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). 11/25/2018 Ligand Substitution and Activation

Dissociative substitution in ML6 16-e ML5 complexes are usually fluxional; the reaction proceeds with partial inversion, partial retention of stereochemistry. 11/25/2018 Ligand Substitution and Activation

Ligand substitution reactions: associative Example: Sometimes the solvent is involved: (cis-platin !) 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation Ligand rearrangement Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions: 11/25/2018 Ligand Substitution and Activation

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. 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation Initiation by added reductant. Sometimes, radical abstraction produces a 17-e species (see C103). 11/25/2018 Ligand Substitution and Activation

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. 11/25/2018 Ligand Substitution and Activation

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 11/25/2018 Ligand Substitution and Activation

Ligand activation: electrophilic and nucleophilic attack Electron-rich metal fragment: ligands activated for electrophilic attack. N4: strong s-donor 11/25/2018 Ligand Substitution and Activation

Ligand activation: electrophilic and nucleophilic attack Electron-poor metal fragment: ligands activated for nucleophilic attack. CO: strong p-acceptor 11/25/2018 Ligand Substitution and Activation

Electrophilic attack on ligand Hapticity may increase or decrease. Formal oxidation state of metal may increase. 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation Electrophilic addition Electrophilic abstraction Alkyl exchange also starts with electrophilic attack. 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation Competition: attack of electrophile on metal (may be followed by shift to ligand) 11/25/2018 Ligand Substitution and Activation

Electrophilic attack on metal Can be the start of oxidative addition Key reaction in the Monsanto acetic acid process: 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation 11/25/2018 Ligand Substitution and Activation

Nucleophilic attack on ligand Nucleophile "substitutes" metal Þ hapticity usually decreases Oxidation state mostly unchanged Competition: nucleophilic attack on metal usually leads to substitution 11/25/2018 Ligand Substitution and Activation

Nucleophilic abstraction 11/25/2018 Ligand Substitution and Activation

Nucleophilic addition Key reaction of the Wacker process: 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation 11/25/2018 Ligand Substitution and Activation

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: 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation 11/25/2018 Ligand Substitution and Activation

Ligand Substitution and Activation Could acetaldehyde be formed directly as vinyl alcohol ? Perform reaction in D2O: 11/25/2018 Ligand Substitution and Activation