1. Elements of organometallic chemistry

2. Complexes containing M-C bonds Complexes with  -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 L2L2 L2XL2X L3L3

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

6. Why 18 electrons? antibonding

7. 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

8. Organometallic Chemistry Fundamental Reactions

9. Reaction  (FOS)  (CN)  (NVE) Association-Dissociation of Lewis acids 0±10 Association-Dissociation of Lewis bases 0±1 Oxidative addition-Reductive elimination ±2 Insertion-deinsertion 000 Fundamental reaction of organo-transition metal complexes

10.  (FOS) = 0;  (CN) = ± 1;  (NVE) = 0 Lewis acids are electron acceptors, e.g. BF 3, AlX 3, ZnX 2 This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts Association-Dissociation of Lewis acids

11.  (FOS) = 0;  (CN) = ± 1;  (NVE) = ±2 Association-Dissociation of Lewis bases A Lewis base is a neutral, 2e ligand “L” (CO, PR 3, H 2 O, NH 3, C 2 H 4,…) 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

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

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

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

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

16. 6CO ligands x 2  e each 12  bonding e “ligand character” “18 electrons” non bonding anti bonding “metal character” Mo(CO) 6

17. 6  ligands x 2e each 12  bonding e “ligand character” non bonding anti bonding “metal character” Mo(CO) 6  -only bonding The bonding orbitals will not be further modified

18. t 2g egeg Mo(CO) 6  -only Mo(CO) 6  + π Energy gain (empty π-orbitals) oo ’o’o  ’o >  o π-bonding may be introduced as a perturbation of the t 2g /e g set: Case 1: CO empty π-orbitals on the ligands M  L π-bonding (π-back bonding) t 2g (π) t 2g (π*) egeg

19. Metal carbonyls may be mononuclear or polynuclear

20. Synthesis of metal carbonyls

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

22. Effect of charge Effect of other ligands

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

24. 13 C NMR spectroscopy 13 C is a S = 1/2 nucleus of natural abundance 1.108% For metal carbonyl complexes  170-290 ppm (diagnostic signals) Very long T 1 (use relaxation agents like Cr(acac) 3 and/or enriched samples)

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

26. Metal complexes of phosphines PR 3 as a ligand Generally strong  donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available

27. Tolman’s electronic and steric parameters of phosphines

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

29. Metal hydride and metal-dihydrogen complexes Terminal hydride (X ligand) Bridging hydride (  -H ligand, 2e-3c) Coordinated dihydrogen (  2 -H 2 ligand) Hydride ligand is a strong  donor and the smallest ligand available

30. Synthesis of metal hydride complexes

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

32. 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

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

34. Metal dihydrogen complexes If back-donation is strong, then the H-H bond is broken (oxidative addition) Very polarized  +,  - Characterized by NMR (T 1 measurements)

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

36. Metal-olefin complexes 2 extreme structures metallacyclopropane π-bonded only sp 3 sp 2 Zeise’s salt

37. Effects of coordination on the C=C bond CompoundC-C (Å)M-C (Å) C2H4C2H4 1.337(2) C 2 (CN) 4 1.34(2) C2F4C2F4 1.31(2) K[PtCl 3 (C 2 H 4 )]1.354(2)2.139(10) Pt(PPh 3 ) 2 (C 2 H 4 )1.43(1)2.11(1) Pt(PPh 3 ) 2 (C 2 (CN) 4 )1.49(5)2.11(3) Pt(PPh 3 ) 2 (C 2 Cl 4 )1.62(3)2.04(3) Fe(CO) 4 (C 2 H 4 )1.46(6) CpRh(PMe 3 )(C 2 H 4 )1.408(16)2.093(10) C=C bond is weakened (activated) by coordination

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

39. [PtCl 4 ] 2 - + C 2 H 4  [PtCl 4 (C 2 H 4 )] - + Cl - Synthesis of metal-olefin complexes RhCl 3.3H 2 O + C 2 H 4 + EtOH  [(C 2 H 4 ) 2 Rh(  -Cl) 2 ] 2

40. Reactions of metal-olefin complexes

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

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

43. Metal alkyl, carbene and carbyne complexes

44. Main group metal-alkyls known since old times (Et 2 Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward W(CH 3 ) 6 Ti(CH 3 ) 6 PtH(C  CH)L 2 Cp(CO) 2 Fe(CH 2 CH 3 ) 6 [Cr(H 2 O) 5 (CH 2 CH 3 ) 6 ] 2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable)

45. Reactions of transition-metal alkyls