Metal carbonyls high-pressure synthesis

Syntheses from Dry Metals and Salts. Only metaUic nickel and iron react direcdy with CO at moderate pressure and temperatures to form metal carbonyls. A report has claimed the synthesis of Co2(CO)g in 99% yield from cobalt metal and CO at high temperatures and pressures (91,92). The CO has to be absolutely free of oxygen and carbon dioxide or the yield is drastically reduced. Two patents report the formation of carbonyls from molybdenum and tungsten metal (93,94). Ruthenium and osmium do not react with CO even under drastic conditions (95,96). 

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

In a collaborative effort with Dr. B. T. Heaton from the Iftiiverslty of Rent at Canterbury, England, we have recently applied this technique W to measure spectra of rhodium carbonyl clusters under high pressure of CO and H2. In connection with the efforts by the petrochemical Industry to find catalytic syntheses which use CO and H. there is increasing evidence that transition metal carbonyl clusters are Involved in the catalytic synthesis of ethylene glycol and in the Fischer Tropsch and related reactions. 

In contrast, metal clusters have several active centers or can form multi-electron systems. Metal clusters such as Rh (CO)i6, Rh4(CO)i2, It4(CO)i2, Ru3(CO)i2, and more complex structures have been successfully tested in carbonylation reactions. Rhodium clusters catalyze the conversion of synthesis gas to ethylene glycol, albeit at very high pressures up to now. 

Most of the carbonyls can be prepared by the direct combination of the metal with carbon monoxide. It is necessary that the metal be in a very active state as when freshly reduced from the oxide or a salt of the metal. While finely divided, freshly reduced nickel combines readily with carbon monoxide at room temperature and atmospheric pressure (synthesis 75) other metals require more elevated temperatures (up to 400 ) and very high pressures (up to 700 atm.). Cobalt nitrosyl tricarbonyl is produced when specially prepared cobalt is treated with a mixture of carbon monoxide and nitric oxide. 

In general high pressure and temperature are required for these reactions to occur. However there are significant examples of reactions catalysed at atmospheric pressure, in part icular for the synthesis of isocyanates (4.2.5.). In the majority of cases the most important steps of these reactions are supposed to be the deoxygenation of the nitro function by carbon monoxide iving a nitrene residue bound to the metal centre, followed by insertion of carbon monoxide into the metal-nitrene bond. This is a likely hyphotesis since nitrene complexes can be obtained by stoichiometric reactions of nitro compounds with metal carbonyls. Conversion of the imido metal complex to the observed... 

Previous syntheses of this compound often gave low yields from the reaction of OsO with carbon monoxide in the gas phase or by the reaction of OSO4 with carbon monoxide in xylene under hi pressure (128 atm) and temperatures (175 °C). Improved synthesis of this cluster was obtained by the use of OSO4 in MeOH under carbon monoxide pressure, with the white oxycarbonyl 0s404(C0)i2 as a side product We now describe an improved low-pressure synthesis of Os3(CO)i2 from OSO4 in EtOH in very high yield. Caution. Metal carbonyls are toxic and should be handled in a well-ventilated fume hood. 

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