L3: Metal Alkene complexes

Question 1

Alkene complexes as ligands

Answer 1

• eta-2; dihapto ligand
• 2-electron donor, simplest of pi-complexes

Question 2

General features of bonding w/ alkene ligands in transition metal pi-complexes

Answer 2

1. Potential for L-> M donation and M-> L back-donation interactions
2. L->M donor and M-> L acceptor interactions both involve ligand orbitals that are of pi-symmetry wrt intra-ligand bonding

Question 3

Dewar-chatt-duncanson model

Answer 3

• sigma donation form alkene pi-orbital to metal sigma-orbital (s, pz or dz2
• pi back-donation form metal d-orbital to alkene pi* orbital

Question 4

Consequences of metal->alkene back donation; what factors govern bonding model?

Answer 4

• Weakens and lengthens C=C bond
• Alkene substituents end away from metal
• more metal-> alkene back-donation tends towards metallacyclopropane extreme (sp3 hybridised carbons)
• Extent of this is governed by a) How electron rich a metal centre is and b) how electron poor the alkene is

Question 5

Methods of synthesis of alkene complexes x5

Answer 5

1. Ligand substitution
2. Reduction of salt and alkene
3. Addition to a coordinatively unsaturated complex
4. Hydride extraction from an alkyl complex
5. Metal Vapour synthesis

Question 6

Ligand substitution reactions types

Answer 6

1. Substitution of a halide ion; often facilitated by a ‘halide scavenger’ that reacts to give either a non-coordinating counter-ion (e.g. AlCl4-) or an insoluble halide salt
2. Thermal or photochemical loss of a CO

Question 7

Hydride extraction from an alkyl complex explanation

Answer 7

A beta-hydrogen of the alkyl ligand is remove using a reagent that functions as a hydride anion extractor e.g. [Ph₃C]BF₄

Question 8

How does metal vapour synthesis work? Key merit of this method

Answer 8

Under high vacuum, metal atom vapours are co-condensed w/ excess gaseous ligand onto the walls of a reactor cooled by liquid nitrogen. On warming to room temperature, metal complexes are formed in competition with metal aggregation.
This method requires specialised apparatus but can yield complexes which are difficult/ impossible to access via other routes

Question 9

Methods of reactivity of alkene complexes

Answer 9

1. Alkene displacement: substitution by another 2-electorn ligand is a common reaction e.g ethene for PPh3
2. Nucleophilic attack on alkene: coordination of an alkene, particularly to an electron-poor metal centre activates it towards nucleophilic attack