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Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count. PowerPoint Presentation
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Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count.

Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count.

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Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count.

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  1. Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count.

  2. Now looking at compounds having a charge of +1 to obey 18 e rule. Elec count: 4 (M) +2 (NO) +12 (L6) = 18 NO+ is isoelectronic to CO X increases O N by 1 Elec Count: 4 (M) + 4 (L2) + 10 (L5)

  3. Actors and spectators Actor ligands are those that dissociate or undergo a chemical transformation (where the chemistry takes place!) Spectator ligands remain unchanged during chemical transformations They provide solubility, stability, electronic and steric influence (where ligand design is !)

  4. Organometallic Chemistry 1.2 Fundamental Reactions

  5. Fundamental reaction of organo-transition metal complexes FOS: Formal Oxidation State; CN: Coordination Number NVE: Number of valence electrons

  6. 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

  7. 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 Crucial step in many ligand exchange reactions For 18-e complexes, only dissociation is possible For <18-e complexes both dissociation and association are possible but the more unsaturated a complex is, the less it will tend to dissociate a ligand

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

  9. Oxidative addition-reductive elimination Vaska’s compound H becomes H- Concerted reaction via Ir: Group 9 cis addition CH3+ has become CH3- SN2 displacement trans addition Also radical mechanisms possible

  10. Oxidative addition-reductive elimination Not always reversible

  11. Insertion-deinsertion D(FOS) = 0; D(CN) = 0; D(NVE) = 0 Mn: Group 7 Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons

  12. Migratory Insertion Also promoted by including bulky ligands in initial complex

  13. Insertion-deinsertion The special case of 1,2-addition/-H elimination A key step in catalytic isomerization & hydrogenation of alkenes or in decomposition of metal-alkyls Also an initiation step in polymerization

  14. Attack on coordinated ligands Very important in catalytic applications and organic synthesis

  15. Some examples of attack on coordinated ligands Electrophilic addition Nucleophilic addition Electrophilic abstraction Nucleophilic abstraction

  16. Brooklyn College Chem 76/76.1/710GAdvanced Inorganic Chemistry(Spring 2009) Unit 6 Organometallic Chemistry Part 2. Some physical and chemical properties of important classes of coordination and organometallic compounds Suggested reading: Miessler/Tarr Chapters 13 and 14

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

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

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

  20. Metal carbonyls may be mononuclear or polynuclear

  21. Synthesis of metal carbonyls

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

  23. Effect of charge u(free CO) 2143 cm-1 Lower frequency, weaker CO bond Effect of other ligands PF3 weakest donor (strongest acceptor) PMe3 strongest donor (weaker acceptor)

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

  25. 13C NMR spectroscopy 13C is a S = 1/2 nucleus of natural abundance 1.108% 1.6% as sensitive as 1H only For metal carbonyl complexes d 170-290 ppm (diagnostic signals) Very long T1 (use relaxation agents like Cr(acac)3 and/or enriched samples)

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

  27. 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

  28. Metal complexes of phosphines Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3 > PCl3 > PF3 Can be measured by IR using trans-M(CO)(PR3) complexes Steric properties: Rigid structures create chiral complexes apex angle of a cone that encompasses the van der Waals radii of the outermost atoms of the ligand

  29. Tolman’s electronic and steric parameters of phosphines

  30. Typical reactions of metal-phosphine complexes Ligand substitution: presence of bulky ligands (large cone angles) can lead to more rapid ligand dissociation Very important in catalysis Mechanism depends on electron count

  31. 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 H2 as ligand involves -donation and π-back donation

  32. Synthesis of metal hydride complexes

  33. 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

  34. 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

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

  36. Metal dihydrogen complexes Characterized by NMR (T1 measurements) Very polarized d+, d- back-donation to s* orbitals of H2 the result is a weakening and lengthening of the H-H bond in comparison with free H2 If back-donation is strong, then the H-H bond is broken (oxidative addition)

  37. Metal-olefin complexes 2 extreme structures sp3 sp2 Zeise’s salt π-bonded only metallacyclopropane Net effect weakens and lengthens the C-C bond in the C2H4 ligand (IR, X-ray)

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

  39. 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

  40. Synthesis of metal-olefin complexes [PtCl4]2- + C2H4 [PtCl3(C2H4)]- + Cl- RhCl3.3H2O + C2H4 + EtOH  [(C2H4)2Rh(m-Cl)2]2

  41. Reactions of metal-olefin complexes

  42. Metal alkyl, carbene and carbyne complexes

  43. Metal-alkyl complexes 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 Blocking kinetically favorable pathways allows isolation of stable alkyls

  45. Metal-carbene complexes L ligand Late metals Low oxidation states Electrophilic X2 ligand Early metals High oxidation states Nucleophilic

  46. Fischer-carbenes

  47. Schrock-carbenes Synthesis Typical reactions + olefin metathesis (we will speak more about that)

  48. Grubbs carbenes Excellent catalysts for olefin metathesis

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