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Transition-metal Organometallics

Transition-metal Organometallics. Peter H.M. Budzelaar. Transition metals are never on time. Early Transition Metals Groups 3,4. Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18 e Polar and very reactive M-C bonds (to alkyl and aryl)

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Transition-metal Organometallics

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  1. Transition-metal Organometallics Peter H.M. Budzelaar

  2. Transition metals are never on time Transition-metal Organometallics

  3. Early Transition MetalsGroups 3,4 • Strongly electrophilic and oxophilic • Few redox reactions (exception: Ti) • Nearly always < 18e • Polar and very reactive M-C bonds(to alkyl and aryl) • Few d-electrons: • preference for "hard" s-donors (N/O/F) • weak complexation of p-acceptors (olefins, phosphines) • Typical catalysis: Polymerisation Transition-metal Organometallics

  4. "Middle" Transition MetalsGroups 5-7 • Many accessible oxidation states • Mostly 18e • Ligands strongly bound • Strong, not very reactive M-C bonds • Preference for s-donor/p-acceptor combinations (CO!) • Typical catalysis: Alkene and alkyne metathesis Transition-metal Organometallics

  5. Late Transition MetalsGroups 8-10 (and 11) • Many accessible oxidation states • Mostly 18e or 16e16e common for square-planar complexes • Easy ligand association/dissociation • Weak, not very reactive M-C bonds • Even weaker, reactive M-O/M-N bonds • Preference for s-donor/weak p-acceptor ligands (phosphines) • Typical catalysis: Hydroformylation Transition-metal Organometallics

  6. Front- and Back-benchers Transition-metal Organometallics

  7. Going down... 1st row: • often unpaired electrons • different spin states (HS/LS) accessible • "highest possible" oxidation states not very stable • MnO4- is a strong oxidant 2nd/3rd row: • nearly always "closed shell" • virtually same atomic radii (except Y/La) • highest oxidation states fairly stable • ReO4- is hardly oxidizing • 2nd row often more reactive than 3rd Transition-metal Organometallics

  8. M-H and M-C s-bonds Transition-metal Organometallics

  9. Synthesis of metal alkyls • Metathesis • Electrophilic attack on metal • Insertion Transition-metal Organometallics

  10. Synthesis of metal alkyls • Oxidative addition • often starts with electrophilic attack Transition-metal Organometallics

  11. Decomposition of metal alkyls Dominant: b-hydrogen elimination Alternatives: • homolysis • a/g/d-eliminations • reductive elimination (especially with H or another alkyl) • ligand metallation Transition-metal Organometallics

  12. How to prevent b-hydrogen elimination ? • No b-hydrogen CH3, CH2CMe3, CH2SiMe3, CH2Ph • No empty site cis to alkyl • Product of elimination unstable Transition-metal Organometallics

  13. How to prevent b-hydrogen elimination ? • Planar transition state inaccessible even for 5-membered metallacycles b-elimination is difficult !(basis of selective ethene trimerization) Transition-metal Organometallics

  14. Reactions of metal alkyls • Insertion, of both polar and non-polar C=X bonds: • olefins, acetylenes, allenes, dienes • (ketones etc) • CO, isocyanides • Reductive elimination Transition-metal Organometallics

  15. Synthesis of metal hydrides • Metathesis • b-elimination • Protonation / oxidative addition Transition-metal Organometallics

  16. Structure of WH6(PiPr2Ph)3 Transition-metal Organometallics

  17. Synthesis of metal hydrides • Hydrogenolysis Transition-metal Organometallics

  18. Reactivity of metal hydrides • "Hydride is the smallest alkyl" • Can react as H+ or H- • HCo(CO)4 nearly as acidic as H2SO4 • Cp2ScH gives H2 with acids, alcohols, ... • Insertion reactions • CO insertion rare (endothermic!) Transition-metal Organometallics

  19. Metal aryls Usually much more stable than alkyls Synthesis: • Metathesis • Oxidative addition Reaction/decomposition: • Reductive elimination Transition-metal Organometallics

  20. The other ligands... Common ligands for transition metals:p ligands, CO, phosphines General characteristic: s-donating, p-accepting, "soft" Transition-metal Organometallics

  21. p ligands, CO, phosphines s-donor character:phosphines > alkenes, CO p-acceptor character:CO > alkenes > phosphines Depends strongly on the substituents on P, C=C ! Transition-metal Organometallics

  22. Donor and acceptor orbitals s-donor p-acceptor alkene CO phosphine p p* p* LP s* LP Transition-metal Organometallics

  23. p ligands, CO, phosphines • CO is one of the best p-acceptors (p-acids) • isocyanides are stronger donors, weaker acceptors • PMe3 very weak p-acceptor, good s-donor • PF3 nearly as strongly p-acidic as CO • C2H4 weak p-acceptor • C2(CN)4 very strong p-acceptor • even forms stable radical anions Transition-metal Organometallics

  24. Synthesis of CO and p-ligand complexes Stable, neutral ligands:generate empty site(s) in presence of free ligand 1.13 1.37 1.417 1.410 2.141 2.223 1.841 1.913 1.157 1.141 Transition-metal Organometallics

  25. Synthesis of CO and p-ligand complexes Variation:reductive synthesis Transition-metal Organometallics

  26. Synthesis of CO and p-ligand complexes Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl) Transition-metal Organometallics

  27. Synthesis of CO and p-ligand complexes Transition-metal Organometallics

  28. Modification of p ligandsby H+/H- addition/abstraction Transition-metal Organometallics

  29. Reactivity of p-ligand complexes Ligand activated for nucleophilic attack both internal and external Transition-metal Organometallics

  30. Reactivity of p-ligand complexes Change of hapticity Transition-metal Organometallics

  31. More than 18 e ? 1.47 1.51 1.40 2.33 2.45 1.22 1.15 1.99 2.97 1.95 2.30-2.32 2.46 1.40 2.35 2.32 1.41 1.41-1.44 1.40 Transition-metal Organometallics

  32. s complexes A s bond as 2-electron donor for a metal. • H2 complexes (non-classical hydrides) • C-H bonds Usually intramolecular Sometimes intermolecular In "matrix" Characterized by IR Transition-metal Organometallics

  33. s complex ? 2.10 1.92 Transition-metal Organometallics

  34. s complex ? Transition-metal Organometallics

  35. s complexes • C-Si bonds 2.63 3.22 103° Transition-metal Organometallics

  36. s complexes • Si-H bonds • etc: B-H, Sn-Cl, P-H, .. ?? Transition-metal Organometallics

  37. s complexes A s complex is an (arrested) intermediate for oxidative addition: Stable s complexes are formed when the metal is notp-basic enough to enable completion of the addition. Transition-metal Organometallics

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