Reaction with Cyclopentadiene and Other Olefins

Chelating olefins such as cycloocta-1,5-diene, cyclooctatetraene, or dicyclopentadiene have not yielded isolable complexes by direct reaction with nickel carbonyl, probably for reasons outlined in Section III, B. From this it should not be concluded that such complexes are incapable of existence. Using reactions in which nickel carbonyl is not the reactant it has been possible to prepare not only complexes of cyclooctadiene and cyclooctatetraene, but also of simple monoolefins (Section IX). 

Norbornadiene when heated with nickel carbonyl also does not afford a complex, but is dimerized and carbonylated (Section VIII, C). Cyclopentadiene when refluxed with nickel carbonyl yields red, crystalline (m.p. 41-42° C), volatile, and air-sensitive, 7r-cyclopentenylcyclopentadienyl-nickel (XIII) (39, 40, 41). This complex may also be obtained from nickel-ocene by reducing it with sodium amalgam in absolute ethanol (40). The mechanism of its formation from nickel carbonyl is unknown. One might speculate that it is formed via the intermediate (XII) 

A methyl-substituted compound (XIII) is obtained similarly from nickel carbonyl and methylcyclopentadiene (41a). Analogous to (XIII) is 7r-allyl-cyclopentadienylnickel, which was prepared from 7r-allylnickel (II) bromide and sodium cyclopentadienide (41b). 

Reactions with Vinyl Compounds [Bis(acrylonitrile)-nickel and Derivatives] 

Well-defined 1 1 and 1 2 adducts with triphenylphosphine are formed with ease 42,45, 46). The yellow 1 1 adduct is more stable than bis(acrylo-nitrile)-nickel itself (dec. p. 185° C) and represents one of the first examples of a nickel complex with the coordination number three. The 2 1 adduct is monomeric in benzene and has a dipole moment of 6 Debye units (46). Unstable bis(pyridine) adducts were also isolated, but were found to decompose at room temperature, depositing metallic nickel (46). 

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