Metal-Alkene complexes

Question 1

Describe the role and significance of Zeise’s salt in organometallic chemistry.

Answer 1

Zeise’s salt, K[Pt(C2H4)Cl3], discovered in 1827, was the first example of a metal-alkene complex, illustrating key coordination between a metal and an alkene. This complex set a foundation for understanding organometallic bonding and reactions in transition metals.
K[PtCl4]+C2H4+H2O→K[Pt(C2H4)Cl3]+HCl

Question 2

What process is described by the Wacker process in organometallic chemistry?

Answer 2

The Wacker process involves regioselective nucleophilic addition using a Pd2+ catalyst

Demonstrating an important application of metal catalysis in organic synthesis.

Question 3

Explain the structural difference between metal-alkene complexes and metallacyclopropanes.

Answer 3

Metal-alkene complexes feature a π-bonded alkene, maintaining the double bond character.

Metallacyclopropanes are formed when the metal inserts into the C=C double bond, creating a single-bonded, three-membered metal-cyclopropane ring.
M+CH2=CH2→[M-CH2-CH2-M] (3 membered ring)

Question 4

Diagram and explain the Dewar-Chatt-Duncanson model of bonding in metal-alkene complexes.

Answer 4

This model describes bonding in metal-alkene complexes as involving σ-donation from C lone pair to the metal and π-back-donation from the metal d-orbitals to the π* orbitals of the alkene.

Sigma donation doesn’t affect C=R bond

Question 5

Describe the significance of ligand substitution in metal-alkene chemistry.

Answer 5

Ligand substitution in metal-alkene complexes facilitates the exchange of ligands around the metal centre, which is crucial for catalytic processes and the synthesis of complex organometallic structures.

Question 6

Explain the mechanism of insertion into the M-H bond using Wilkinson’s catalyst.

Answer 6

Wilkinson’s catalyst, RhCl(PPh3)3, facilitates the insertion of alkenes into M-H bonds, a key step in hydrogenation reactions where the alkene is converted to an alkane by adding hydrogen across the double bond.

Question 7

What is the effect of strong back-bonding in metal-alkene complexes?

Answer 7

Strong back-bonding, as seen in complexes like [Ru(C2H4)(PMe3)4], can lead to significant weakening of the C=C bond due to extensive π-back-donation, which can alter the reactivity and stability of the complex.

Making terminal C more susceptible to nucleophilic attack

Question 8

Explain the role of metal cations in activating olefins toward nucleophilic attack.

Answer 8

Metal cations withdraw electron density from the olefin, increasing its electrophilic character and making it more susceptible to attack by nucleophiles, a principle utilized in many catalytic processes including the Wacker process.

Polarises the C=C bond

Question 9

How does the geometry of a metal-alkene complex compare to a typical alkene?

Answer 9

In metal-alkene complexes, the bond lengths and angles can be altered due to metal coordination. For example, in Zeise’s salt, the Pt-C2H4 bonding causes changes in the ethylene geometry, influencing reactivity and physical properties.

Question 10

Describe the influence of electron-withdrawing substituents on back-bonding in metal-alkene complexes.

Answer 10

Electron-withdrawing substituents on the alkene increase the electron-accepting capacity of the π* orbitals, enhancing π-back-donation from the metal.

This results in stronger back-bonding.

Question 11

What is the typical effect of back-bonding on the metal-carbon σ-bonds in metal-alkene complexes?

Answer 11

Back-bonding generally strengthens the metal-carbon σ-bonds by increasing the electron density at the metal, which can stabilize the complex and affect its reactivity and structural properties.

Question 12

How does the aromatic nature of polyolefins influence their coordination to metal centres?

Answer 12

Aromatic polyolefins, due to their delocalized π-electrons, can coordinate more effectively with metal centres, donating a set number of π-electrons that typically matches the coordination sites available on the metal.

Question 13

What are the key differences in reactivity between metal-alkene and metal-arene complexes?

Answer 13

Metal-arene complexes tend to exhibit different reactivity patterns due to the aromatic system, which can affect electron density and reactivity at the ring, unlike the more straightforward π-bonding seen in metal-alkene complexes.

Question 14

Explain the concept of ‘slippage’ in metal-alkene complexes.

Answer 14

Slippage refers to the movement of the alkene within the coordination sphere of the metal, altering the bonding angle and potentially the electron distribution across the metal and alkene, affecting reactivity and stability.

Question 15

What role do geometric factors play in back-bonding?

Answer 15

Geometric factors such as the orientation and distance between the metal and alkene, as well as the spatial arrangement of ligands around the metal centre, critically influence the extent and effectiveness of back-bonding.

Question 16

Discuss the impact of polyolefins in organometallic chemistry.

Answer 16

Polyolefins, through insertion reactions, contribute to the growth of polymer chains in organometallic catalysis, showcasing the industrial and synthetic importance of metal-mediated polymerization processes.

Question 17

What is the significance of the Wacker process in industrial chemistry?

Answer 17

The Wacker process is crucial in the industrial production of ketones from alkenes, showcasing a practical application of palladium-catalysed oxidation reactions that are vital for synthesizing important organic intermediates.

Question 18

Describe the role of ligand exchange reactions in the stability of metal-alkene complexes.

Answer 18

Ligand exchange reactions can significantly influence the stability and reactivity of metal-alkene complexes by altering the electronic and steric environment around the metal centre, affecting the overall properties of the complex.

Question 19

How does the insertion of alkenes into M-H bonds contribute to catalysis?

Answer 19

This insertion is a key step in catalytic cycles, particularly in hydrogenation reactions, where it facilitates the addition of hydrogen across unsaturated bonds, crucial for converting alkenes to alkanes.

Question 20

Explain the concept of regioselective nucleophilic addition in the context of Wacker Chemistry.

Answer 20

In Wacker Chemistry, regioselectivity refers to the preferential addition of nucleophiles at specific positions on the alkene, dictated by the orientation of the alkene and the electronic environment created by the metal catalyst.

Question 21

What is the effect of strong vs. weak back-bonding on the physical properties of metal-alkene complexes?

Answer 21

Strong back-bonding can lead to increased reactivity and decreased thermal stability due to the weakening of the C=C bond, whereas weak back-bonding maintains more of the alkene’s original character and stability.

Question 22

How do metal-alkene complexes activate olefins towards further chemical transformations?

Answer 22

Metal-alkene complexes activate olefins by altering their electron density and molecular orbitals, making them more reactive towards nucleophilic attack, oxidation, and polymerization reactions.

Question 23

Detail the structural changes in alkene upon coordination to a metal.

Answer 23

Upon coordination, the alkene’s C=C bond length may increase slightly due to π-back-donation, and the bond angles around the alkene carbons may also adjust to accommodate the metal’s coordination preferences.

Question 24

How does the insertion of alkenes into metal-alkyl bonds affect polymer growth?

Answer 24

The insertion of alkenes into metal-alkyl bonds is crucial for catalysing the growth of polymer chains. This process involves the alkene inserting into a metal-carbon bond, generating a new metal-alkyl bond and a vacant site at the metal for further coordination and insertion of another alkene molecule, facilitating continuous chain growth.

Question 25

What is the importance of metal-arene complexes in organometallic chemistry?

Answer 25

Metal-arene complexes are important due to their stability and the aromatic system’s ability to delocalize electron density, which influences their reactivity and makes them useful in various catalytic, synthetic, and materials applications. Their bonding and reactivity mirror those of metal-alkene complexes but are influenced by the aromatic character.

Question 26

Explain the significance of π-back donation in terms of complex stability and chemical reactivity.

Answer 26

π-back donation enhances complex stability by providing additional electron density to the alkene, reducing its electron deficiency and thus stabilizing the molecule. However, it also makes the alkene more reactive by weakening the C=C bond, which can facilitate further chemical transformations.

Question 27

Discuss the structural effects of electron-rich vs. electron-poor metals in metal-alkene complexes.

Answer 27

Electron-rich metals can donate more electron density via π-back donation, leading to stronger bonding interactions and potentially weaker C=C bonds. Electron-poor metals may engage in less effective π-back donation, resulting in complexes where the alkene retains more of its original character and strength of the C=C bond.

Question 28

What factors influence the extent of back-bonding in metal-alkene complexes?

Answer 28

The extent of back-bonding is influenced by the electron density at the metal, the electron-donating or -withdrawing nature of substituents on the alkene, the spatial arrangement of ligands around the metal, and the orbital overlap between the metal and the alkene.

Question 29

Describe the impact of steric effects on the structure and reactivity of metal-alkene complexes.

Answer 29

Steric effects, arising from bulky ligands or substituents, can significantly alter the structure by affecting the orientation and distance between the metal and the alkene. These changes can influence the efficiency of σ-donation and π-back donation, thereby affecting the reactivity and stability of the complex.

Question 30

What are some common applications of metal-alkene complexes in industrial chemistry?

Answer 30

Metal-alkene complexes are extensively used in catalysis, particularly in processes like olefin polymerization, hydroformylation, and hydrogenation. Their ability to activate and transform alkenes underlies their utility in synthesizing a wide range of chemical products.

Question 31

Explain how the reactivity at the ring in arenes differs from simple alkene reactivity in metal complexes.

Answer 31

Reactivity at the ring in arenes involves electrophilic aromatic substitution reactions that are influenced by the metal’s ability to modulate the electron density across the aromatic system. This differs from simple alkene reactivity, where additions typically occur at the carbon-carbon double bond.