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31.5: π-Propenyl Complexes of Nickel

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    A considerable body of highly useful chemistry based on nickel has been developed, largely by the German chemist, G. Wilke. Many of these reactions involve what are called \(\pi\)-propenyl (\(\pi\)-allyl) complexes and their formation has a close analogy in the formation of ferrocene from cyclopentadienylmagnesium compounds and ferric chloride (Section 31-1). Treatment of \(\ce{NiBr_2}\) with two moles of 2-propenylmagnesium bromide gives a stable (albeit oxygen sensitive) substance of composition \(\ce{(C_3H_5)_2Ni}\):

    Roberts and Caserio Screenshot 31-5-1.png

    Unlike \(\ce{C_3H_5MgBr}\), the metal compound has a very complex proton NMR spectrum. Analysis of the spectrum indicates it arises from a mixture (75:25) of two \(\ce{(C_3H_5)_2Ni}\) isomers with each isomer having its \(\ce{C_3H_5}\) groups in a rigid planar arrangement as follows:

    Roberts and Caserio Screenshot 31-5-2.png

    These facts can be accommodated by the trans- and cis-di-\(\pi\)-propenylnickel structures, \(20\). Di-\(\pi\)-propenylnickel has many interesting reactions, among which are the following examples:

    Roberts and Caserio Screenshot 31-5-3.png

    The \(\pi\)-propenyl-type structures are more stable for nickel than for other metals such as iron. With 1,3-butadiene, \(\ce{Fe(CO)_5}\) forms a double \(\pi\) complex, whereas \(\ce{Ni(CO)_4}\) produces a bis-\(\pi\)-propenyl-type structure, \(21\):

    Roberts and Caserio Screenshot 31-5-4.png

    With more 1,3-butadiene, \(21\) is converted first to \(22\), which after rearrangement reacts with 1,3-butadiene to give back \(21\) with liberation of trans,trans,trans-1,5,9-cyclododecatriene:

    Roberts and Caserio Screenshot 31-5-5.png

    The overall sequence thus provides a catalytic route for the cyclic trimerization of 1,3-butadiene.

    Ethene and alkynes react with \(21\) in the presence of excess carbon monoxide to give ten-membered ring compounds, whereas the reaction of \(21\) with excess carbon monoxide results in formation of a mixture of six- and eight-membered rings:

    Roberts and Caserio Screenshot 31-5-6.png

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 31.5: π-Propenyl Complexes of Nickel is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.