Osmium Dioxide

Chemical transport of osmium dioxide is carried out in a transport furnace wired to provide two independently controllable zones. The transport tube containing oxide, oxygen, and by-product sodium chloride is centered in the two-zone furnace with half of the tube in each of the separate zones. Thermocouples for temperature control are placed at the ends of the tube. It is important for optimum quality and size of product crystals that the growth (empty) zone of the transport tube be free of nucleation sites. To ensure that no microscopic seeds of osmium dioxide arc in the growth zone, reverse transport conditions arc imposed by heating the growth zone to 960°, while 

The sealed ampule is then slowly heated (at a rate of about S07h) in a muffle furnace to 300° and left overnight. 

The temperature is then raised to 650° for an additional 3 h. This treatment results in complete decomposition of the chlorate and formation of golden osmium dioxide powder in one end of the tube. An oxygen pressure of about 0.2 atm results from the excess of sodium chlorate, and about 0.036 g of sodium chloride is present as the byproduct of chlorate decomposition. The oxygen is useful as the transporting agent during subsequent crystal growth sodium chloride serves no useful function in the reaction, but is not detrimental. 

Osmium dioxide is golden and crystals formed in the growth zone have an equidimensional habit and are about 2 mm across a polyhedral face. X-ray powder diffraction patterns taken on powdered ciwstals can be indexed on the basis of a tetragonal unit cell with Oq = 4.4968 A and Cq = 3.1820 A. The oxide exhibits metallic conductivity (psoox 6x 10 Q cm) and is Pauli paramagnetic. Resistivity ratios (P300 k Pa 2 k) typical crystals are about 200-300. 

Weight percent oxygen calcd. for OsOj 14.40. Found 14.49. 

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The cleavage of the intermediate osmate ester has presented problems in the past, and a variety of procedures have been developed, including the use of mannitol and strong aqueous base, refluxing aqueous alcoholic sodium bisulfite,formaldehyde and ascorbic acid. A much milder method involves simply bubbling hydrogen sulfide into a solution of the osmate ester osmium dioxide is precipitated rapidly, and the desired organic... 

Presumably the osmium tetroxide attacks the double bond in the usual fashion, is split off in situ as osmium dioxide, and is subsequently reoxidized to the tetroxide by the oxidizing agent which is also present. 

During the preparation of osmium dioxide, the ampoule containing the reaction mixture must be cooled dining sealing operations to prevent violent reaction occurring. Subsequent heating, first to 300 and then 600°C must also be effected slowly. 

The bulk metal is stable in oxygen at ordinary temperatures but reacts at 200°C, forming osmium tetroxide. When further heated to 800 to 1,500°C in air or oxygen, the tetroxide converts to volatde trioxide, OsOs. When the metal is heated in a stream of 0s04 vapor, osmium dioxide is produced ... 

Heating osmium tetroxide with osmium metal forms osmium dioxide ... 

The orange-colored solution, containing, among other things, potassium ruthenate, was treated with nitric acid, whereupon a black precipitate of osmium dioxide containing from fifteen to twenty per cent of ruthenium oxide was thrown down as a velvety deposit. Klaus distilled this with aqua regia, taking care to condense the osmium tetroxide. The residue... 

Diammino-osmo-hydroxide, [OsO(NH3)2](OH)2, is prepared from osmium tetroxide and excess of concentrated aqueous ammonia. The yellow liquid obtained is heated in a closed vessel at 50° C. till it becomes dark brown in colour and a black powder is deposited. It is then exposed to the atmosphere and evaporated at low temperature, when a blackish-brown powder is deposited. The dry powder decomposes explosively on heating, with evolution of nitrogen. It dissolves in acids yielding the corresponding salts, and from these the base may be precipitated on the addition of aqueous alkali hydroxide. On boiling with a solution of alkali, however, the base is decomposed, ammonia is liberated, and a precipitate of osmium dioxide remains.1... 

Reaction Mixtures. Osmium tetroxide can be reduced to the dioxide by reacting it with an olefin to form the osmate ester and then bubbling hydrogen sulfide through the solution. The black precipitate of osmium dioxide is removed by filtration. Package Os02 for disposal or recycle.5... 

Osmium dioxide is characterised by its remarkable power of forming what are commonly known as osmyl derivatives of general formula M2(0s02)X1. 

Colloidal osmium may also be obtained by reduction of colloidal osmium dioxide. The last named is prepared (see p. 220) by reduction with hydrazine hydrate of potassium osmate solution suspended in lanolin, which serves as protective colloid. The product is dissolved in petroleum, precipitated with alcohol and reduced in a current of hydrogen at 50° to 60° C. A solution containing 21 per cent, of osmium has been obtained in this manner.5... 

Osmium Dioxide, 0s02. results when salts of tetravalent osmium are heated with sodium carbonate in a current of carbon dioxide, and bv heating finely divided osmium in the vapour of osmium tetroxide.4... 

Pyrophoric Osmium Dioxide.—The oxide prepared by reduction of alkali osmates with alcohol or by hydrolysis of ammonium chlorosmate is not pure, but contains small quantities of organic substances or alkalies. When dry it is liable to ignite upon exposure to air, and even to detonate if warmed4 to 800° C. directly it meets the air. 

Hydrated Osmium Dioxide exists in two stages of hydration. The dihydraie, 0s02.2H20, is obtained by the reduction of alkali osmates with alcohol ... 

Colloidal Osmium Dioxide.—It has already been mentioned that osmium dioxide, obtained either by reduction cf an alkali osmate with alcohol or by hydrolysis of an alkali chlorosmate, is precipitated in the colloidal form. 

The precipitated osmium dioxide discloses its unsaturated nature by exploding when heated with organic substances. E. von Meyer5 describes the preparation of an osmium oxysulphide by passage of hydrogen sulphide through a solution of osmium tetroxide. 

Osmium Di-ammine Hydroxide, OsO(NH3)2(OH)2, results when the tetroxide is dissolved in concentrated aqueous ammonia and heated in a closed vessel to 50° C. On opening the vessel and evaporating the excess of ammonia, osmosammine hydroxide is obtained as a dark brown powder.2 It unites with acids to form salts, and upon being heated readily decomposes. When boiled with alkalies ammonia is evolved and hydrated osmium dioxide remains. ... 

Alkalies precipitate hydrated osmium dioxide, Os(OH)4, as a brownish red mass, whilst grey anhydrous osmium dioxide is obtained on fusing potassium hexachlor osmate, K30sC16, with sodium carbonate. 

Single crystals of tungsten and rhenium dioxides are grown by a procedure that is analogous to that previously described for osmium dioxide, except that iodine is used as the transport agent. The reaction is presumed to involve an oxyiodide and to be of the type ... 

The emerald green solid, OsOFe. m. p. 59-2, b. p. 100-6. is the major product of the fluorinatlon of osmium dioxide. Vapour pressure-temperature relationships for the compound are 32—59 (solid), log Pmu + 9-064 ... 

The only known fluorine containing compounds of octa-valent osmium are osmium trioxide difluoride, OsOgF, and its salts. Osmium dioxide tetrafluoride, OsOjFg, and osmium oxide hexafluoride, OsOFg, are unknown. 

In order to better understand the factors which are involved in limiting the oxidation state of osmium, a search for high valent oxyfluorides was undertaken. In particular, since six-co-ordinate osmium occurs in the readily formed hexafluoride, both osmium dioxide tetra-fluoride and osmium oxide pentafluoride appeared possible. 

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