CHEM 522 Chapter 04

1. CHEM 522Chapter 04 Carbonyl, Phosphine complexes and Ligand Substitution Reaction

2. Bonding • σ Donation • π Back bonding

6. From IR it is possible to tell how good is the metal as a π base

7. Preparation of CO Complexes • Direct reaction of metal with CO • CO replace weakly bonded ligands

8. Preparation of CO Complexes • From CO and a reducing agent (like Na, S2O42- and CO)

9. Preparation of CO Complexes • From a reactive carbonyl compound followed by desertion

10. Metal Carbonyls Reactions • Nucleophilic attack at carbon • Reaction wit Me- give carbenes • Reaction with Me3NO give a free bonding site for metal

11. Metal Carbonyls Reactions • Nucleophilic attack at carbon [Cp(NO)(PPh3)ReCO]+ Cp(NO)(PPh3)Re(CHO)

12. Metal Carbonyls Reactions • Electrophilic attack at oxygen Cl(PR3)4ReCO Cl(PR3)4ReCOAlMe3

13. Metal Carbonyls Reactions • Migratory insertion MeMn(CO)5 (PMe3)(CO)4Mn

14. Bridging CO Groups

15. Unequivalent Bridging CO

16. triply Bridging CO

17. Isonitriles • M=C=N-R • Stabilize higher oxidation state [Pt(CNPh)4]2+ no [Pt(CO)4]2+ is known • The lone pair in CO is almost nonbonding while in CNR it is more of antibonding, so when σ donation take place the CN bond become stronger, π back donation weaken the bond, so the shift in the IR will depend on the strength of σ or π donation. (unlike CO)

18. Isonitriles • M=C=N-R • If back bonding is not strong, M-CΞNR should be linear • M=C=N-R bent molecule is also known which means strong back bonding • NbCl(CO)(CNR)(dmpe). The ligand is bent at N (129o-144o)

19. Thiocarbonyls • CS ligand • CS is not stable by itself above -160oC • It is known in some compounds as a ligand bonding through C • Also bridging CS is also known • Usually prepared from CS2 RhCl(PPh3)3 Trans-RhCl(CS)(PPh3)2 + SPPh3

20. Thiocarbonyls • Frequency range • Free CS is 1273 • μ3 CS 1040-1080 • μ2 CS 1100-1160 • M-CS 1160-1410

21. Nitrosyls • NO is a stable free radical • Also as NO+ in NOBF4 • NO+ is isoelectronic with CO • It can bind as NO+ and it will be three electron donor • When NO is bent then it will be one electron donor

22. NO is a fifteen electron molecule • with one unpaired electron residing in the π* molecular orbital: (σ1)2(σ1*)2(σ2)2(σ2*)2(σ3)2(πx, πy)4(πx*, πy*)1(σ*3) • This electronic configuration explains the high reactivity of the NO molecule, particularly the formation of nitrosonium cation (NO+) on oxidation and the reduction to nitroxide anion (NO–), making it a "non-innocent" ligand • Most of the known stable "nitrosyl" complexes are assumed to contain the diamagnetic π acceptor ligand nitrosonium, NO+,but there are cases when NO• or NO– (nitroxide) can be reasonably postulated as ligands in transition metal complexes. • Establishing the actual form of coordinated NO often requires a variety of physical methods such as IR, EPR, NMR, UV/VIS, X-rays, resonance Raman, magnetic circular dichroism (MCD), etc., and theoretical calculations.

23. NO Bonding • NO binds in two ways • Either as NO+ then it will give linear molecule and will be three electron donor • Or as NO- then it will give bent molecule and will be one electron donor

24. Reaction When NO+ is added it makes reaction with Nu- more probable

25. Electron Count • When NO change from linear to bent both the number of electron on the metal and the oxidation state of the metal will change • CoCl2L2(lin-NO) • CoCl2L2(bent-NO)

27. Electron Count

28. Preparation - NO+ is a powerful oxidation agent - Migratory insertion is also possible for NO

29. Phosphine Ligands • Phosphine ligands have the general formula PR3 • where R = alkyl, aryl, H, halide etc. • Closely related are phosphite ligands which have the general formula P(OR)3. • Both phosphines and phosphites are neutral two electron donors that bind to transition metals through their lone pairs. • There are many examples of polydentate phosphine ligands, some common examples of which are shown below.

32. Bonding

33. π Acidity

34. π Acidity Ti2+ is a d2 ion in octahedral field so it should be paramagnetic, however it is diamagnetic. The reason is because of the strong back bonding

35. Tolman Cone Angle

36. Tolman Cone Angle The stronger donor phosphine increase the electron density on metal which increase it on CO by back donation

37. Cone angles for some common phosphine ligands are:

38. Factors Effecting Bonding • There are two important factors effecting the bonding of the phosphines • Electronic • Steric • The advantage of using bulky ligands compounds of low coordination number can be formed [Pt(PCy3)2]

39. Cis and trans phosphines Chelates

40. Dissociative Substitution

41. Dissociative Substitution Usually the larger the cone angle the faster the dissociation This mechanism is usually preferred for 18-electron molecule Transition state has a positive ΔS‡ and in the range 10-15 eu (entropy unit)

42. stereochemistry • Oh can go to SP or distorted TBP (DTBP)

43. stereochemistry • Oh can go to SP or distorted TBP • ML6 d6 seems to prefer SP or DTBP • ML6 d8 seems to prefer TBP

44. stereochemistry • Phosphines usually do not replace all CO in the complex • The fac structure is usually prefer over the mer for electronic reason

45. Dissociative Substitution • Bulky ligands usually enhance dissociation • Protonation can be used to remove an alkyl or hydride group • Weakly bonded solvent is a good leaving group W(CO)5(thf) + PPh3 W(CO)5(PPh3)

46. Associative Mechanism LnM  LnM-L’  Ln-1M-L’ This mechanism is usually adapted for 16 e complexes

47. The Trans Effect • This is observed in square planar complexes where the incoming ligand will occupy certain position depending on the ligand trans to it

48. The Trans Effect • The solvent may have some effect

49. Ligand Rearrangement • This take place for 18-e complexes

50. Ligand Rearrangement • This also observed for indenyl complexes better than their Cp analogs because of the benzene ring