Carbon monoxide, reaction with osmium

Dodeeacarbonyltriosmium was first obtained from the reaction of osmium (VIII) tetraoxide with carbon monoxide in the gas phase.6 Yields by this method are low. More recently, Bradford and Nyholm10 have obtained this carbonyl in high yields from the reaction of osmium tetraoxide with carbon monoxide in xylene under conditions of moderate pressure (128 atmospheres) and temperature (175°). The method... 

Isonitriles are isoelectronic with carbon monoxide bnt homoleptic see Homoleptic Compound) isonitrile complexes are more difficnlt to prepare than carbonyl complexes. The reduction of OsXg with ethanol in the presence of methylisonitrile gives OsX2(CNMe)4. The osmium(II) isonitrile complex [Os(CNMe)6] + results from the alkylation of [Os(CN)6]" with MeOS02CF3. More generally, [Os(CNR)6] + can be prepared by reaction of 0s2(02CMe)4Cl2 with alkyl isonitriles. 

Pentacarbonyliron was prepared independently by Mond [2] and Berthelot [3] in 1891 by the direct reaction of finely ground iron with carbon monoxide. It is an air-stable liquid at room temperature which is used as a starting material for iron(O) chemistry (Scheme 10.1). Corresponding pentacarbonyls of ruthenium and osmium have found less application due to their facile decomposition to M3(CO)i2. Since Fe(CO)5 is commercially available and inexpensive, there is no need to synthesize it in the labora-... 

In Table 2.28, the activation parameters observed for the reaction of Os3(CO)io(NCMe) with PPha as well as those obtained for substitution reactions of the trimetal dodecacarbonyls of iron, osmium, and ruthenium are shown. The relatively high activation enthalpies and positive AS values observed in this Table agree with the dissociative mechanism described above. The features in Table 2.28 also reflect the much greater liability of Os3(CO)ii(NCMe) compared with Os3(CO)i2. This probably arises from a weaker bond strength of osmium with acetonitrile than with carbon monoxide. 

The only study on the direct reaction of carbon monoxide with osmium pincer complexes was reported by Milstein and coworkers in 2001 [52]. In this case, the pincer (PCP)Os(II) complex (131) was reacted with CO in benzene at room temperature to afford quantitatively the carbon monoxide adduct (132). This species further reacted with an additional equivalent of CO to yield the bis-carbonyl (PCP)Os(II) pincer complex (133) with the elimination of PPhs. Additionally, complex (133) can be directly generated by pressurizing the pincer compound (131) with 3 bar of CO or from the reaction of complex (134) with CO under mild conditions. The structure of... 

A Belgian patent (178) claims improved ethanol selectivity of over 62%, starting with methanol and synthesis gas and using a cobalt catalyst with a hahde promoter and a tertiary phosphine. At 195°C, and initial carbon monoxide pressure of 7.1 MPa (70 atm) and hydrogen pressure of 7.1 MPa, methanol conversions of 30% were indicated, but the selectivity for acetic acid and methyl acetate, usehil by-products from this reaction, was only 7%. Ruthenium and osmium catalysts (179,180) have also been employed for this reaction. The addition of a bicycHc trialkyl phosphine is claimed to increase methanol conversion from 24% to 89% (181). 

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

The formation of carbido-carbonyl cluster compounds with ruthenium and osmium appears to be common in pyrolysis reactions the basic reaction may be viewed as the transformation of the coordinated carbon monoxide to carbide and carbon dioxide. Small variations in... 

Osmium tetroxide reacts with phenyl Grignard reagent to produce a reactive intermediate, serving as a synthetic route to prepare polyphenyl osmium complexes. For example, reaction with o-tolyl magnesium bromide, o-tolMgBr, forms a purple, tetracoordinated osmium ort/io-tolyl complex, Os(o-tol)4, which reacts with trimethylphosphine or carbon monoxide to yield osmium n-aryl complexes. 

Only a very few compounds have been prepared that contain more than one coordinated N2 molecule. Similarly, there are only one or two complexes known so far in which both N2 and CO are coordinated to the same metal site. Taube (28) prepared the first bis-nitrogen compound, cis-[Os (NH3)4(N2)2] by the reaction of nitrous acid with [Os (NH3)5N2]. Neither of these two osmium-N2 compounds can be prepared using N2 as a reagent, but the carbonyl analogue of one of them, [Os (NH3)sCO] has now been prepared (29) by the careful reduction of an osmium (III) complex with amalgamated zinc in the presence of carbon monoxide. Treatment of this compound with nitrous acid gives cw-[Os(NH3)4CO N2] (29). 

As many carbonate complexes are synthesized usually in aqueous solution under fairly alkaline conditions, the possibility of contamination by hydroxy species is often a problem. To circumvent this, the use of bicarbonate ion (via saturation of sodium carbonate solution with COj) rather than the carbonate ion can often avoid the precipitation of these contaminants. Many other synthetic methods use carbon dioxide as their starting point. Transition metal hydroxo complexes are, in general, capable of reacting with CO2 to produce the corresponding carbonate complex. The rate of CO2 uptake, which depends upon the nucleophilicity of the OH entity, proceeds by a mechanism that can be regarded as hydroxide addition across the unsaturated C02. There are few non-aqueous routes to carbonate complexes but one reaction (3), illustrative of a synthetic pathway of great potential, is that used to prepare platinum and copper complexes. Ruthenium and osmium carbonate complexes result from the oxidation of coordinated carbon monoxide by dioxygen insertion (4). ... 

Osmium tetraoxide (10 g) and ethyl alcohol (200 mL) are placed in the autoclave in that order. Carbon monoxide (CP grade) (80 atm) is then added and the reaction mixture is heated at 175 °C for 7 h. When cold (< 12 h) the pressure is released (behind an explosive proof shield). The large hexagonal yellow crystals of dodecacarbonyltriosmium are separated by filtration and washed with ice-cold ethanol (3 x 10-mL portions). The product obtained is dried under vacuum for at least 3 h (20°C, 10 mmHg). The sample thus obtained is sufficiently pure for most purposes. Further purification may be carried out by recrystallization from hot benzene. Yield 9.7-10.0 g, 97-99.5%. Other minor products from this reaction are the trinuclear derivatives HOs3(CO)io(OMe) and Os3(CO)io(OMe)2. ... 

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