transition metal alkene complex

Complexes of ethylene are particularly common. Examples include 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.

{{See also|(Diene)iron tricarbonyl}}

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).

File:Ni(cod)2.png|Bis(cyclooctadiene)nickel(0), a catalyst and source of "naked nickel."

File:Zeise'sSalt.png|The first alkene complex, the anion in Zeise's salt.

File:Rh2Cl2 coe 4.svg|Chlorobis(cyclooctene)rhodium dimer, source of "RhCl".

File: Crabtree.svg|Crabtree's catalyst, a very active catalyst for hydrogenation.

File: (benzylideneacetone)iron-tricarbonyl-2D-skeletal.png|(Benzylideneacetone)iron tricarbonyl, source of "Fe(CO)₃".

File:CHTMo(CO)3.png|Mo(C₇H₈)(CO)₃, a complex of cycloheptatriene.

File:Fe(cot)2.svg|Fe(C₈H₈)₂, a complex of cyclooctatetraene

File:Mo(nbd)(CO)4.png|(Norbornadiene)molybdenum tetracarbonyl, a source of "Mo(CO)₄"

File:XylyleneFe(CO)3.svg|(Xylylene)Fe(CO)₃, illustrating the stabilization of a labile alkene by complexation

File:AcacRh(C2H4)(C2F4).svg

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.{{cite journal |doi=10.1039/C29710000197 |title=The Crystal Structures of Ethylene and Tetrafluoroethylene Complexes of Rhodium(I) |year=1971 |last1=Evans |first1=J. A. |last2=Russell |first2=D. R. |journal=Journal of the Chemical Society D: Chemical Communications |issue=4 |page=197}}

File:DCDmodel.png|Orbital interactions in a metal-ethylene complex, as described by the Dewar–Chatt–Duncanson model

File:M-C2H4.png|Two extreme depictions of M---C₂H₄ interactions.

The barrier for the rotation of the alkene about the M-centroid vector is a measure of the strength of the M-alkene pi-bond. Low symmetry complexes are suitable for analysis of these rotational barriers associated with the metal-ethylene bond.In CpRh(C₂H₄)(C₂F₄), the ethylene ligand is observed to rotate with a barrier near 12 kcal/mol but no rotation is observed for about the Rh-C₂F₄ bond.{{cite journal |doi=10.1021/ja01038a021 |title=Bond Character and Conformational Equilibration of Ethylene- and Tetrafluoroethylenerhodium Complexes from Nuclear Magnetic Resonance Spectra |year=1969 |last1=Cramer |first1=Richard |last2=Kline |first2=Jules B. |last3=Roberts |first3=John D. |journal=Journal of the American Chemical Society |volume=91 |issue=10 |pages=2519–2524}}

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.

Metal alkene complexes are intermediates in many or most transition metal catalyzed reactions of alkenes: polymerization., hydrogenation, hydroformylation, and many other reactions.Piet W. N. M. van Leeuwen "Homogeneous Catalysis: Understanding the Art", 2004, Wiley-VCH, Weinheim. {{ISBN|1-4020-2000-7}}

File:WackerMechWiki3.gif involves Pd-alkene complex intermediates. ]]

Since alkenes are mainly produced as mixtures with alkanes, the separation of alkanes and alkenes is of commercial interest. Separation technologies often rely on facilitated transport membranes containing Ag⁺ or Cu⁺ salts that reversibly bind alkenes.{{cite journal |doi=10.1016/j.jiec.2008.04.014 |title=A Review on Olefin/Paraffin Separation Using Reversible Chemical Complexation technology |year=2008 |last1=Azhin |first1=Maryam |last2=Kaghazchi |first2=Tahereh |last3=Rahmani |first3=Mohammad |journal=Journal of Industrial and Engineering Chemistry |volume=14 |issue=5 |pages=622–638}}

In argentation chromatography, stationary phases that contain silver salts are used to analyze organic compounds on the basis of the number and type of alkene (olefin) groups. This methodology is commonly employed for the analysis of the unsaturated content in fats and fatty acids.{{cite web |url=https://lipidlibrary.aocs.org/lipid-analysis/silver-ion-chromatography-of-lipids |title=Principles of Silver Ion Complexation with Double Bonds |author=Boryana Nikolova-Damyanova}}

Metal-alkene complexes are uncommon in nature, with one exception. Ethylene affects the ripening of fruit and flowers by complexation to a Cu(I) center in a transcription factor.Jose M. Alonso, Anna N. Stepanova "The Ethylene Signaling Pathway" Science 2004, Vol. 306, pp. 1513-1515. {{doi|10.1126/science.1104812}}

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Category:Coordination chemistry

Category:Organometallic chemistry

Category:Transition metals