LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION

1. LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION

3. LIGAND SUBSTITUTION

4. LIGAND SUBSTITUTION Basic premise about metal-catalyzed reactions: • Reactions happen in the coordination sphere of the metal • Reactants (substrates) come in, react, and leave again • Binding or dissociation of a ligand is oftenthe slow, rate-determining step Ligand Substitution

5. LIGAND SUBSTITUTION This premise is not always correct, but it appliesin the vast majority of cases. Notable exceptions: • Electron-transfer reactions • Activation of a single substrate for external attack • peroxy-acids for olefin epoxidation • CO and olefins for nucleophilic attack

6. 2 MAIN MECHANISTIC PATHWAYS ASSOCIATIVE (A): LnML’ + L’’ slow LnML’L’’ fast LnML’’ + L’ DISSOCIATIVE (D): L’’ LnML’ slow LnM fast LnML’’ + L’

7. DISSOCIATIVE LIGAND SUBSTITUTION Example: Factors influencing ease of dissociation: • 1st row < 2nd row > 3rd row • d8-ML5 > d10-ML4 > d6-ML6 • stable ligands (CO, olefins, Cl-) dissociate easily(as opposed to e.g. CH3, Cp). Ligand Substitution

8. DISSOCIATIVE SUBSTITUTION at ML6 16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry. The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques. Ligand Substitution

9. ASSOCIATIVE LIGAND SUBSTITUTION Example: Sometimes the solvent is involved. Reactivity of cis-platin: Ligand Substitution

10. trans influence and trans effect • In square planar complexes, some ligands direct substitution to a position trans to themselves. • When reaction is controlled by factors influencing the ground state energy of the complex – trans influence • Reaction is controlled by factors affecting the transition state energy.

11. trans influence Ligands that form strong  bonds or  tend to weaken the metal ligand bond trans to the metal. In the ground state this property is called the trans influence. H- > PR3 > SCN- > I-, CH3-, CO, CN- > Br- > Cl- > NH3 > OH-

12. trans kinetic effect • Tendency of certain ligands to direct incoming groups to trans position with reactions under kinetic control. C2H4, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3> OH-

13. overall trans effect CO, CN-, NO C2H4 > PR3, H- > CH3-, S=C(NH2)2 > Ph- NO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O

14. Exercise 7.1

15. ASSOCIATIVE LIGAND SUBSTITUTION Example: Sometimes the solvent is involved. Reactivity of cis-platin: Ligand Substitution

16. rate Rate = ks[ML4] = k1 [ML4][Y] Mechanism: Square pyramidal – trigonal bipyramid – with retention of configuration.

17. Associative substitution with 18 e systems • Can occur if the metal can delocalize a pair of electrons onto one of its ligands

18. LIGAND REARRANGEMENT Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactionsof an apparently saturated complex: Ligand Substitution

19. DISSOCIATIVE LIGAND SUBSTITUTION Example: Rate = k [ML6] Ligand Substitution

20. Rate of substitution of Ligands Rate of substitutions of a particular ligand is a function of ligand type. Ligands that are nuetral in their free state dissociate rather easily.

22. Redox-induced ligand substitution Unlike 18-e complexes, 17-e and 19-e complexes are labile. Oxidation and reduction can induce rapid ligand substitution. • Reduction promotes dissociative substitution. • Oxidation promotes associative substitution. • In favourable cases, the product oxidizes/reducesthe starting material Þredox catalysis. Ligand Substitution

23. Redox-induced ligand substitution Initiation by added reductant. Sometimes, radical abstractionproduces a 17-e species Ligand Substitution

24. Photochemical ligand substitution Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (Ligand Field transition, LF).This often results in fast ligand dissociation. Requirement: the complex must absorb, so it must have a colour! or use UV if the complex absorbs there Ligand Substitution