Cobalt diolefin complexes

Numbers in parentheses indicate yield for the respective reactions. Diolefin complexes of cobalt dimers are prepared from the direct reaction of Co2(CO)s with the diolefin ... 

However, the fact that complexes in which R = Ph, F, M02CH, and C2F6 also initiate polymerization of acrylonitrile suggests that chemistry at cobalt may be involved instead. The displacement of carbon monoxide ligands by diolefins to give stable complexes of type RCCo3(CO)7(nor-bornadiene) in the case of norbornadiene has been reported (18, 76), so that such a process is entirely possible. 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

Cobalt forms few complexes of monoolefins, although diolefins react with Co2(CO)g without cleavage of the Co( t-CO)2Co bridge . Similarly, monoolefins do not displace CO from rhodium carbonyl, although diolefins react with Rh6(CO)ig to form a complex series of products . 

This simple division into polymerization and dimerization catalysts does not apply if the growing chain stabilizes as a jt-allyl—metal complex, as is the case with conjugated diolefins (i ). Certain cobalt complexes, for instance, are perfectly able to polymerize butadiene (19). 

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