Ethylene hexacarbonyls

The first catalysis of an olefin metathesis reaction was reported by Banks and Bailey in 1964 (56). They reported that activated molybdenum hexacarbonyl on alumina converted propylene, for example, into ethylene and 2-butene at 150°C and 30 atm. Oxides of rhenium are also powerful heterogeneous catalysts. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

Banks and Bailey 77> noted that the occurence of disproportionation, polymerization, and isomerization over similar catalysts, or simultaneously over the same catalyst, suggests a similarity of mechanism. They noted that this is not to say that one can predict a given catalyst will promote one or more of these reactions or that a given catalyst known to promote one of these reactions also will promote another. Banks and Bailey 77> proposed that the ability of the catalyst to shift hydrogen atoms is a key factor in determining the reaction course. When they contacted ethylene with a series of catalysts prepared by supporting Group VI hexacarbonyls on alumina, they obtained different products with the different hexacarbonyls (Table 10). 

Table 10. Products obtained from ethylene over group VI metal hexacarbonyls...

Olefin metathesis—the catalytically induced redistribution of alkyli-dene groups between olefins— is a rather recently discovered process. The first example was the conversion of propylene into ethylene and 2-butene in the presence of molybdenum hexacarbonyl on alumina (82). 

Table 1. Ethylene reactions over group VI metal hexacarbonyls O)...

The photochemical reaction of Group VI metal hexacarbonyls with mono-enes was studied several years ago, but only recently has the reaction of one of the hexacarbonyls, W(CO)s in fact, with conjugated dienes been investigated. The W(CO)6 proves to be an effective catalyst for cis trans isomerization of such dienes compounds of the type W(CO)6(diene), which must act as intermediates in such isomerizations, have been isolated and their reactions studied. Under normal conditions iron pentacarbonyl reacts with acetylene to produce a complicated mixture of products, but under irradiation in an argon matrix at — 256 °C the product is but-l-en-3-yne, complexed to the iron through the carbon-carbon triple bond only. This iron-alkyne-carbonyl compound is presumably an intermediate in the reaction under normal thermal conditions. In an argon matrix at — 256 C, iron pentacarbonyl reacts with ethylene to give Fe(CO)4(QH4). ... 

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