Reactions of Organometallic Complexes

• Substitution

• Oxidative Addition

• Reductive Elimination

• Migratory Insertion

• β - Elimination

1
 M.C. White, Chem 153 Ligand Exchange
Mechanism -3-Mechanisms
Week of September 18th, 2
Ligand Exchange Mechanisms
Associative ligand substition: is often called square planar substition because16 e-, d8 square planar complexes generally undergo ligan

provides Associative
a lower energy
Ligand Exchange Mechanisms
substitution via an associative mechanism (the M-Nu bond is formed before the M-X bond breaks). The intermediate is 18e- and therefo
via dissociative
ligandroute to the product
substition: than square
is often called a 14e- planar
intermediate formed
substition because16 e-, d8 square substitution (the generally
planar complexes M-X bond is fully
undergo brok
ligand
Associative
before the M-Nu bondligand
substitution substitution
via begins to form).
an associative Analogous
mechanism (the in bond
many
M-Nu Substitution
ways oftheligand
to SN 2 before
is formed reactions. M-X bondin breaks).
complex with other
The intermediate ligand
is 18e- and therefore
provides a lower energy route to the product than a 14e- intermediate formed via dissociative substitution (the M-X bond is fully broken
before empty,
Nu the M-Nu non-bonding
bond pz orbital
begins to form). can actinasmany
Analogous an ways to SN 2 reactions.
acceptor orbital for the e- density of the
incoming nucleophile
empty, non-bonding p orbital can act as an
Nu z
acceptor orbital for the e- density of the
incoming nucleophile
Nu L

L Nu
L L L L L
L L Nu L
M M L M M M
L L L X L LL L Nu LL
L X
M
L
M L M XNu M M
L Nu
L X L X L Nu L Nu
L X X
L
18 e- intermediates X 16 e-
d8 16 e-
16 e- 18 e- intermediates 16 e-
M = Ni(II), Pd(II),
M = Ir(I).
Pt(II), Rh(I), Ni(II), Pd(II),
Pt(II), Rh(I), Ir(I).

Dissociative ligand substitution is

Dissociative ligand substitution is
Dissociative
most favored in ligand substitution
coordinatively saturated
- most favored in 10
coordinatively saturated L L LL L L
18e complexes- (e.g. d tetrahedral,
10 d6 d6
18e complexes (e.g.
10 tetrahedral, d6 octahedral
doctahedral). d tetrahedral, X-X-
In the
octahedral). In the dissociative
dissociative L L L L LL L
L LL L L
Nu:
Nu:
mechanism, the M-X
mechanism, the bond
M-X bond is fully
is fully M M M
M MM
broken before the M-Nu
broken before bond bond
the M-Nu formsforms
L XX LL LL Nu Nu
thereby thereby
avoidingavoiding an energetically L
an energetically
unfavorable
unfavorable
-
20e- 20eintermediate.
intermediate. L L LL L L
AnalogousAnalogous
in many in manyways waysto Sto S 1
N1 N 18e-
reactions.reactions. 18e-18e- 16e-
16e- 18e-
Note that in all ligand substition M = Ru(II), Co(III),
Note thatprocesses,
in all ligand
theresubstition
is no oxidation M =Rh(III),
Ru(II),Ir(III)
Co(III),
processes,
statethere is no
change oxidation
at the metal center. Rh(III), Ir(III)
state change at the metal center.
Substitution

 Examples:

3
 Oxidative Addition (OA)

• Increases formal oxidation by two units

Coordinatively unsaturated Oxidation (loss of two electrons)
complex in a relatively low
oxidation state • Increases its coordination number by two
(16e becomes 18e)
• General representation:

[Y] Oxidation Addition

[Y+2]

• Example:

Oxidative Addition is common for 16-electron square planar metal complexes 4

ctron count of (n+2)
the metal complex (e.g. 18e- The two M-L s bonds undergoing redu
(n+2)to 16 e-).X-
croscopicL M
reverse
RE is of
or
theoxidative L M
addition R
where
form two M-L sviabonds are broken to form
General Oxidative addition mechanisms
er. Currently,
on
ronsOA
intoMechanisms:
the metal
most common way to C-C bonds transition metal complexes
eral X d electron count. This is reflected in a two unit decrease in the m
companied by a decrease
Concerted (generally in the metal's
for non-polar coordination number by
substrates) 2 units. The
Nucleophilic result
displacemen
lex (e.g. 18e- to 16 e-). The two M-L s bonds undergoing reductive elimination mu
ion where two M-L s bonds are broken‡to form one substrate A s concerted
bond. REmechanism is
ommon way to A
form C-C bonds Avia transition metal complexes. A
. This isn
reflected in a two unit decrease in the metal's oxidation thought
state. Theto operate
(n+2)
tal'sLxcoordination
M + n n
numberLby xM2 units. The result L isxM LxM
primarily
a 2 unit decrease in the+ non-polar Lx
for
-L s bonds
polar substrates)
B
undergoing Nucleophilic
reductiveBelimination must bedisplacement
oriented
B
(generally
substrates
cis to each for polar
(i.e.XH-H,C-H,
transition metal complexes. Si-H, B-H) with electron
A ‡ A TS
3-centered cis addition rich, low valent metals.
A d + d- ‡
LxM(n+2)
Mn Nucleophilic displacement x
n substrates)
(generally forLpolar
M +
Radical (both non-polar and polar) L x M n
A X L
B X
B
d+ d ‡ A
-
A
ntered Mn +
LxTS cis addition
n L Mn
LxM xA X+ LxM(n+2) L M(n+1)
+
xA + XA - C LxM
X C
Radical (both non-polar and polar)
A
A
LxM n
+ LxM(n+1) A C LxM(n+2)
C A C
+1)
A C L M(n+2)
Oxidative Addition to Vaska’s Complex

HX

Cl
X 7
 Overview of Oxidative Addition

Mechanism Type of A-B Stereochemistry

Concerted Fairly non-polar substrates: cis-addition
H-H, R3C-H, R3Si-H
SN2 Polarized substrates: trans-addition
R3C-X
Also Cl2, Br2, I2
Radical R3C-X, R3Sn-X -
Ionic H-X (largely dissociated in -
solution)

 Non-polar substrates (e.g. H-H, C-H, Si-H) → Concerted

 Alkyl halides → Nucleophilic (SN2) or Radical
 Halogens (Cl2, Br2, I2) → Nucleophilic (SN2)
 Acids (HCl, HBr, HI) → Ionic

8
 Oxidative Addition: Mechanism
 Mechanism: Concerted Pathway – Non Polar X–Y with multiple bond (e.g. O2, CH2=CH2)

π-complex transition state

 Mechanism: Non-Concerted Pathway – Polar X–Y

1. SN2 Mechanism

X and Y need not be cis

9
 Oxidative Addition: Mechanism
 Mechanism: Concerted Pathway – Non Polar X–Y with multiple bond (e.g. O2, CH2=CH2)

π-complex transition state

 Mechanism: Non-Concerted Pathway – Polar X–Y

1. SN2 Mechanism

X and Y need not be cis

10
 Oxidative Addition: Mechanism

M.C. White, Chem 153 Mechanism -16- Week of S

 Mechanism: Concerted Pathway – Non Polar X–Y with single bond (e.g. H2, aryl halide)
OA: Concerted 3-centered (non-polar substra
H
H p-backbonding>>
s-donation
M M
oxidative addition
H
H

s-complex metal dihydride

s-complex: σ-complex transition

intermolecular binding of astate Ciss-bond
substrate via it's product formed
to a metal complex. s-complexes
thought to
[Threebe along the
centre pathway
TS for
also oxidative
proposed] addition of non-polar substrates to low valent, e-rich m
complexes. Analogous to the Dewar-Chatt-Duncanson model for olefin metal-bonding, s-bonding is thou
to occur via a 2 way donor-acceptor mechanism that involves s-donation from the bonding s-electrons of
substrate to empty s-orbital of the metal and p-backbonding from the metal to the s* orbitals of
substrate. These bonding principles have been applied to non-polar s-bonds such as H-H, C-H, Si-H, B
and even C-C bonds.

H H ‡ H
H
LxM n + LxM n LxM n LxM (n+2) >1.6Å
H H H
H 11
0.74Å 0.84Å
 Oxidative addition
• Factors that determine the tendency of a complex to give oxidative
additions
– Tendency to oxidation
– The electron richer the metal, the easier the oxidation
– Good donors (e. g., trialkylphosphines) favor oxidative addition.

• The tendency to give oxidative addition increases upon descending

in a triad: Co(I) < Rh(I) < Ir(I) (mainly because the M–L-bonding
energy increases in this order)
• Relative stability of the coordination numbers
• Bulky ligands favor low coordination numbers, and therefore
disfavor oxidative addition reactions (if they do not dissociate!)
• Strength of the newly formed M–X- and M–Y-bonds relative to X–Y
 Oxidative Addition: Mechanism – Non-Concerted Pathway
Inversion at carbon has been found in substituted halides

Reactivity of Metal
More nucleophilic the metal,
the greater its reactivity in SN2
additions

2. Radical Mechanism (one electron transfer) – Requires radical initiator

14
 Reductive Elimination (RE)
 Reductive elimination is the reverse of Oxidative addition
• Decreases formal oxidation by two units
Coordinatively saturated Reduction (gain of two electrons)
complex in a relatively high
oxidation state • Decreases its coordination number by two

• General representation:

Reductive Elimination
[Y+2]

• Example:

15
 Reductive Elimination (RE)
 Reductive elimination is favoured for the complexes having:
1. Higher formal positive charge on metal – electron poor (High oxidation state
metal or poor donor ligands)
2. Presence of bulky group (ligand) on metal – Steric hinderance
3. cis-configuration of the leaving groups
and the organic product formed should be stable.

 Reductive elimination efficient for: d6 –> PdIV, PtIV, RhIII, IrIII and d8 –> NiII, PdII, AuIII
 Mechanism is not well studied. Because, complexes that undergo RE are generally less stable.

 Concerted RE occurs with retention of configuration:

16
 Migratory Insertion
 Migratory Insertion: Reaction of cisoidal anionic ligand (e.g. hydride or alkyl group) and
neutral ligand (e.g. CO) on the metal complex couple to generate a new coordinated
anionic ligand
vacant site

 General Features:
1. No change in the formal oxidation state of the metal
2. Two groups that react must be cisoidal to one another
3. A vacant coordination site is generated during a migratory insertion, which gets
occupied by the incoming ligand in order to stop the back elimination reaction.

17
 Migratory Insertion
 Anionic and neutral ligands that can undergo migratory insertion with one another:
Anionic: H–, R– (alkyl), Ar– (aryl), acyl–, O2– (oxo)
Neutral: CO, Alkene, Alkynes, carbenes

 Type of Migratory Insertion:

Migrated group and metal are

separated by one atom
– Occurs in CO and carbenes (η1)

Migrated group and metal are

separated by two atom
– Occurs in alkenes and alkynes (η2)
18
1,1-Migratory Insertion: Mechanism

Structure and stereochemistry (cis) of the product –

• migration occurs between the groups in cis to
each other and
• incoming *CO does not insert into the Mn-Me
bond

– Involves the anionic ligand doing a – Involves the neutral ligand moving over to
nucleophilic-like attach on the neutral where the anionic ligand is coordinated and
ligand. “inserting” into anionic ligand–M bond
19
 1,1-Migratory Insertion: Mechanism

– Calderazzo’s Study

 Observation: Products A, B and C are formed, but not D.

This supports the mechanism involving Me migration

http://chemwiki.ucdavis.edu/?title=Inorganic_Chemistry/Organometallic_Chemistry/F 20
undamentals/Migratory_Insertion:_Introduction_%26_CO_Insertions
 1,2-Migratory Insertion: Mechanism

– Mainly involves the alkene and alkynes as neutral ligands

1,2-Migratory
insertion

Basis of almost all transition

metal based polymerization
catalysts.

21
 Elimination
 Elimination is the reverse of migratory insertion reactions
 Types of elimination:
β
α

 Key features:
1. No change in formal oxidation state
2. Must have empty orbital or ligand that can easily dissociate to open up an empty
orbital cisoidal to the group that undergoing elimination
22
 β-Hydride Elimination Canand Mechanism
not adobe syn-coplanar
conformation:
 β-Hydride elimination reaction can provide a facile route for decomposition of alkyl-
containing compounds.
M

3. There is a vacant coordination site (orbital) cis to th

 Mechanism:

II.a. Ligands with C-atom attached to metal

Alkyl ligands

 β-hydrogen elimination ß-Hydride

does not elimination
happen whenoccurs for nearly all transition metal-alkyls that meet
the following requirements:
• the alkyl has no β-hydrogen (as in unit
2. The M-C-C-H PhCH 2, Me
can 3CCH
adopt 2, Me3SiCH
a roughly 2)
syn-coplanar conformation that
brings the ß-hydrogen close to the metal center.
• the β-hydrogen on the alkyl is unable to approach the metal (as in C≡C-H)
M
• Can notbecome
the M–C–C–H unit cannot adobe syn-coplanar
coplanar
conformation:
23
M
 Exercise
Draw products of oxidative addition of the following compounds to (PPh3)2Pd.

A) HBr B) H2 C) I2 D) CH3Br

Classify the following reactions as oxidative addition, reductive elimination, migratory

insertion and elimination.

24
Catalysis
• Catalysis is the increase in the rate of a chemical reaction due to the participation
of an additional substance called a catalyst – Coined by J. J. Berzelius in 1835
• Catalyst – a chemical substance that increases the rate of a chemical reaction
without itself undergoing any permanent chemical change.
 Increases the reaction rate and with less energy – providing new pathway
 Not consumed - Tiny amount is enough – Can be recycled
 Tolman Loop: A reaction involving a true catalyst can be represented
by a closed loop

Jöns Jacob Berzelius

1779 – 1848 25
Catalysis
• Catalysis is the increase in the rate of a chemical reaction due to the participation
of an additional substance called a catalyst – Coined by J. J. Berzelius in 1835
• Catalyst – a chemical substance that increases the rate of a chemical reaction
without itself undergoing any permanent chemical change.
 Increases the reaction rate and with less energy – providing new pathway
 Not consumed - Tiny amount is enough – Can be recycled
 Tolman Loop: A reaction involving a true catalyst can be represented
by a closed loop

Jöns Jacob Berzelius

1779 – 1848 26
Types of Catalysis

• Majorly of two type

Homogeneous
Heterogeneous Catalysis Catalysis

Pt/C

27
Homogeneous and Heterogeneous Catalysis

Homogeneous Catalysis Heterogeneous Catalysis

Activity (relative to metal content) High Variable
Selectivity High Variable
Reaction conditions Mild Harsh
Service life of catalysts Variable Long
Sensitivity towards catalyst poisons Low High

Diffusion problems None May be important

Catalyst recycling Expensive Not necessarily
Variability of steric and electronic Possible Not possible
properties of catalysts

Mechanistic understanding Plausible under random More or less impossible

conditions

28
The turnover frequency, f (to express the efficiency of a
catalyst Q).

provided the rate of the un-catalysed reaction is negligible

A catalyst that results in a fast reaction even in low concentrations has a high turnover
frequency

The turnover number: the number of cycles for which a

catalyst survives. If it is to be economically viable, a catalyst
must have a large turnover number.
 Turnover Number (TON)
The absolute number of ‘passes through the catalytic cycle’ before the catalyst
becomes deactivated.
Academic chemists sometimes report only the turnover number when the
catalyst is very slow, or decomposes quite rapidly.
Industrial chemists are interested in both TON and TOF. A large TON (e.g.,
106 - 1010) indicates a stable, very long-lived catalyst. TON is defined as the
amount of reactant (moles) divided by the amount of catalyst (moles) times the %
yield of product.

# moles (equivalents) reactant (substrate)

Turnovers 
# moles (equivalents) catalyst
10 mole % catalyst = 10 turnovers
5 mole % catalyst = 20 turnovers
1 mole % catalyst = 100 turnovers
0.1 mole % catalyst = 1000 turnovers
0.01 mole % catalyst = 10,000 turnovers
 If I had 7.7 equivalents of reactant and 1 equivalent of Pd-
CATALYST
equivalents reactant 7.7
max turnovers    7.7
equivalents catalyst 1.0
If ONE assumes 100% yield.

The actual number of turnovers needs to be reduced by the % yield, which they
report as 64%, so the actual number of turnovers is:

actual turnovers  7.7  0.64  4.9

What about the TOF?
How long they ran the reaction to get their 64% yield: 18 hours at 1 atm of CO.
The TOF is the number of turnovers divided by the time:

4.9 turnovers
TOF   0.27hr  1
18 hr
 Homogeneous Catalysis

• Hydrogenation –> Alkene to alkane

• Hydroformylation –> Alkene to aldehyde with one carbon

homologation

• Monsanto Process –> Methanol to acetic acid

• Wacker Process –> Alkenes to ketones (hydration of alkene)

32
 Summary
 Hydrogenation

 Hydroformylation

 Monsanto Process

 Wacker Oxidation

33
 Hydrogenation

H2
R R
 Although, hydrogenation of alkene is thermodynamically feasible, it does not occur at
room temperature – rate negligible in the absence of catalyst
 In the presence of transition metal catalyst based on either Ni, Cu, Pd or Pt, the reaction
is fast and complete.

Cat.
R H2 R

 The first example of an effective and rapid homogeneous catalyst for hydrogenation of
alkenes, active at room temperature and atmospheric pressure is the Wilkinson’s catalyst

Chlorotris(triphenylphosphine)rhodium(I)
(Wilkinson’s Catalyst)
Independently discovered by G Wilkinson and R Coffey
Square planar, 16 e–, d8 complex (1964)

34
 Rh – Group 9
Hydrogenation
 Mechanism

Wilkinson’s Catalyst

Stereoview of [(Ph3P)3Rh]+ 16 e–,

Analogous complexes with alkylphosphine ligands are inactive, presumably because they are
35
more strongly bound to the metal atom and do not readily dissociate
 Hydrogenation
 Relative reactivity of alkenes

• cis-alkene reacts faster than trans-alkene

• trans-alkene reacts faster than internal and branched alkenes

36
Heterogeneous catalysis

Physisorption Chemisorption
Heterogeneous Catalysis:
Polymerization Reactions
Polymerization Reactions
 Ziegler-Natta Polymerization

Aufbau reaction
Ziegler-Natta Polymerization
 Ziegler-Natta Polymerization

First example of a crystalline isotactic polypropylene

Z-N catalyst: High molecular weight linear polymers with

stereoregularity
 Ziegler-Natta Polymerization: Mechanism

Z-N Catalyst
a transition metal (Group IV metals, like Ti,
Zr, Hf) compound
and an organoaluminum compound (co-
catalyst).

e.g. TiCl4 + Et3Al or TiCl3 + AlEt2Cl.

 TiCl3+AlEt3 catalyst

 The titanium chloride compound has a crystal structure in which

each Ti atom is coordinated to 6 chlorine atoms.
 On the crystal surface, a Ti atom is surrounded by 5 chlorine
atoms with one empty orbital to be filled.
 Mechanism of Ziegler–Natta Polymerization
1. Initiation, 2. Propagation 3. Termination
1. Initiation

C2H5
*Cl Ti + Cl-Al(C2H5)2
*Cl *Cl
*Cl
 Mechanism of Ziegler-Natta catalytic polymerization

1. Initiation, 2. Propagation 3. Termination

Initiation step The polymerization reaction is initiated by forming alkene-

metal complex. When a vinyl monomer like propylene comes to the active
metal center, it can be coordinated to Ti atom by overlapping their orbitals.

Interaction between metal – d orbital with pi-bonding alkene .

2. Propagation

Ti Ti

Ti
Ti
Migratory Insertion
Migratory Insertion Transition state
2. Propagation

CH2=CH2

CH2=CH2

Ti Ti

Polymerization
Termination step
Termination is the final step of a chain-growth polymerization,
forming desired polymers products.

Figure illustrates several termination approaches

developed with the aid of co-catalyst AlEt3.
 Stereo-selectivity

Stereochemistry of polymers made from ZN-catalyst can be well regulated

by rational design of ligands. By using different ligand system, either
syndiotactic or isotactic polymers can be obtained.
 Ziegler-Natta Polymerization: Mechanism

Courtesy: https://pslc.ws/macrog/ziegler.htm
 Ziegler-Natta Polymerization: transition metal
dependent stereoregularity
VCl4/Al(C2H5)2Cl

Courtesy: https://pslc.ws/macrog/ziegler.htm
 Ziegler-Natta Polymerization: transition metal
dependent stereoregularity

Courtesy: https://pslc.ws/macrog/ziegler.htm