1. Dissociative Associative
Principal mechanisms of ligand exchange in octahedral complexes
Dissociative
Associative

2. (5-coordinated intermediate)
MOST COMMON
Dissociative pathway
(5-coordinated intermediate)
Associative pathway
(7-coordinated intermediate)

3. Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L

4. Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L

5. Inert and labile complexes
Some common thermodynamic and kinetic profiles
Exothermic
(favored, large K)
Large Ea, slow reaction
Exothermic
(favored, large K)
Large Ea, slow reaction
Stable intermediate
Endothermic
(disfavored, small K)
Small Ea, fast reaction

6. Labile or inert?
LFAE = LFSE(sq pyr) - LFSE(oct)

7. Why are some configurations inert and some are labile?

8. Substitution reactions in square-planar complexes
the trans effect
(the ability of T to labilize X)

9. Synthetic applications
of the trans effect
Cl- > NH3, py

10. Mechanisms of ligand exchange reactions in square planar complexes

11. Electron transfer (redox) reactions
-1e (oxidation)
M1(x+)Ln + M2(y+)L’n
M1(x +1)+Ln + M2(y-1)+L’n
+1e (reduction)
Very fast reactions (much faster than ligand exchange)
May involve ligand exchange or not
Very important in biological processes (metalloenzymes)

12. Reactions ca. 100 times faster than ligand exchange
Outer sphere mechanism
[Fe(CN)6] [IrCl6]2-
[Fe(CN)6] [IrCl6]3-
[Co(NH3)5Cl]+ + [Ru(NH3)6]3+
[Co(NH3)5Cl] [Ru(NH3)6]2+
Reactions ca times faster
than ligand exchange
(coordination spheres remain the same)
r = k [A][B]
Tunneling
mechanism

13. Inner sphere mechanism
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
[Co(NH3)5Cl)] [Cr(H2O)6]2+
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
[CoIII(NH3)5(m-Cl)CrII(H2O)6]4+
[CoII(NH3)5(m-Cl)CrIII(H2O)6]4+
[CoIII(NH3)5(m-Cl)CrII(H2O)6]4+
[CoII(NH3)5(m-Cl)CrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+
[CoII(NH3)5(H2O)]2+
[Co(H2O)6] NH4+

14. than outer sphere electron transfer (bridging ligand often exchanged)
Inner sphere mechanism
Reactions much faster
than outer sphere electron transfer
(bridging ligand often exchanged)
r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3)
Tunneling
through bridge
mechanism

15. Organometallic Chemistry
Brooklyn College
Chem 76/76.1/710G Advanced Inorganic Chemistry (Spring 2008)
Unit 6
Organometallic Chemistry
Part 1
General Principles
Suggested reading:
Miessler/Tarr Chapters 13 and 14

16. 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. The d-block transition metals

19. Main types of common ligands

20. A simple classification of the most important ligandsX L L2
L2X L3

21. 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

23. Why is this relevant?
Stable mononuclear diamagnetic complexes
generally contain 18 or 16 electrons
The reactions of such complexes
generally proceed through 18- or 16-electron intermediates
Although many exceptions can be found, these are very useful practical rules
for predicting structural and reactivity properties
C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337.

24. Why 18 electrons?
antibonding

25. 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
generally unstable

26. A closer look at some important ligands

27. Typical -donor ligands

28. Other important C-donor ligands

29. Other important ligands

30. Other important ligands

31. The M-L-X game

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

34. 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)

35. 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 !)

36. Organometallic Chemistry 1.2 Fundamental Reactions

37. Fundamental reaction of organo-transition metal complexes
D(FOS)
D(CN)
D(NVE)
Association-Dissociation of Lewis acids ±1
Association-Dissociation of Lewis bases ±2
Oxidative addition-Reductive elimination
Insertion-deinsertion
FOS: Formal Oxidation State;
CN: Coordination Number
NVE: Number of valence electrons

38. 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

39. 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

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

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

42. Oxidative addition-reductive elimination
Not always reversible

43. 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

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

45. 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

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

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