Metal Olefin Complexes

1. https://drive.google.com/file/d/0BygsoAhFInJONndjaS1GTndYbTg/view?usp=sharing

2. https://www.youtube.com/watch?v=Osn2zrRJu5g

3. https://drive.google.com/file/d/0BygsoAhFInJOLWNESXRSTjAwdmM/view?usp=sharing

4. https://drive.google.com/file/d/0BygsoAhFInJOQXQwZ2JWM1pyekE/view?usp=sharing

5. https://drive.google.com/file/d/0BygsoAhFInJOdHRxM3QwcWRDQ2c/view?usp=sharing

General Information

Alkene or olefin ligands are common in organotransition metal chemistry. In fact, the first organotransition metal complex, Zeise's salt (K[PtCl₃(C₂H₄]·H₂O) was an alkene complex although its true nature was not unambiguously determined until about 100 years after its discovery.

Bonding and Structure in Alkene Complexes

The bonding in alkene complexes is described by the Dewar-Chatt-Duncanson model, which provides us with a bonding picture not unlike that seen in carbonyl or phosphine complexes. A sigma-type donation from the C=C pi orbital with concomitant pi-backbonding into an empty pi* orbital on the ethylene presents us with a synergistic bonding situation: the greater the sigma donation to the metal, the greater the pi-backbonding:

The greater the electron density back-donated into the pi* orbital on the alkene, the greater the reduction in the C=C bond order. An alternative way of stating this would be to say that the hybridization of the alkene carbon changes from sp² to sp³ as back-donation increases. Either formalism describes two limiting structures: a planar olefin adduct and a metallocyclopropane. X-ray crystallographic studies confirm that the as the C-C bond length increases, the CH₂ plane is distorted from the ideal planar geometry of an alkene:

The structural distortion of a bound alkene can also be detected by NMR: the J_CH of alkene-like sp² carbons is typically around 160 Hz whereas sp³-like carbons have a J_CH around 120 Hz. Unlike carbonyl stretching frequencies, the C=C IR band (around 1500 cm^-1) is usually weak and not well-correlated to C-C bond length.Electronic factors play a large role in the binding of alkenes to transition metals. For example, tetrafluoroethylene will bind more tightly than ethylene to a low valent metal complex because the presence of electron-withdrawing groups on the olefin results in poorer sigma donation and lowers the energy of the pi* orbital (providing better overlap for backbonding). Likewise, ethylene (like carbon monoxide) is a poor ligand for d⁰ metal complexes because there are no d-electrons to engage in back-bonding.
The stability of alkene complexes also depends on steric factors as well. An empirical ordering of relative stability would be:

tetrasubstituted < trisubstituted < trans-disubstituted < cis-disubstituted < monosubstituted < ethylene.

Synthesis of Alkene Complexes

Alkene complexes can be synthesized by several methods:

1. Ligand substitution reactions.
2. Reduction of a higher valent metal in the presence of an alkene.
3. From alkyls and related species:
• reductive elimination (of an allyl and hydride, for example).
• hydride abstraction from alkyls
• protonation of sigma-allyls
• from epoxides (indirectly)

vvReactions of Alkene Complexes

The bonding of an alkene to a transition metal can activate the ligand to electrophilic or nucleophilic attack depending on the nature and charge of the metal center. For example, if there is a high formal charge on the metal center then the olefin is subject to attack by nucleophiles at the face opposite the metal (giving trans addition). Likewise, electron rich metal centers in low oxidation states are activated for attack by electrophiles at the C-C bond.The reaction chemistry of these complexes is quite broad and could form another textbook in itself. For some other examples of alkene reactivity look at the sections on olefin metathesis, olefin polymerization and insertion reactions.If the metal to ligand π−back donation component is smaller than the ligand to metal σ−donation, then the lengthening of the C−C bond in the metal bound olefin moiety is observed. This happens primarily because of the fact that the alkene to metal σ−donation removes the C=C π−electrons away from the C−C bond of the olefin moiety and towards the metal center, thus, decreasing its bond order and increasing the C−C bond length. Additionally, as the metal to ligand π−back donation increases, the electron donation of the filled metal dπ orbital on to the π* orbital of the metal bound olefin moiety is
enhanced. This results in an increase in the C−C bond length. The lengthening of the C−C bond in metal bound olefin complex can be correlated to the π−basicity of the metal. For example, for a weak π−basic metal, the C−C bond lengthening is anticipated to be small while for a strong π−basic metal, the C−C lengthening would be significant. Another implication of ligand−metal π−back donation is in the observed change of hybridization at the olefinic C atoms from pure sp2, in complexes with no metal to ligand π−back donation, to sp3, in complexes with significant metal to ligand π−back donation, is observed. The change in hybridization from sp2 to sp3 centers of the olefinic carbon is accompanied by the substituents being slightly bent away from the metal center in the final metalacyclopropane form (Figure 3). This change in hybridization can be conveniently detected by 1H and 13C NMR spectroscopy. For example, in case of the metalacyclopropane systems, which have strong metal to ligand π−back donation, the vinyl protons appear 5 ppm (in the 1H NMR) and 100 ppm (in the 13C NMR) high field with respect to the respective position of the free ligands.
An interesting fallout of the metal to ligand π−back bonding is the tighter binding of the strained olefins to the metal center as observed in the case of cyclopropene and norbornene. The strong binding of these cyclopropene and norbornene moieties to the metal center arise out of the relief of ring strain upon binding to the metal. Lastly, in the metal−olefin complexes having very little π−back bonding component, the chemical reactivities of the metal bound olefin appear opposite to that of a free olefin. For example, a free olefin is considered electron rich by virtue of the presence of π−electrons in its outermost valence orbital and hence it undergoes an electrophilic attack. However, the metal bound olefin complexes having predominantly σ−donation of the olefinic π−electrons and negligible metal to ligand π−back donation, the olefinic C becomes positively charged and hence undergoes a nuclophilic attack. This nature of reversal of olefin reactivity is called umpolung character.

Synthesis Metal alkene complexes are synthesized by the following methods.
i. Substitution in low valent metals
ii. Reduction of high valent metal in presence of an alkene
iii. From alkyls and related species
Reaction of alkenes The metal alkene complexes show the following reactivities. i. Insertion reaction These reactions are commonly displayed by alkenes as they insert into metal−X bonds yielding metal alkyls. The reaction occurs readily at room temperature for X = H, whereas for other elements (X = other atoms), such insertions become rare. Also, the strained alkenes and alkynes undergo such insertion readily.
ii. Umpolung reactions
Umpolung reactions are observed only for those metal−alkene complexes for which the metal center is a poor π−base and as a result of which the olefin undergoes a nuclophilic attack.

iii. Oxidative addition
Alkenes containing allylic hydrogens undergo oxidative addition to give a allyl hydride complex

1. Reactions of Alkene Complexes

   The bonding of an alkene to a transition metal can activate the ligand to electrophilic or nucleophilic attack depending on the nature and charge of the metal center. For example, if there is a high formal charge on the metal center then the olefin is subject to attack by nucleophiles at the face opposite the metal (giving trans addition). Likewise, electron rich metal centers in low oxidation states are activated for attack by electrophiles at the C-C bond.The reaction chemistry of these complexes is quite broad and could form another textbook in itself. For some other examples of alkene reactivity look at the sections on olefin metathesis, olefin polymerization and insertion reactions.If the metal to ligand π−back donation component is smaller than the ligand to metal σ−donation, then the lengthening of the C−C bond in the metal bound olefin moiety is observed. This happens primarily because of the fact that the alkene to metal σ−donation removes the C=C π−electrons away from the C−C bond of the olefin moiety and towards the metal center, thus, decreasing its bond order and increasing the C−C bond length. Additionally, as the metal to ligand π−back donation increases, the electron donation of the filled metal dπ orbital on to the π* orbital of the metal bound olefin moiety is
   enhanced. This results in an increase in the C−C bond length. The lengthening of the C−C bond in metal bound olefin complex can be correlated to the π−basicity of the metal. For example, for a weak π−basic metal, the C−C bond lengthening is anticipated to be small while for a strong π−basic metal, the C−C lengthening would be significant. Another implication of ligand−metal π−back donation is in the observed change of hybridization at the olefinic C atoms from pure sp2, in complexes with no metal to ligand π−back donation, to sp3, in complexes with significant metal to ligand π−back donation, is observed. The change in hybridization from sp2 to sp3 centers of the olefinic carbon is accompanied by the substituents being slightly bent away from the metal center in the final metalacyclopropane form (Figure 3). This change in hybridization can be conveniently detected by 1H and 13C NMR spectroscopy. For example, in case of the metalacyclopropane systems, which have strong metal to ligand π−back donation, the vinyl protons appear 5 ppm (in the 1H NMR) and 100 ppm (in the 13C NMR) high field with respect to the respective position of the free ligands.
   An interesting fallout of the metal to ligand π−back bonding is the tighter binding of the strained olefins to the metal center as observed in the case of cyclopropene and norbornene. The strong binding of these cyclopropene and norbornene moieties to the metal center arise out of the relief of ring strain upon binding to the metal. Lastly, in the metal−olefin complexes having very little π−back bonding component, the chemical reactivities of the metal bound olefin appear opposite to that of a free olefin. For example, a free olefin is considered electron rich by virtue of the presence of π−electrons in its outermost valence orbital and hence it undergoes an electrophilic attack. However, the metal bound olefin complexes having predominantly σ−donation of the olefinic π−electrons and negligible metal to ligand π−back donation, the olefinic C becomes positively charged and hence undergoes a nuclophilic attack. This nature of reversal of olefin reactivity is called umpolung character.

   Synthesis Metal alkene complexes are synthesized by the following methods.
   i. Substitution in low valent metals
   ii. Reduction of high valent metal in presence of an alkene
   iii. From alkyls and related species
   Reaction of alkenes The metal alkene complexes show the following reactivities. i. Insertion reaction These reactions are commonly displayed by alkenes as they insert into metal−X bonds yielding metal alkyls. The reaction occurs readily at room temperature for X = H, whereas for other elements (X = other atoms), such insertions become rare. Also, the strained alkenes and alkynes undergo such insertion readily.
   ii. Umpolung reactions
   Umpolung reactions are observed only for those metal−alkene complexes for which the metal center is a poor π−base and as a result of which the olefin undergoes a nuclophilic attack.
   
   iii. Oxidative addition
   Alkenes containing allylic hydrogens undergo oxidative addition to give a allyl hydride complex

   
   

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