Metal-Alkene complexes Flashcards
(31 cards)
Describe the role and significance of Zeise’s salt in organometallic chemistry.
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
What process is described by the Wacker process in organometallic chemistry?
The Wacker process involves regioselective nucleophilic addition using a Pd2+ catalyst
Demonstrating an important application of metal catalysis in organic synthesis.
Explain the structural difference between metal-alkene complexes and metallacyclopropanes.
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)
Diagram and explain the Dewar-Chatt-Duncanson model of bonding in metal-alkene complexes.
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
Describe the significance of ligand substitution in metal-alkene chemistry.
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.
Explain the mechanism of insertion into the M-H bond using Wilkinson’s catalyst.
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.
What is the effect of strong back-bonding in metal-alkene complexes?
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
Explain the role of metal cations in activating olefins toward nucleophilic attack.
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
How does the geometry of a metal-alkene complex compare to a typical alkene?
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.
Describe the influence of electron-withdrawing substituents on back-bonding in metal-alkene complexes.
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.
What is the typical effect of back-bonding on the metal-carbon σ-bonds in metal-alkene complexes?
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.
How does the aromatic nature of polyolefins influence their coordination to metal centres?
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.
What are the key differences in reactivity between metal-alkene and metal-arene complexes?
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.
Explain the concept of ‘slippage’ in metal-alkene complexes.
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.
What role do geometric factors play in back-bonding?
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.
Discuss the impact of polyolefins in organometallic chemistry.
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.
What is the significance of the Wacker process in industrial chemistry?
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.
Describe the role of ligand exchange reactions in the stability of metal-alkene complexes.
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.
How does the insertion of alkenes into M-H bonds contribute to catalysis?
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.
Explain the concept of regioselective nucleophilic addition in the context of Wacker Chemistry.
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.
What is the effect of strong vs. weak back-bonding on the physical properties of metal-alkene complexes?
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.
How do metal-alkene complexes activate olefins towards further chemical transformations?
Metal-alkene complexes activate olefins by altering their electron density and molecular orbitals, making them more reactive towards nucleophilic attack, oxidation, and polymerization reactions.
Detail the structural changes in alkene upon coordination to a metal.
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.
How does the insertion of alkenes into metal-alkyl bonds affect polymer growth?
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.
