Ligand substitution reactions: dissociative

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). 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. Ligand Substitution and Activation

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

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

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

8. 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 Ligand Substitution and Activation

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

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

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

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

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

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

15. 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 Ligand Substitution and Activation

17. Nucleophilic abstraction Ligand Substitution and Activation

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

19. Ligand Substitution and Activation

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

21. Ligand Substitution and Activation

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