Chapter 3. Carbonyls, phosphine complexes and substitution reactions

1
Chapter 3
Carbonyls, Phosphine Complexes and Substitution
Reactions
2
Outline
1. Metal complexes of CO 2. Metal complexes of
CO-like ligands 3. Phosphines as ligands 4.
Substitution reactions a). Dissociative
substitution b). Associative substitution
References and suggested readings

• The Organometallic Chemistry of the Transition
  Metals, Robert H. Crabtree, 4th Edition, 2001,
  Chapter 4.
• 2. Organometallic Chemistry, G. O. Spessard, G.
  L. G. L. Miessler, Prentice-Hall New Jersey,
  1997, Chapter 4.
• 3. Organotransition Metal Chemistry, Akio
  Yamamoto, 1986. Chapters 4.5, 6.1.

3
1. Metal complexes of CO

• Why CO chemistry?
• Current industrial raw material for
  carbon-based chemicals
• Petroleum oil
• Another possible carbon source coal
• coal ------------gt organic compound ?
• C (coal) H2O ---------gt H2 CO
  (water gas)

4
Electronic structure of free CO (1) In terms
of valence bond theory
m 0, why?
dipole moment
5
(2) In terms of MO theory
Consider s-s, p-p mixing only
LUMO
HOMO
HOMO 3s orbital (a little antibonding character,
localized on C) LUMO 2p orbital (antibonding
character, localized on C)
6
B. Electronic structure of CO complexes
CO is not only a s donor, but also a p acceptor.
How many p bonds can a CO form with a transition
metal ion?
7

• Depending on the metal and other ligands, the
  nature of the CO ligand in LnM-CO can vary.
• - If L are good p acids or the complex is
  cationic, then s-donation will be more important
  (A)
• - If the L are s-donors or the complex is
  anionic, then the metal will be a strong p-donor.
  (C)

8
Q2. In metal complexes, we usually observed
Electronegativity consideration
In term of orbital interaction
9
Effect of complexion
s donation p
backdonation overall d(M-C) d(C-O)
u(CO) e- density on C e- density on O
Expected reactivity
10
C. Preparation of CO complexes
(1) From CO

• Metal CO

Colorless liquid, b. p. 34 oC
yellow liquid, b. p. 103 oC
11

• Metal Salts reducing agent CO gt 18e
  complexes
• Typical reducing agents Na, Al, H2, AlR3,
  CO ...

12

• Unsaturated compounds CO

13
(2) From a reactive organic carbonyl compound.
Example 1.
Mechanism?
14
Example 2.
Mechanism?
15
D. Structures of M-CO compounds.
The three principal coordination modes of CO
m2-complexes
m3-complexes
16
Other less common coordination modes
Semibriding CO
s/p- bridge CO
17
E. Characterization of M-CO compounds
13C NMR.
d(CO), ppm 150-220 230-280
IR u(CO), cm-1
2143 1850- 1700- 1600- 2150
1860 1700
18
(No Transcript)
19
Molecular symmetry and IR bands.
C
D
How many IR bands due to u(CO)?
20
For a given, molecules, the numbers of expected
IR bands can be determined by group theory. e.g.
21
F. Reactions of metal carbonyls
(1) Substitution
useful starting material
22
(No Transcript)
23
(2) Nucleophilic attack on carbon
24
(No Transcript)
25
(3) Electrophilic attack at oxygen
e.g.
(4) Migratory insertion reactions
26
(4) Oxidative decarbonylation
27
2. Metal complexes of CO-like ligands
A. Isocyanide complexes (metal isonitrile)
(1) In terms of composition and structures,
metal isonitriles are very similar to metal
carbonyls. e.g. M(CO)6 M(CO)5
M2(CO)10 M(CO)4 M(CNR)6 M(CNR)5
M2(CNR)10 M(CNR)4 M Cr, Mo W Fe, Ru, Os
Mn, Tc, Re Ni, Pd, Pt
28
(2) In terms of chemical properties, they are
also very similar.
29
(3) Difference between CO and CN-R.
Which one is a stronger s donor, CO or
CNR? CN-R Which one is stronger p acceptor, CO
or CNR? CO
CN-R can form complexes with metals in higher
oxidation state. Pt(CO)42 does not
exist. Pt(CNR)42 is stable.
M-CNR complexes are more reactive toward both
nucleophiles and electrophiles.
In general
30
Exercise.
Provide the products for the following reactions.
31
B. CS complexes C?S unstable (more
reactive) M-C?S stable Examples
Chemical properties of M-CS complexes Very
similar to M-CNR complexes (More reactive than CO
complexes).
32
(No Transcript)
33
C. NO (nitrosyl) complexes
Lewis structure NO NO (isoelectronic to CO)
MO description
34
Bonding in M-NO. How many e- can NO donate?
35
Q1. What is the electron count for the following
complexes?
36
Q2. How could we differentiate linear or bent NO ?
, u(NO), cm-1 1610-1830 1520-1720
Q3. Can we predict if a NO is linear to bent?
But, it is not always true. e.g.
37
Preparation of NO complexes
(1) From NO gas Cr(CO)6 excess NO -----gt
Cr(lin-NO)4 6 CO Fe(CO)5 excess NO -----gt
Fe(CO)2(NO)2 3 CO Note 2 NO for 3 CO. Why?
(2) From NOBF4, e.g.
Note 1 NO for 1 CO.
38
Some chemical properties of M-NO complexes
a. Line M-NO and bent M-NO interconversion
important in substitution reactions.
b. They are less reactive towards nucleophiles
and electrophile than CO.
c. M-NO complexes can also undergo insertion
reactions.
VE ?
39
(No Transcript)
40
(No Transcript)
41
3. Phosphines as ligands
Phosphines are probably the most important
organometallic ligands because their steric and
electronic properties can be easily modified.
A. Nature of M-PR3 bonds
PR3 can function as s-donor and a p-acceptor.
How do phosphines function as p-acceptor?
Early belief.
However No experimental evidence supports
it. Theoretical calculation shows that dp-pp
interaction is not important here.
42
Current belief The p-accepting properties of
PR3 is due to d(M) -gt s(P-R).
Phosphines, like amines, are strong s-donors.
Unlike amines, they are also p-acceptors.
43
Evidences for dp (M)-gts (P-R) bonding.
Q1. If there is dp(M)-gts(P-R) bonding,
As a result, the backdonation from metal to PR3
plays a Important role.
44
Q2. Trans-TiMe2(dmpe)2 is diamagnetic why?
45
B. Electronic effect of substitutents in PR3

• Q1. Consider the ligands PCl3, PPh3, PMePh2,
  PMe3.
• What is the order of s-donating ability ?
• What is the order of p-accepting ability?
• s-donating ability
• p accepting ability
• Electron-donating groups on P will
• increase s-donating ability
• decrease p accepting ability
• overall, increase the basicity of PR3

p acceptor ability
PR3 P(NR2)3 lt PAr3 lt P(OR)3 lt P(OAr)3 lt PCl3 lt
CO PF3
46
Q2. Why is PF3 a stronger p-acceptor than PMe3?
Electron-withdrawing group X make the s(P-R)
have lower energy, thus increase the p accepting
ability.
47
Q3. Which of the following complexes would you
expect to have the lowest C-O stretching
frequency? (a) Ni(CO)4 (b) Ni(CO)3(PPh3)
(c) Ni(CO)3(PMe3) (d) Ni(CO)3(PPhMe2)
48
The electronic parameter? The electron donating
or withdrawing ability of phosphines can be
correlated with the CO stretching frequency of
monophosphine metal carbonyls. While any metal
carbonyl can be used, LNi(CO)3 is used most
commonly.
49
C. Steric influence of ligands.
PR3 Ligands have varying size, depending on R.
Tolman introduced the concept of cone angle to
describe the size of PR3.
Can cone angle be larger than 180o ?
50
(No Transcript)
51
Importance of cone angles. Cone angle
determines the maximum number of ligands that can
be attached to a metal. e.g. PCy3, P(i -Pr)3
----gt at most M(PR3)2 (q 170o, 160o) PPh3
--------gt M(PPh3)3 or M(PPh3)4 (rare) (q
145o) PMe3 --------gt M(PMe3)6 is possible. (q
118o) Useful when steric effect is considered,
e.g. in substitution reaction rates
Phosphines tend to be sterically demanding, and
thus steric effects dominate coordination - Bulky
phosphines will tend to bind trans to one
another - The presence of several bulky
phosphines will result in deviations from ideal
geometries.
52
Summary. Electronic ans steric properties of
typical phosphines.
53
Chelating phosphinesChelating phosphines are
widely used to control structure and reactivity.
Chelating diphosphines preferentially coordinate
in cis fashion, while 2 monophosphines would
adopt a trans geometry.Natural Ligand Bite
Angle (ßn) The ligand preferred P-M-P angle for
a chelating diphosphine. This can be determined
from x-ray structures or by molecular modeling.
(Chem. Rev., 2000, 100, 2741-2769)
54
4. Substitution reactions
Introduction to substitution reactions LnM-X Y
--------gt LnM-Y X Types of substitution
reactions LnM-X Y- --------gt LnM-Y
X- LnM-X L' --------gt LnM-L' X- LnM-L'
X--------gt LnM-X L' LnM-L' L"-------gt
LnM-L" L' 2e ---gt 2e
55
Examples
56
(No Transcript)
57
Major mechanisms for substitution
reactions? LnM-L' L"-------gt LnM-L" L'
Dissociative substitution LnM-L' ----------gt LnM
L' rate determining step LnM L''
--------gt LnM-L" Associative
substitution LnM-L' L'' -------gt LnML'L"
rate determining step LnML'L" -------gt
LnM-L" L' Interchange mechanism
Notes Associative substitution undergoes an
intermediate while interchange mechanism
undergoes a transition state.
58
A. Dissociative substitution
Q1. What is the most important feature for D
mechanism? LnM-L' ----------gt LnM L'
slow, rate determining step LnM L''
--------gt LnM-L" fast
Q2. What types of complexes are likely to undergo
substitution reactions via D mechanism?
59
Q3. Stereochemistry retained or lost?
60
Q4. What affect reaction rates?
Q4A. Effect of metal. Consider the reactions
below. Which one is fast? (a) Ni(CO)4 PPh3
-----gt Ni(CO)3(PPh3) CO (b) Cr(CO)6 PPh3
-----gt Cr(CO)5(PPh3) CO (a). Ni(0), d10, no
CFSE. Ni-CO bond is weaker. The
substitution reactions of d3, d6 (low spin)
octahedral complexes are usually very slow. Q4B.
Consider the reactions below. Which one is
fast? (a) Fe(CO)5 PPh3 -----gt Fe(CO)4(PPh3)
CO (b) Fe(CO)5- PPh3 -----gt Fe(CO)4(PPh3)-
CO
61
Q4C. Effect of leaving ligand. What is the likely
product?
Also chelating ligand dissociate less
readily than monodentate ligands.
62
Q4D. Effect of trans ligand. What is the likely
product?

• If bulky PR3 is used instead of PMe3, A may be
  favored due to steric effect.
• Q4E. Effect of spectator ligand (steric effect).
  Consider the reactions below. Which one is fast?
• (a) Ni(PPh3)4 CO -----gt Ni(CO)(PPh3)3 PPh3
• (b) Ni(PMe3)4 CO -----gt Ni(CO)(PMe3)3 PMe3
• (c) Ni(CO)(PMe3)3 PMe3 ---gtNi(CO)2(PMe3)2
  PMe3

63
B. Associative substitution
Q1. What is the most important feature for A
mechanism?
LnM-L' L'' -------gt LnML'L" rate
determining step LnML'L" -------gt LnM-L" L'
Rate depends on both LnM-L' and L" rate
KLnM-L'L" DS lt O
Q2. What types of complexes are likely to
undergo substitution reactions via A
mechanism?
16e species normally undergo D mechanism.
64

• lt 18 e complexes. e.g.
• V(CO)6 PPh3 -----------gt V(CO)5(PPh3) CO
• 17e
• V(CO)6- PPh3 -----------gt V(CO)5(PPh3)-
  CO 18e slow

• 18e complexes that can rearrange to 16e species.
  e.g.

65

• Other complexes capable of such rearrangement

Q3. What affect reaction rates?
66
Exercise 1. Consider the following associative
substitution reactions. Which one reacts faster,
PMe3 or PPh3? V(CO)6 PR3 -----------gt
V(CO)5(PR3) CO PMe3, It is a better s donor,
V-P bond is stronger. Which one reacts faster,
PCy3 or PPh3? Exercise 2. 19e or 17e usually
undergo substitution reactions fast than 18e
complexes. Suggest a reason.