1. Ligand substitution reactions: dissociative
Example:
Factors influencing ease of dissociation:
1e row < 2e row > 3e row
d8-ML5 > d10-ML4 > d6-ML6
stable ligands (CO, olefins, Cl-) dissociate easily (as opposed to e.g. CH3, Cp).
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Ligand Substitution and Activation

2. Dissociative substitution in ML6
16-e ML5 complexes are usually fluxional; the reaction proceeds with partial inversion, partial retention of stereochemistry.
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Ligand Substitution and Activation

3. Ligand substitution reactions: associative
Example:
Sometimes the solvent is involved:
(cis-platin !)
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Ligand Substitution and Activation

4. Ligand Substitution and Activation
Ligand rearrangement
Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions:
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Ligand Substitution and Activation

5. Ligand substitution reactions: redox-induced
For 17-e ⇋ 19-e usually fast equilibrium:
Reduction promotes dissociative substitution.
Oxidation promotes associative substitution.
In favourable cases, the product oxidizes/reduces the starting material Þ redox catalysis.
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Ligand Substitution and Activation

6. Ligand Substitution and Activation
Initiation by added reductant.
Sometimes, radical abstraction produces a 17-e species (see C103).
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Ligand Substitution and Activation

7. Ligand substitution reactions: photochemical
Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (LF transition). This results in fast ligand dissociation.
Requirement: the complex must absorb, so it must have a colour.
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Ligand Substitution and Activation

8. Ligand Substitution and Activation
Some ligands have a low-lying p* orbital and undergo MLCT excitation. This leads to easy associative substitution.
The excited state is formally (n-1)-e !
M-M bonds dissociate easily on irradiation
Þ (n-1)-e associative substitution
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Ligand Substitution and Activation

9. Ligand activation: electrophilic and nucleophilic attack
Electron-rich metal fragment: ligands activated for electrophilic attack.
N4: strong s-donor
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Ligand Substitution and Activation

10. Ligand activation: electrophilic and nucleophilic attack
Electron-poor metal fragment: ligands activated for nucleophilic attack.
CO: strong p-acceptor
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Ligand Substitution and Activation

11. Electrophilic attack on ligand
Hapticity may increase or decrease.
Formal oxidation state of metal may increase.
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Ligand Substitution and Activation

12. Ligand Substitution and Activation
Electrophilic addition
Electrophilic abstraction
Alkyl exchange also starts with electrophilic attack.
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Ligand Substitution and Activation

13. Ligand Substitution and Activation
Competition: attack of electrophile on metal
(may be followed by shift to ligand)
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Ligand Substitution and Activation

14. Electrophilic attack on metal
Can be the start of oxidative addition
Key reaction in the Monsanto acetic acid process:
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Ligand Substitution and Activation

15. Ligand Substitution and Activation
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Ligand Substitution and Activation

16. Nucleophilic attack on ligand
Nucleophile "substitutes" metal
Þ hapticity usually decreases
Oxidation state mostly unchanged
Competition: nucleophilic attack on metal
usually leads to substitution
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Ligand Substitution and Activation

17. Nucleophilic abstraction
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Ligand Substitution and Activation

18. Nucleophilic addition
Key reaction of the Wacker process:
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Ligand Substitution and Activation

19. Ligand Substitution and Activation
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Ligand Substitution and Activation

20. Ligand Substitution and Activation
How can you distinguish between internal and external attack of OH- ?
Use trans-CHD=CHD and trap the intermediate Pd-C-C-OH with CO:
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Ligand Substitution and Activation

21. Ligand Substitution and Activation
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Ligand Substitution and Activation

22. Ligand Substitution and Activation
Could acetaldehyde be formed directly as vinyl alcohol ?
Perform reaction in D2O:
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Ligand Substitution and Activation