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6.2.6.2: Metal Allyl and Diene Complexes

  • Page ID
    360910
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    Learning Objectives

    In this lecture you will learn the following

    • The metal−allyl complexes.
    • The metal−diene complexes.
    • The metal−cyclobutadiene complexes.
    • The respective metal−ligand interactions.

    The allyl ligand is often referred to as an “actor” ligand rather than a “spectator” ligand. It binds to metals in two ways i.e. in a η1 (monohapto) form and a η3 (trihapto) form (Figure \(\PageIndex{1}\)). (i). In its monohapto (η1) form, it behaves as an anionic 1e−donor X type of a ligand analogous to that of a methyl moiety while (ii) in a trihapto (η3) form, it acts as an anionic 3e−donor LX type of a ligand.

    clipboard_e279119337d70beadaebf8d218522df0e.png
    Figure \(\PageIndex{1}\): Metal−allyl interaction.

    Metal−allyl interaction

    Of particular interest are the molecular orbitals namely Ψ1, Ψ2 and Ψ3 of the allyl ligand that interact with the metal in a metal allyl complex. The energy of these molecular orbitals increase with the increase in the number of nodes. Of the three, the Ψ1 and Ψ2 orbitals usually engage in ligand to metal σ−donation, with Ψ1 involving in a dative L−type bonding and Ψ2 participating in a covalent X−type bonding with the metal d orbitals (Figure \(\PageIndex{2}\)).

    clipboard_efaf5bcee569a699a4e9aed597f0fa319.png
    Figure \(\PageIndex{2}\): Metal−allyl interactions.

    Synthesis of the metal allyl complexes

    The metal allyl complexes are synthesized by the following methods.

    i. From an alkene complex as shown below.

    \[\ce{Mo(dpe)2(η2 -propene) <=> Mo(dpe)2( η3-allyl)H} \nonumber \]

    ii. By a nucleophilic attack of an allyl compound as shown below.

    clipboard_e566bd258ee75493d751a0d1c80411c5f.png

    iii. By an electrophilic attack of an allyl compound as shown below.

    clipboard_e25ab61483bb56d73c10b22b97d0545ec.png

    iv. From a diene complex as shown below.

    clipboard_e4de1b627951940595f1526901c5ced0c.png

    Reactions of metal allyl complexes

    The reactivities of the metal allyl complexes toward various species are illustrated below.

    i. Reaction with nucleophiles

    clipboard_eab7d8cb912f31ff7d4981c8f91fa62d6.png

    ii. Reaction with electrophiles

    clipboard_e888ca62546cebfd6a124010b4357bb62.png

    iii. Insertion reaction

    clipboard_e16adc520a23f9180a0c76e4e22e48492.png

    iv. Reductive elimination

    clipboard_ee78f1bd74b23f1c78d17ff6161b8c78a.png

    Diene complexes

    1,3−Butadiene is a 4e−donor ligand that binds to a metal in a cisoid conformation. The Dewar−Chatt model, when applied to 1,3−butadiene, predicts that the ligand may bind to metal either as a L22) donor type, similar to that of an alkene, or as an LX22π) donor type, similar to that of a metalacyclopropane form. The L2 binding of 1,3−butadiene is rare, e.g. asin (butadiene)Fe(CO)3, while the LX2 type binding is more common, e.g. as in Hf(PMe3)2Cl2. An implication of the LX2 type binding is in the observed shortening of the C2−C3 (1.40 Å) distance alongside the lengthening of the C1−C2(1.46 Å) and C3−C4 (1.46 Å) distances (Figure \(\PageIndex{3}\)).

    clipboard_e5929d261e2aa3fe0473d5151c133fdd7.png
    Figure \(\PageIndex{3}\): Metal−diene interaction in cisoid binding.

    The molecular orbitals of the 1,3−butadiene ligand comprises of two filled Ψ1 (HOMO−1) and Ψ2(HOMO) orbitals and two empty Ψ3 (LUMO) and Ψ4 (LUMO+1) orbitals. In a metal−butadiene interaction the ligand to metal σ−donation occurs from the filled Ψ2 orbital of the 1,3−butadiene ligand while the metal to ligand π−back donation occurs on to the empty Ψ3 orbital of the 1,3−butadiene ligand (Figure \(\PageIndex{4}\)).

    clipboard_e7f973a838456a604ebc43db2542f7c1c.png
    Figure \(\PageIndex{4}\): Metal−diene interaction.

    Though cisoid binding is often observed in metal butadiene complexes, a few instances of transoidbinding is seen in dinuclear, e.g. as in Os3(CO)10(C4H6), and in mononuclear complexes e.g. as in Cp2Zr(C4H6) (Figure \(\PageIndex{5}\)).

    clipboard_e3f34d30597461942ee4ba3a8d99ef022.png
    Figure \(\PageIndex{5}\): Metal−diene interaction in transoid binding.

    Synthesis of metal butadiene complex

    Metal butadiene complexes are usually prepared by the same methods used for synthesizing metal alkene complexes. Two noteworthy synthetic routes are shown below.

    clipboard_ea3135f5238062f415dd0a0869431522e.png

    Metal cyclobutadiene complexes

    Cyclobutadiene is an interesting ligand because of the fact that its neutral form, being anti−aromatic (−electrons), is unstable as a free molecule (Figure \(\PageIndex{6}\)), but its dianionic form is stable because of being aromatic (−electrons). Consequently, the cyclobutadiene ligand is stabilized by significant metal to ligand π−back donation to the vacant ligand orbitals.

    clipboard_e04ad3e684aa14ddd1fc0db84d156792d.png
    Figure \(\PageIndex{6}\): Electronic structure of cyclobutadiene ligand.

    A synthetic route to metal cyclobutadiene complex is shown below.

    clipboard_eb7b49b3ee9de08a966f55627a3cefc88.png

    Problems

    1. The hapticities displayed by an allyl moiety in binding to metals are?

    Ans: 1 and 3.

    2. Identify which molecular orbitals of an allyl moiety engage in σ−interaction with a suitable d orbital of a metal in a η3−metal allyl complex?

    Ans: Ψ1 and Ψ2.

    3. Predict the product of the reaction.

    clipboard_e69aa6f4ba37cc4cc384c69d1f6add537.png

    Ans:

    clipboard_eb2a85500917efaf442850fb2d48e3601.png

    4. Identify which molecular orbitals of a butadiene moiety engage in σ−interaction with a suitable dorbital of a metal in a η4−metal butadiene complex?

    Ans: Ψ2.

    Self Assessment test

    1. Predict the product of the reaction.

    \[\ce{<(--Ni--)> + CO2 -> } \nonumber \]

    Ans:

    clipboard_ed985f427d661793631158d6bd1d9eeb9.png

    2. Identify which molecular orbitals of a butadiene moiety engage in π−interaction with a suitable dorbital of a metal in a η4−metal allyl complex?

    Ans: Ψ3.

    3. Mention the type of orientations displayed by butadiene ligands for binding to metal.

    Ans: Cisoid (common) and transoid (rare).

    4. Comment on the number of π−electrons present in the cyclobutadiene moiety of a metal cyclobutadiene complex.

    Ans: 6 π−electrons.

    Summary

    Allyl, 1,3−butadiene and cyclobutadiene together constitute an important class of σ−donor/π−acceptor ligands that occupy a special place in organometallic chemistry. The complexes of these ligands with metals are important intermediates in many catalytic cycles and hence an understanding of their interaction with metal is of significant importance. In this context, the synthesis, characterization and the reactivities of the organometallic complexes of these ligands are described alongside the respective metal−ligand interactions.


    6.2.6.2: Metal Allyl and Diene Complexes is shared under a CC BY-SA license and was authored, remixed, and/or curated by LibreTexts.

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