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

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

3. Early Transition Metals Groups 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 Metals Groups 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 Metals Groups 8-10 (and 11)
Many accessible oxidation states
Mostly 18e or 16e 16e 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
phosphine CO
alkene 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
1.37
1.13
1.410
1.417
2.223
2.141
1.913
1.841
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-), substitution or oxidative addition (allyl)
Transition-metal Organometallics

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

28. Modification of p ligands by 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
1.95
1.15
2.97
1.99
2.46
1.40
2.35
2.32
1.41
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 not p-basic enough to enable completion of the addition.
Transition-metal Organometallics