Transition metal alkene complex

The simplest monoalkene is ethene. Many complexes of ethene are known, including Zeise's salt (see figure), Rh₂Cl₂(C₂H₄)₄, Cp*₂Ti(C₂H₄), and the homoleptic Ni(C₂H₄)₃. Substituted monoalkene include the cyclic cyclooctene, as found in chlorobis(cyclooctene)rhodium dimer. Alkenes with electron-withdrawing groups commonly bind strongly to low-valent metals. Examples of such ligands are TCNE, tetrafluoroethylene, maleic anhydride, and esters of fumaric acid. These acceptors form adducts with many zero-valent metals.[1]

Butadiene, cyclooctadiene, and norbornadiene are well-studied chelating agents. Trienes and even some tetraenes can bind to metals through several adjacent carbon centers. Common examples of such ligands are cycloheptatriene and cyclooctatetraene. The bonding is often denoted using the hapticity formalism. Keto-alkenes are tetrahapto ligands that stabilize highly unsaturated low valent metals as found in (benzylideneacetone)iron tricarbonyl and tris(dibenzylideneacetone)dipalladium(0).

• Metal alkene complexes.
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  Bis(cyclooctadiene)nickel(0), a catalyst and source of "naked nickel."
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  The first alkene complex, the anion in Zeise's salt.
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  Chlorobis(cyclooctene)rhodium dimer, source of "RhCl".
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  Crabtree's catalyst, a very active catalyst for hydrogenation.
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  (Benzylideneacetone)iron tricarbonyl, source of "Fe(CO)₃".
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  Mo(C₇H₈)(CO)₃, a complex of cycloheptatriene.
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  Fe(C₈H₈)₂, a complex of cyclooctatetraene
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  (Norbornadiene)molybdenum tetracarbonyl, a source of "Mo(CO)₄"
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  (Xylylene)Fe(CO)₃, illustrating the stabilization of a labile alkene bby complexation

Structure of (acac)Rh(C₂H₄)(C₂F₄), distances (red) in picometers.[3]

The bonding between alkenes and transition metals is described by the Dewar–Chatt–Duncanson model, which involves donation of electrons in the pi-orbital on the alkene to empty orbitals on the metal. This interaction is reinforced by back bonding that entails sharing of electrons in other metal orbitals into the empty pi-antibonding level on the alkene. Early metals of low oxidation state (Ti(II), Zr(II), Nb(III) etc.) are strong pi donors, and their alkene complexes are often described as metallacyclopropanes. Treatment of such species with acids gives the alkanes. Late metals (Ir(I), Pt(II)), which are poorer pi-donors, tend to engage the alkene as a Lewis acid–Lewis base interaction. Similarly, C₂F₄ is a stronger pi-acceptor than C₂H₄, as reflected in metal-carbon bond distances.[3]

• Bonding images
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  Orbital interactions in a metal-ethylene complex, as described by the Dewar–Chatt–Duncanson model
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  Two extremes depictions of M---C₂H₄ interactions.

Alkene ligands lose much of their unsaturated character upon complexation. Most famously, the alkene ligand undergoes migratory insertion, wherein it is attacked intramolecularly by alkyl and hydride ligands to form new alkyl complexes. Cationic alkene complexes are susceptible to attack by nucleophiles.[1]

Catalysis

Metal alkene complexes are intermediates in many or most transition metal catalyzed reactions of alkenes: polymerization., hydrogenation, hydroformylation, and many other reactions.[5]

The mechanism of the Wacker process involves Pd-alkene complex intermediates.

Metal-alkene complexes are uncommon in nature, with one exception. Ethene affects the ripening of fruit and flowers by complexation to a Cu(I) center in a transcription factor.[8]