatively rare example of a metal oxo complex that obeys the 18e rule.
Preparation
The compound was first reported by Edmond Frémy in 1844.[8] Potassium osmate is prepared by reducing osmium tetroxide with ethanol:[9]
- 2 OsO4 + C2H5OH + 5 KOH → CH3CO2K + 2 K2[OsO2(OH)4]
Alkaline oxidative fusion of osmium metal also affords this salt.[4]
Properties
Potassium osmate slowly decomposes in aqueous solution, producing osmium tetroxide.[10][3] It is more stable in cold water under alkaline conditions, but hot water or acid accelerates decomposition. When exposed to sunlight for prolonged periods of time, it decomposes and produces a sooty powder along with the odor of the tetroxide.[1][11][5] It does not dissolve in alcohol or ether, but in aqueous solution it forms a complex with alcohol, producing a pink color.[11][5][12]
Reactions
Potassium osmate reacts with acids to produce osmyl salts, such as potassium osmyl chloride and the osmyl bromide. It reacts with oxalic acid to produce potassium osmyl oxalate.[5][1]
- K2[OsO2(OH)4] + 4 HCl → K2[OsO2Cl4] + 4 H2O
- K2[OsO2(OH)4] + 2 H2C2O4 → K2[OsO2(C2O4)2 + 4 H2O
Potassium osmate reacts with potassium nitrite to yield potassium osmyl nitrite, and it reacts with ammonium salts to produce osmyl tetra-ammine compounds such as [OsO2(NH3)4]SO4 and [OsO2(NH3)4]Cl2.[5][13]
Potassium osmate can be reduced to osmium dioxide with alcohol.[1][5]
- K2[OsO2(OH)4] + C2H5OH → OsO2(H2O)2 + 2KOH + CH3CHO
It can also be used to prepare ammonium hexachloroosmate(IV).[citation needed]
Potassium osmate can be used to prepare many other compounds. If potassium osmate is refluxed with HCl, it can be reduced to hexachloroosmic acid. When refluxed with HBr, it can be reduced to hexabromoosmic acid.[citation needed] It reacts with hydrogen sulfide to produce osmium disulfide, OsS2, and with sulfurous acid to form potassium osmium sulfite.[5][11]
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Applications
Potassium osmate is used as a catalyst for the Sharpless dihydroxylation of olefins. In this application, it shuttles to Os(VIII) derivative, which adds to the olefin. Osmium tetroxide functions equivalently.
Related compounds
Other osmates are known, such as the highly water soluble sodium osmate, and insoluble osmates such as those of barium, strontium, calcium, and lead.[5][11]
When osmium tetroxide is treated with an alkali hydroxide, or even when potassium osmate is oxidized in alkaline medium, the perosmate anion is formed, [OsO4(OH)2]2-, which possesses a yellow color in solution. When treated with alcohol, perosmates are reduced to osmates, [OsO2(OH)4]2-, yielding a pink color in solution.[5][12][14] Perosmate salts share the same valency as osmium tetroxide, as opposed to perruthenates.[14] Unlike the tetroxide however, they are not volatile.[12] The perosmates of sodium, potassium, rubidium, cesium, and barium are known.[12][15]
- OsO4 + 2KOH → K2[OsO4(OH)2][16]
When osmium tetroxide is dissolved in anhydrous methanol containing potassium hydroxide, a blue solution of dipotassium tetramethyl osmate (DTMO) is obtained. DTMO was used as a staining agent for electron microscopy.[17]
- OsO4 + 5CH3OH + 2KOH → K2[OsO2(OCH3)4] + H2CO + 4H2O
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See also
References
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