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Elements of organometallic chemistry

Elements of organometallic chemistry. Elements of organometallic chemistry. Complexes containing M-C bonds Complexes with p -acceptor ligands Chemistry of lower oxidation states very important Soft-soft interactions very common Diamagnetic complexes dominant Catalytic applications.

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Elements of organometallic chemistry

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  1. Elements of organometallic chemistry

  2. Elements of organometallic chemistry Complexes containing M-C bonds Complexes with p-acceptor ligands Chemistry of lower oxidation states very important Soft-soft interactions very common Diamagnetic complexes dominant Catalytic applications 18-electron rule (diamagnetic complexes) Most stable complexes contain 18 or 16 electrons in their valence shells Most comon reactions take place through 16 or 18 electron intermediates

  3. A simple classification of the most important ligands X LX L L2 L2X L3

  4. Method A Method B Ignore formal oxidation state of metal Count number of d electrons for M(0) Add d electrons + ligand electrons (B) Determine formal oxidation state of metal Deduce number of d electrons Add d electrons + ligand electrons (A) Counting electrons The end result will be the same

  5. antibonding Why 18 electrons?

  6. Organometallic complexes 18-e most stable 16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II)) <16-e OK but usually very reactive > 18-e possible but rare

  7. Organometallic Chemistry Fundamental Reactions

  8. Fundamental reaction of organo-transition metal complexes

  9. Association-Dissociation of Lewis acids D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0 Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2 This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts

  10. Association-Dissociation of Lewis bases D(FOS) = 0; D(CN) = ± 1; D(NVE) = ±2 A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…) in this case the metal is the Lewis acid For 18-e complexes, dissociative mechanisms only For <18-e complexes dissociative and associative mechanisms are possible

  11. Vaska’s compound Oxidative addition-reductive elimination D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2 Very important in activation of hydrogen

  12. Insertion-deinsertion D(FOS) = 0; D(CN) = 0; D(NVE) = 0 Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons

  13. Metal Carbonyl Complexes CO as a ligand strong s donor, strong π-acceptor strong trans effect small steric effect CO is an inert molecule that becomes activated by complexation to metals

  14. Frontier orbitals “C-like MO’s” Larger homo lobe on C

  15. anti bonding “metal character” non bonding 12 s bonding e “ligand character” Mo(CO)6 “18 electrons” 6CO ligands x 2s e each

  16. anti bonding “metal character” non bonding 12 s bonding e “ligand character” Mo(CO)6s-only bonding The bonding orbitals will not be further modified 6 s ligands x 2e each

  17. Do D’o > Do D’o Energy gain π-bonding may be introduced as a perturbation of the t2g/eg set: Case 1: CO empty π-orbitals on the ligands ML π-bonding (π-back bonding) t2g (π*) t2g eg eg t2g t2g (π) Mo(CO)6 s-only Mo(CO)6 s + π (empty π-orbitals)

  18. Metal carbonyls may be mononuclear or polynuclear

  19. Synthesis of metal carbonyls

  20. Characterization of metal carbonyls IR spectroscopy (C-O bond stretching modes)

  21. Effect of charge Effect of other ligands

  22. The number of active bands as determined by group theory

  23. 13C NMR spectroscopy 13C is a S = 1/2 nucleus of natural abundance 1.108% For metal carbonyl complexes d 170-290 ppm (diagnostic signals) Very long T1 (use relaxation agents like Cr(acac)3 and/or enriched samples)

  24. Typical reactions of metal carbonyls Ligand substitution: Always dissociative for 18-e complexes, may be associative for <18-e complexes Migratory insertion:

  25. Metal complexes of phosphines PR3 as a ligand Generally strong s donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available

  26. Tolman’s electronic and steric parameters of phosphines

  27. Typical reactions of metal-phosphine complexes Ligand substitution: Very important in catalysis Mechanism depends on electron count

  28. Metal hydride and metal-dihydrogen complexes Terminal hydride (X ligand) Bridging hydride (m-H ligand, 2e-3c) Coordinated dihydrogen (h2-H2 ligand) Hydride ligand is a strong s donor and the smallest ligand available

  29. Synthesis of metal hydride complexes

  30. Characterization of metal hydride complexes 1H NMR spectroscopy High field chemical shifts (d 0 to -25 ppm usual, up to -70 ppm possible) Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz Coupling between inequivalent hydrides J(H-H) = 1-10 Hz Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans IR spectroscopy n(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging n(M-H)/n(M-D) = √2 Weak bands, not very reliable

  31. Some typical reactions of metal hydride complexes Transfer of H- Transfer of H+ A strong acid !! Insertion A key step in catalytic hydrogenation and related reactions

  32. Bridging metal hydrides Anti-bonding Non-bonding 4-e ligand 2-e ligand bonding

  33. Metal dihydrogen complexes Characterized by NMR (T1 measurements) Very polarized d+, d- If back-donation is strong, then the H-H bond is broken (oxidative addition)

  34. NMR characterization of organometallic complexes If X = CO 1H NMR 1 n(CO) band 2 n(CO) bands 1 n(CO) band

  35. Metal-olefin complexes 2 extreme structures sp3 sp2 π-bonded only metallacyclopropane Zeise’s salt

  36. Effects of coordination on the C=C bond C=C bond is weakened (activated) by coordination

  37. Characterization of metal-olefin complexes IR n(C=C) ~ 1500 cm-1 (w) NMR 1H and 13C, d < free ligand X-rays C=C and M-C bond lengths indicate strength of bond

  38. Synthesis of metal-olefin complexes [PtCl4]2- + C2H4 [PtCl4(C2H4)]- + Cl- RhCl3.3H2O + C2H4 + EtOH  [(C2H4)2Rh(m-Cl)2]2

  39. Reactions of metal-olefin complexes

  40. Metal cyclopentadienyl complexes Metallocenes (“sandwich compounds”) Bent metallocenes “2- or 3-legged piano stools”

  41. Cp is a very useful stabilizing ligand Introducing substituents allows modulation of electronic and steric effects

  42. Metal alkyl, carbene and carbyne complexes

  43. Main group metal-alkyls known since old times (Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward Ti(CH3)6 W(CH3)6 PtH(CCH)L2 Cp(CO)2Fe(CH2CH3)6 [Cr(H2O)5(CH2CH3)6]2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable)

  44. Reactions of transition-metal alkyls

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