Organometallic Reactions and Catalysis

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CONTENTS
 𝑂𝑟𝑔𝑎𝑛𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑖𝑐 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
 𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐴𝑑𝑑𝑎𝑡𝑖𝑜𝑛
 𝐵𝑖𝑛𝑢𝑐𝑙𝑒𝑟 𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛𝑠
 𝐶𝑦𝑐𝑙𝑜𝑚𝑒𝑡𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛
 𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐶𝑜𝑢𝑝𝑙𝑖𝑛𝑔
 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑣𝑒 𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
 𝐵𝑖𝑛𝑢𝑐𝑙𝑒𝑟 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑣𝑒 𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
 𝑆𝑖𝑔𝑚𝑎 𝐵𝑜𝑛𝑑 𝑀𝑒𝑡𝑎𝑡ℎ𝑒𝑠𝑖𝑠
 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
 𝐶𝑎𝑟𝑏𝑜𝑛𝑦𝑙𝑎𝑡𝑖𝑜𝑛
 𝐻𝑦𝑑𝑟𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑎𝑡𝑖𝑜𝑛
 𝛽 𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
 𝑆𝑡𝑎𝑏𝑙𝑒 𝐴𝑙𝑘𝑦𝑙𝑠
 𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠 𝑎𝑛𝑑 𝑂𝑟𝑔𝑎𝑛𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑖𝑐
𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
 𝐻𝑒𝑐𝑘 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
 𝑂𝑥𝑜 𝑝𝑟𝑜𝑐𝑒𝑠𝑠 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
 𝑊𝑖𝑙𝑘𝑖𝑛𝑠𝑜𝑛’𝑠 𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑡 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚

𝑂𝑟𝑔𝑎𝑛𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑖𝑐 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
Type 1 Type 2
Reactions Involving
Gain or Loss of Ligands
Reactions Involving
Modification of
Ligands
 Dissociation Reaction.
 Oxidative Addition.
 Reductive Elimination.
 Sigma-bond metathesis.
 Insertion.
 Hydride Elimination.
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𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛𝑠 𝐼𝑛𝑣𝑜𝑙𝑣𝑖𝑛𝑔
𝐺𝑎𝑖𝑛 𝑜𝑟 𝐿𝑜𝑠𝑠 𝑜𝑓 𝐿𝑖𝑔𝑎𝑛𝑑𝑠
 Many reactions of organometallic compounds involve a change in metal
coordination number by a gain or loss of ligands .
 If the oxidation state of the metal is retained, these reactions are considered
addition or dissociation reactions.
 If the metal oxidation state is changed, they are termed oxidative additions or
reductive eliminations.
 In classifying these reactions, it is often necessary to determine oxidation
states of the metals.
Type of Reaction
Change in Coordination
Number
Change in Formal Oxidation
State of Metal
Addition Increase None
Dissociation Decrease None
Oxidative addition Increase Increase
Reductive elimination Decrease Decrease

 These reactions involve an increase in both the oxidation state and the
coordination number of the metal.
 Also, there is increase in electron count of resulting organometallic
compound.
 Oxidative addition (OA) reactions are essential steps in many catalytic
processes.
 The reverse reaction, designated reductive elimination (RE) , is also very
important.
 These reactions are described schematically by the following:
𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐴𝑑𝑑𝑎𝑡𝑖𝑜𝑛 𝑂. 𝐴
LnM + A B LnM
A
B
∆𝑶𝑺 = +𝟐
∆𝑪𝑵 = +𝟐
∆𝑬𝑪 = +𝟐
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 Metal should be electron rich and is in low oxidation state, so it can be
easily oxidize.
 Mainly 16 e- organometallic compound easily give this reaction due to
formation of stable 18 e- organometallic compound.
 Also, some 18 e- organometallic compound give this reaction but their
should be dissociation of ligands of that compound in first step so it can
become 16 e- organometallic compound.
 Metal having d0 electron does not give this reaction.
 Oxidative addition requires that the metal complex have a vacant
coordination site. So, square planar organometallic complex give this
reaction with resulting octahedral geometry.
 Many of d8 metal complexes have a square planar geometry.
𝐹𝑎𝑣𝑟𝑎𝑏𝑙𝑒 𝑐𝑜𝑛𝑑𝑎𝑡𝑖𝑜𝑛 𝑓𝑜𝑟 𝑂. 𝐴
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 In square planar complex the ligands (A-B) add oxidatively in two possible
way i.e. cis addition and trans addition.
 The stereochemistry of addition depend on the class of A-B and which
mechanistic pathway the reaction follow (concert , SN2, Radical ).
𝑃𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑆𝑡𝑒𝑟𝑖𝑜𝑐ℎ𝑒𝑚𝑖𝑠𝑡𝑟𝑦
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𝐶𝑙𝑎𝑠𝑠 𝑜𝑓 𝐴 − 𝐵
Class 1
Non-Polar
Class 2
Polar
Class 3
A-B bond retained
 H2
 R-H
 Ar-H
 R3Si-H
 H-X
 R-X
 RCO-X
 X2
 ArSO2-Cl
 O==O
 S==S
 ==
 RN3
 Acetylene
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𝑂. 𝐴 𝑜𝑓 𝑉𝑎𝑠𝑘𝑎′
𝑠 𝑐𝑜𝑚𝑝𝑙𝑒𝑥 𝑤𝑖𝑡ℎ
𝑑𝑖𝑓𝑓𝑟𝑒𝑛𝑡 𝐴 − 𝐵
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𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑡𝑖𝑐 𝑃𝑎𝑡ℎ𝑤𝑎𝑦𝑠 𝐹𝑜𝑟 𝑂. 𝐴
Pathway 1
Concerted
Pathway 2
SN2
Pathway 3
Radical
A-B
Class 1 and 3
A-B
Class 2
A-B
Class 2
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 Concerted, or three-centre, oxidative addition is really an associative
reaction in which the incoming ligand first binds as a σ complex.
 Then A−B bond break as a result of strong back donation from the metal
into the A−B σ∗ orbital.
 Class 1 type i.e. nonpolar reagents, such as H2, or compounds containing
C−H and Si−H bonds all tend to react via a transition state—or more
probably an intermediate—of this type.
 Step a is associative step, involves formation of a σ complex.
 Step b is the oxidative part of the reaction in which metal electrons are
formally transferred to the σ∗ orbital of A−B
𝐶𝑜𝑛𝑐𝑒𝑟𝑡𝑒𝑑 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
LnM + A B LnMLnM
A
B
A
B3/27/2020

𝑂. 𝐴 𝑖𝑛 𝑉𝑎𝑠𝑘𝑎’𝑠 𝑐𝑜𝑚𝑝𝑙𝑒𝑥 𝑏𝑦
𝑐𝑜𝑛𝑐𝑒𝑟𝑡𝑒𝑑 𝑝𝑎𝑡ℎ𝑤𝑎𝑦
 The best-studied case is the addition of H2 to 16e square planar d8
species, such as IrCl(CO)(PPh3)2, to give 18e d6 octahedral dihydrides.
 In oxidative addition of H2 to Vaska’s complex, the trans-Cl(CO) set of
ligands usually folds back to become cis in the product
 As a powerful π acceptor, the CO prefers to be in the equatorial plane of
the resulting TBP transition state.
 High-trans-effect ligands such as CO prefer to be in the equator of a TBP,
or in this case of a TBP-like geometry.
Ir+1, d8, 16e-
Ir+1, d8, 18e- Ir+3, d6, 18e-
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C𝑙𝑎𝑠𝑠 1 𝑐𝑜𝑚𝑝𝑢𝑛𝑑𝑠 𝑜𝑡ℎ𝑒𝑟
𝑡ℎ𝑒𝑛 𝐻2
 The C−H and Si−H bonds of various hydrocarbons and silanes can also oxidatively
add to metals by concerted pathway.
 Among different types of C−H bonds, those of arenes are particularly prone to do this
because of the high thermodynamic stability of the aryl hydride adduct.
 The C−H bond seems to approach with the H atom pointing toward the metal and then
the C−H bond pivots around the hydrogen to bring the carbon closer to the metal in a
side-on arrangement, followed by C−H bond cleavage.
 The addition goes with retention of stereochemistry at carbon, as expected on this
mechanism.
 Carbon–carbon bonds do not normally oxidatively add, but a classic early case, the
reaction of cyclopropane with Pt(II) to give a Pt(IV) metalacyclobutane shown below,
illustrates how the reaction can be driven by ring strain.
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𝑂. 𝐴 𝑖𝑛 18𝑒− 𝑐𝑜𝑚𝑝𝑙𝑒𝑥
 The 18 e- organometallic compound give O.A but their should be dissociation (removal)
of ligands of that compound in first step by which it become 16 e- organometallic
compound.
 Dissociation of ligand also generate vacant site (coordinative unsaturation) on complex
which favors O.A.
 The term coordinative unsaturation to describe a complex that has one or more open
coordination sites where another ligand can be accommodated.
 For oxidative addition coordinative unsaturation should me their in complex.
 So, in 18 e- organometallic compound step one is generation coordinative unsaturation.
 There is different methods for removal of different ligands to generate coordinative
unsaturation in 18 e- organometallic compound .
CO
Halide
Alkyl
Hydride
hv
Ag+
H+
Ph3C+
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𝐸𝑥𝑎𝑚𝑝𝑙𝑒
hv
18e-
16e-
Retention of Stereochemistry
enantiopure
Generation of
coordinative
unsaturation by removal
of CO
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C𝑙𝑎𝑠𝑠 3 𝑐𝑜𝑚𝑝𝑢𝑛𝑑𝑠 𝑠ℎ𝑜𝑤𝑖𝑛𝑔
𝐶𝑜𝑛𝑐𝑒𝑟𝑡𝑒𝑑 𝑝𝑎𝑡ℎ𝑤𝑎𝑡
 When Vaska’s complex adds O2, the metal reduces the O2 to O2
2−, the peroxide
ion, which coordinates to the Ir(III).
 In O2 reaction go through both the steps but in case of ethylene the reaction
stops in step one.
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C𝑙𝑎𝑠𝑠 3 𝑐𝑜𝑚𝑝𝑢𝑛𝑑𝑠 𝑠ℎ𝑜𝑤𝑖𝑛𝑔
𝐶𝑜𝑛𝑐𝑒𝑟𝑡𝑒𝑑 𝑝𝑎𝑡ℎ𝑤𝑎𝑡
 The reason for ethylene to stop at step 1 is deficiency of pii accepting nature
of ethylene but this deficiency can be removed by introduction of
electronegative substituent in carbon atom of ethylene molecule.
 If the H atoms of ethylene is exchange by the electronegative atoms such as F
then the pi system of double bond favour the pi back donation and the
reaction easily go through step 2
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𝑆𝑜𝑚𝑒 𝐹𝑎𝑐𝑡𝑠 𝑜𝑓
 The reaction accelerated to some extent by electron-releasing ligands which
makes metal more electron reach and sports in back donation of electron
density to sigma star or pi star of incoming ligand.
 For example-
I
Br
Cl
>100
14
0.9
 If we replace X with different
halogens like Cl, Br and I the rate
of reaction changes as follow-
PMe3
PMe2Ph
PMePh2
14
4
1
 If we replace L with different
phosphene like PMe3, PMe2Ph and
PMePh2 the rate of reaction
changes as follow-
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 Solvent polarity does not have much more effect on reaction.
 The reason is formation of non polar transition state.
#
Non polar
Transition state
 How to experimentally determine the reaction is following this
pathway ?
 Changing the solvent from polar to non polar if reaction rate is not
varying too much that will indicate OA undergoes concerted
mechanism.
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 The SN2 pathway adopted for class 2 polarized AB substrates such as alkyl
halides.
 Like the concerted type, they are second-order reactions.
 SN2 mechanism is often found in the addition of methyl, allyl, acyl, and benzyl
halides to species such as Vaska’s complex.
 The metal in Vaska’s complex acts as nucleophile and attack alkyl halide
through electrons present in dz2 (HOMO) orbital of metal.
 There is inversion in stereochemistry if carbon of alkyl halide is chiral.
𝑆𝑁2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
Molecular orbital
energy level of metal
Square planer Vaska’s
complex
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 Reaction of vaska’s complex with MeX.
 In first step metal electron pair of vaska’s complex present in dz2 orbital
of Ir directly attacks the Me–X σ∗ orbital by an in-line attack at the least
electronegative formally to give [Ir(CO)(Cl)(PPh3)2]2+ Me−,and X−
fragments on the ionic model. Me− comes near to metal and start forming
partial bond.
 Then Me get attached to metal forming [Ir(CO)(Cl)(Me)(PPh3)2]+ this is
slow and rate determining step.
 Me group prefers to remain trans to the vacancy in the 16e square
pyramidal intermediate due to high-trans-effect of Me.
 In last step the X− get attached to metal in side of remained vacancy.
 So, the MeX add to metal in trans way.
 All these steps are shown in next slide-
𝐷𝑒𝑡𝑎𝑖𝑙𝑒𝑑 𝑆𝑁2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
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Ir(CO)(Cl)(PPh3)2 CH3 X+
[Ir(CO)(Cl)(PPh3)2]2+
CH3
- X-
𝐷𝑒𝑡𝑎𝑖𝑙𝑒𝑑 𝑆𝑁2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
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𝑆𝑁2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
 If the reacting compound have chiral center then there is inversion if
stereochemistry.
 More the nucleophilic metal grater its reactivity and faster the reaction. So
electron rich substituent accelerate the reaction. Steric hindrance also plays a
important role.
 Example reactivity of Ni0 complex according to these two factor.
Ni(PR3)4 > Ni(PAr3)4 > Ni(PR3)2(alkene)
> Ni(PAr3)2(alkene) > Ni(cod)2 3/27/2020

𝑆𝑛2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
 Steric hindrance at carbon also slow down reaction and decrease reactivity.
 For example reactivity order-
 A batter leaving group X at carbon accelerate the reaction.
 For example reactivity order-
 Due to ionic intermediate the polar solvent favour the pathway so reaction is
accelerate in polar solvent.
 How to experimentally determine the reaction is fallowing this pathway ?
 Changing the solvent from polar to non polar if reaction rate is varying too
much that will indicate OA undergoes SN2 mechanism.
Me-I > Et-I > i-Pr-I
R-I > R-Br > R-Cl
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 Radical mechanisms in oxidative additions were recognized later than the
SN2 and the concerted processes.
 Some OA reaction experimentally give non reproducible kinetic data so
this indicate the presence of radical intermediate in mechanism.
 Also, by radical traps the reaction get stopped which proves presence of
radical intermediate in mechanism.
 If reacting substrate have chiral center in this pathway the
stereochemistry get lost and racemic product get formed.
 Two subtypes of radical process are distinguished i.e nonchain and
chain.
 The nonchain variant is believed to operate in the additions of certain
alkyl halides, RX, to Pt(PPh3)3 (R = Me, Et; X = I; R = PhCH2;X = Br).
 The radical chain has been identified in the case of the reaction of EtBr
and PhCH2Br with the PMe3 analog of Vaska’s complex.
𝑅𝑎𝑑𝑖𝑐𝑎𝑙 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
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𝑁𝑜𝑛𝑐ℎ𝑎𝑖𝑛
 In this pathway at starting one electron transfer from M to the RX σ*to
form M+ and RX−.shown in Eq. 2.
 After X− transfer to M+, the R0 radical is liberated.
 This produces the pair of radicals shown in Eq. 2, which rapidly
recombine to give the product before either can escape from the solvent
cage shown in Eq. 3.
 Eq.2 and Eq. 3 can lead to a chain process if the radicals formed can
escape from the solvent cage without recombination. 3/27/2020

𝐶ℎ𝑎𝑖𝑛
 A radical initiator Q0, (e.g., a trace of air) may be required to set the process
going (Eq. 4 with Q0 replacing R0).
 In Eq. 4 the R0 (Q0 ) with vaska’s complex generates metal-centered radical.
 Metal-centered radical abstracts X0 from the alkyl halide (Eq. 5), to leave the
chain carrier (R0).
 The chain is present in Eqs. 4 and 5.
 To limit the number of cycles possible per R0 the chain termination step is
required So, Eq. 6 is chain terminating step.
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𝐵𝑖𝑛𝑢𝑐𝑙𝑒𝑟 𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛𝑠
 Binuclear oxidative additions, because they involve 1e rather than 2e
changes at the metals, often go via radicals pathways.
 One of the best known examples is shown in as below-
 The rate-determining step is net abstraction of a halogen atom from RX
by the odd-electron d7 Co(II) Eq. 7 .
 The resulting R0 combines with a second Co(II) center Eq. 8 .
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𝑅𝑎𝑑𝑖𝑐𝑎𝑙 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
 Like the SN2 process, the radical mechanism is faster when metal is more
basic, and the more readily electron transfer takes place, which gives the
reactivity order -
 Unlike the SN2 process, the reaction is fast for the 3◦ alkyl halide as compare
to 2◦ alkyl halide due to the more stability of the radical so the reactivity order
is like 3◦ > 2◦ > 1◦ > Me.
 So, bulky alkyl halide like t-BuF prefer radical pathway as compare to SN2
pathway and lite alkyl halide like MeI prefer SN2 pathway over radical
pathway.
 How to experimentally determine the reaction is fallowing this pathway ?
 The radical traps are used to determine that the reaction is using this pathway
or not. If reaction get stop or slow down by radical traps indicate reaction is
following this pathway.
R-I > R-Br > R-Cl
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𝐶𝑜𝑚𝑝𝑎𝑟𝑖𝑠𝑜𝑛
𝑅𝑎𝑑𝑖𝑐𝑎𝑙 𝑃𝑎𝑡ℎ𝑤𝑎𝑦𝑆𝑁2 𝑃𝑎𝑡ℎ𝑤𝑎𝑦𝐶𝑜𝑛𝑐𝑒𝑟𝑡𝑒𝑑 𝑃𝑎𝑡ℎ𝑤𝑎𝑦
Addition Cis-Addition Trans-Addition Trans-Addition
A-B have chiral center Retention of
stereochemistry
Inversion of
stereochemistry
Racemic mixture
Radical traps No effect No effect Stop or slowdown
reaction
Effect of solvent
Changing solvent
polarity no effect rate
of reaction
Changing solvent
polarity effect rate of
reaction
Solvent which do not
fast react with R.
should be used
Reactivity
R-I > R-Br > R-Cl R-I > R-Br > R-Cl
Me-I > Et-I > i-Pr-I Me-I < Et-I < i-Pr-I
-
-
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 The tendency of oxidative addition increase as we move down in group.
Moving down the group the oxidation of metal become easy. So Ir3+ easier
to oxidize than Rh3+ which is easer to oxidize then Co3+.
 A low initial oxidation state is more favourable for oxidative addition to
occur. So, when all the factor are equal Fe0 is easer to oxidize Co+ which is
easier to oxidize than Ni2+ .
Isoelectronic Atoms
Increasing tendency of
oxidative addition
Increasing tendency of
oxidative addition
𝑃𝑟𝑒𝑜𝑑𝑖𝑐 𝑇𝑟𝑒𝑛𝑑𝑠 𝑖𝑛 𝑂. 𝐴
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 These are the reaction that incorporate metals in to organic ring.
 The most common of these are orthometallation i.e oxidative addition in
which the ortho position of aromatic ring become attached to the metal.
Cl PPh3
Ir
Ph2P PPh3
H
Cl PPh3
Ir
Ph2P H
PPh
𝐶𝑦𝑐𝑙𝑜𝑚𝑒𝑡𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛
3/27/2020

 As the intramolecular reaction it result in formation of cyclic product with
transfer of hydrate to metal. So it is also called cyclometalization.
 Often the product of the cyclometalization undergoes reductive
elimination with loss of hydrate.
Cl PPh3
Ir
Ph2P H
PPh
PPh3
Ir
Ph2P
PPh
𝐶𝑦𝑐𝑙𝑜𝑚𝑒𝑡𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛
3/27/2020

 In oxidative coupling, the metal induces a coupling reaction between two
alkene ligands to give a metalacycle.
 The formal oxidation state of the metal increases by two units; hence the
“oxidative” part of the name.
 The electron count decreases by two, but the coordination number stays
the same.
 The reverse reaction, which is perhaps best called “reductive
fragmentation” is more rarely seen.
M M
𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐶𝑜𝑢𝑝𝑙𝑖𝑛𝑔
3/27/2020

CH3 Cr
Cl
Cl
N
Ph2
P
P
Ph2
CH3
Cr
Cl
Cl
N
Ph2
P
P
Ph2
Cr+3, d3, 15e- Cr+5, d1, 13e-
 Simple alkenes will only undergo reaction if metal is 𝝅 basic.
 Alkenes can be activated by electron withdrawing substituents.
𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒 𝐶𝑜𝑢𝑝𝑙𝑖𝑛𝑔
3/27/2020

 Reductive elimination is almost exactly the reverse of oxidative addition
and it decrease both coordination number as well as oxidation state.
 Key requirement for reductive elimination to occur is that the metal
should be in high formal positive charge.
Pt
CH3
CH3
CH3
CH3
Ph2
P
P
Ph2
Pt
CH3
CH3
Ph2
P
P
Ph2
Pt+4, d6, 18e- Pt+2, d8, 16e-
𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑣𝑒 𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
3/27/2020

 A high formal positive charge on metal.
 Presence of bulky group on metal
 Electronicky stable organic product.
 The low valent MLn complex formed after reductive
elimination must be stable.
 𝝅 accepting ligands on the metal generally accelerate
reductive elimination.
 Two eliminating group must be in Cis Position to each other.
𝐹𝑎𝑐𝑡𝑜𝑟𝑠 𝑠𝑝𝑜𝑟𝑡𝑠 𝑅. 𝐸
3/27/2020

𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑛𝑔 𝑔𝑟𝑜𝑢𝑝 𝑖𝑠 𝑖𝑛 𝐶𝑖𝑠
𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 𝑡𝑜 𝑒𝑎𝑐ℎ 𝑜𝑡ℎ𝑒𝑟
 Two examples are shown above in first example eliminating group is
locked in cis way while in second they are locked in trans way.
 It is interesting to check experimentally which one posses reductive
elimination eastly in same environment.
 Next slide shows the result which one posses reductive elimination in
same reaction condition.
3/27/2020

 The Cis substituents preferably involve in reductive
elimination.
 The ligand which eliminate and form organic product belongs
from same molecule. No cross product formed.
 So, this elimination is intramolecular elimination.
 Also. In the crossover experiment, a mixture of cis-
Pd(CH3)2L2 and cis-Pd(CD3)2L2,is thermolyzed and it was
found that only C2H6 and C2D6 are formed,
𝑅𝑒𝑠𝑢𝑙𝑡𝑠 𝑜𝑓 𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡
3/27/2020

Y-type distorted TBP
transition state
 Common general mechanism for reductive elimination in
Milstein’s octahedral d6 species (L = PMe3;R = CH2COMe).
𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
3/27/2020

 In first step the PMe3 trans to the high-trans-effect hydride ligand so it
get lost which generate 5-coordinate intermediate.
 5-coordinate can more readily distort to reach the transition state for
reductive elimination. First it convert in to Y-type distorted trigonal
bipyramidal structure.
 This structure brings the two groups to be eliminated, R and H, very
close together.
 The typical small R−M−H angle 70◦, may facilitate reductive elimination
in proposed transition.
 After reductive elimination, a T-shaped 3-coordinate species is formed.
 In the last step the PMe3 wich removed in first step get coordinated to
metal to foam square planar product.
3/27/2020

Thermochemical data and bond dissociation energy of iridium complex.
+
𝑇ℎ𝑒𝑟𝑚𝑜𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑃𝑟𝑒𝑓𝑟𝑎𝑛𝑐𝑒
3/27/2020

 Oxidative addition and reductive elimination are microscopic reverse of
each other the position of equilibrium depend on thermodynamics of the
process.
 Many metal complex oxidatively add MeI, but few one reductively
eliminate MeI.
 The oxidative addition of RCH2-H is less common wile reductive
elimination of alkane from M(H)CH2R is very often observe.
 Oxidative addition with CH3-I and H-H is feasible while it is
thermodynamically unfavourable with CH3-H and CH3-CH3.
𝑇ℎ𝑒𝑟𝑚𝑜𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑃𝑟𝑒𝑓𝑟𝑎𝑛𝑐𝑒
3/27/2020

 Of course, no one wants to make ethane that way (if at all) but many other
pairs of ligands can be coupled by reductive elimination to synthesize
important organic molecules.
 Reductive elimination is one of the most important methods for the
removal of a transition metal from a reaction sequence, leaving a neutral
organic product.
 So, it is last step of many catalytic process (oxo, Heck, Wilkinson’s
Catalyst etc).
 Like in indole synthesis the last step depend on the reductive elimination
by which the Pd metal get removed from organic system
𝐼𝑚𝑝𝑜𝑟𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑅. 𝐸
An indolePd+2
Pd0
3/27/2020

 Complex with two and more atoms adjacent metal atoms can also
participate in oxidative addition and reductive elimination.
 This often involves both metal centers and each metal changes its
oxidation state by ±1 instead of ± 2.
 The example of binuclear oxidative addition given blow-
+
Mn0,18e-
Mn+1,18e-
𝐵𝑖𝑛𝑢𝑐𝑙𝑒𝑎𝑟 𝑆𝑦𝑠𝑡𝑒𝑚
3/27/2020

+
𝐵𝑖𝑛𝑢𝑐𝑙𝑒𝑎𝑟 𝑆𝑦𝑠𝑡𝑒𝑚
 Also , the reductive elimination of R-R is possible especially in metal –
metal double bond compound with leaving ligand on adjacent metal unit
as shown in example
Mo+3,16e- Mo+2,14e-
3/27/2020

𝑆𝑖𝑔𝑚𝑎 𝐵𝑜𝑛𝑑 𝑀𝑒𝑡𝑎𝑡ℎ𝑒𝑠𝑖𝑠
 Sigma-bond metathesis is a chemical reaction wherein a metal-ligand
sigma bond undergoes metathesis (exchange of parts) with the sigma
bond in some reagent.
 The reaction is mainly observed for complexes of metals with d0
configuration, e.g. complexes of Sc(III), Zr(IV), Nb(IV), Ta(V), etc.
 It is fascinating that some d0 transition-metal complexes are able to
activate C—H bonds without any change in oxidation state.
+ +
3/27/2020

 Metal is first postulated to coordinate the bond to be activated in an η2
fashion.
 Then there is formation of a four-centered transition state.
 Lastly there is an exchange of ligands at the metal.
transition state
sigma-bond
metathesis
net reaction
η2- complex
η2- complex

 Reaction in which any atom or group is inserted between two other atoms
initially bound together.
 In organometallic system ligand or molecular fragment appears to insert
into a metal–ligand bond.
 These reactions are believed to occur by direct single-step insertion but
many “insertion” reactions do not involve a direct insertion step.
 The most studied of these reactions are carbonyl insertions.
A B A X B
X
𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
3/27/2020

+
+
+
𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛
3/27/2020

 There are two main types of insertion—1,1 and 1,2.
 The metal and the X ligand end up bound to the same (1,1) or adjacent
(1,2) atoms of an L-type ligand.
16e-
16e-
18e-
18e-
𝐶𝑙𝑎𝑠𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛
3/27/2020

 In general, η1 ligands tend to give 1,1 insertion and η2 ligands give 1,2
insertion.
 For example, CO gives only 1,1 insertion and on the other hand, ethylene
gives only 1,2 insertion.
 SO2 is the only common ligand that can give both types of insertion; as a
ligand, SO2 can be η1 (S) or η2 (S, O).
 The group undergoing insertion must be cisoidal to each other.
 When the migrating ligand ‘X’reaction usually proceeds with retention
of configuration.
𝐶𝑙𝑎𝑠𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛
3/27/2020

18e- M+
16e- M+
18e- M+ 16e- M+
𝑀𝑖𝑔𝑟𝑎𝑡𝑜𝑟𝑦 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑖𝑛 𝐶𝑂
𝑀𝑖𝑔𝑟𝑎𝑡𝑜𝑟𝑦 𝑉𝑠 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛
3/27/2020

 A migration occurs when the
anionic ligand moves and
perform a nucleophilic like
intramolecular attack on
electrophilic neutral ligand.
 An insertion occurs when the
neutral ligand moves and gets
inserted in to bond between
the metal and anionic ligand.
 Most of the studied cases have been shown that CO undergo
migration not insertion.
𝑀𝑖𝑔𝑟𝑎𝑡𝑜𝑟𝑦 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑖𝑛 𝐶𝑂
𝑀𝑖𝑔𝑟𝑎𝑡𝑜𝑟𝑦 𝑉𝑠 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛
3/27/2020

+
+
+
𝑃𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
Mechanism 1
Mechanism 2
Mechanism 3
3/27/2020

 Reaction of 13CO with CH3Mn(CO)5 yield cis
(CH3CO)Mn(13CO)(CO4)
 13CO is absent in acetyl group which turn down possibility if
mechanism 1.
 13CO is cis to acetyl group means group involve in reaction must be
cis to each other.
 Additional mechanistic information can be gained by studying
reverse reaction shown in nest slide.
𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝐹𝑖𝑛𝑑𝑖𝑛𝑔𝑠
3/27/2020

 25 % of the product formed with out 13CO.
 25 % of the product has 13CO trans to CH3
 50 % of product has 13CO cis to CH3.
 So, CO insertion must be eliminated as mechanistic possibility and methyl
migration is sported by these results.
 Mechanism 3 is actual mechanism.
 Reaction involves ligands cis to each other.
𝑅𝑒𝑠𝑢𝑙𝑡
+
Mechanism 3
Actual Mechanism
3/27/2020

+
 The less energy required to break M-R bond and more energy release
when C-C and M-CO bond formed.
 Since gaseous CO is captured, so it would expected that the entropy term
inhabit spontaneity, but even so larger negative entropy term is
dominant.
∆𝑯 = −𝟓𝟒 𝑲𝑱/𝒎𝒐𝒍
𝑇ℎ𝑒𝑟𝑚𝑜𝑑𝑦𝑛𝑎𝑚𝑖𝑐𝑎𝑙𝑙𝑦
𝐹𝑎𝑣𝑜𝑢𝑟𝑎𝑏𝑙𝑒
3/27/2020

+
 Lewis acid accelerate the alkyl carbonyl migration insertion reaction.
 The Lewis acid get bind to basic oxygen of acyl group.
 Lewis acid lowers the activation energy by this coordination.
𝐿𝑒𝑤𝑖𝑠 𝑎𝑐𝑖𝑑 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
3/27/2020
Catalyst

 The hydrate is transferred to 𝜷 carbon of the alkene to form metal
alkyl complex
 The hydrate, metal and alkene group become coplanar during the
reaction.
 The stereochemistry of both carbon remain retained since it is syn
addition.
 𝛽 hydrogen elimination is reverse of insertion reaction and it
represent the chief decomposition pathway of transition metal alkyl
complex.
𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑜𝑓 𝐴𝑙𝑘𝑒𝑛𝑒𝑠 1,2
𝑖𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 (𝐻𝑦𝑑𝑟𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑎𝑡𝑖𝑜𝑛)
3/27/2020

 For many alkenes, the insertion and hydrate elimination reaction can
be considered to be in equilibrium.
 For alkenes , the equilibrium lies towards the left but for alkenes with
electron withdrawing group like C2F4 the alkyl group is particularly
stable and equilibrium lies to right.
𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 𝑜𝑓 𝐴𝑙𝑘𝑒𝑛𝑒𝑠
1,2 𝑖𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛
3/27/2020

 Insertion is the principal way of building up the chain of an organic
ligand before elimination.
 Common examples include insertion of carbon monoxide, alkenes, and
alkynes, producing metal–acyl, metal–alkyl, and metal–alkenyl complexes
which after elimination produce important organic molecules.
𝐼𝑚𝑝𝑜𝑟𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛
Carbonylation carbometallation
or hydrometallation
alkyne insertion
3/27/2020

 Formation of important organic compound by insertion reaction-
 Carbonylation (the addition of carbon monoxide to organic molecules) is
an important industrial process as carbon monoxide is a convenient one-
carbon feedstock and the resulting metal–acyl complexes can be
converted into aldehydes, acids, and their derivatives.
 The OXO process is the hydroformylation of alkenes such as propene and
uses two migratory insertions to make higher value aldehydes.
𝐼𝑚𝑝𝑜𝑟𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝐶𝑎𝑟𝑏𝑜𝑛𝑦𝑙𝑎𝑡𝑖𝑜𝑛
Important organic molecule
3/27/2020

 1-2 insertion can be use to synthesize important organic molecule as well as
polymers (ziegler natta polymerization).
 Hydrometallation (carbometallation) is important step of many homogenous
catalytic process (oxo, heck , Wilkinson’s Catalyst).
𝐼𝑚𝑝𝑜𝑟𝑡𝑎𝑛𝑐𝑒 𝑜𝑓
ℎ𝑦𝑑𝑟𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑎𝑡𝑖𝑜𝑛
Important step of ziegler natta polymerization
Important step of Wilkinson’s Catalyst
3/27/2020

𝛽 𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝐸𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
 After the successful synthesis of metal alkyls of main group elements,
many attempts were made to form transition metal alkyls.
 All attempts during the 1920 – 1940 to make d block metal alkyls fail.
 This was puzzling because by then almost every non transition element
foam stable alkyls.
 These failures let to the view that transition metal carbon bond were
weak.
 Now it is known that M-C bond is strong (30-65 kcal/mol)
 This is the existence of several decomposition path way that make many
transition metal alkyl unstable.
3/27/2020

 The major decomposition pathway for alkyls is 𝛽 𝑒𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛 which
converts a metal alkyl in to a hydro metal alkene complex.
 The 𝛽 carbon of alkyl bears a hydrogen substituent.
 The M-C-C-H unit can take up roughly coplanar conformation.
 There is a vacant site on the metal symbolized as cis to alkyl.
 The electron count of the product is 2e more then the starting
material.
3/27/2020

 Mainly the 16e- metal show 𝛽 elimination forming 18e- species
 Some 18 e- alkyls do 𝜷 elimination but there is dissociation of some ligand.
 Main group alkyls can also 𝛽 elimination but very slowly.
 Reason is grater ability of d-block metals to stabilize T.S involved agnostic
alkyl complexes.
𝑆𝑡𝑎𝑏𝑙𝑒 𝐴𝑙𝑘𝑦𝑙𝑠
 Stable alkyls are those which block 𝛽 elimination pathway.
3/27/2020

 Alkyls that have no 𝛽 hydrogen.
 Alkyls for which the 𝛽 hydrogen is unable to approach.
 Alkyls in which the M-C-C-H unit not become syn coplanar.
3/27/2020

 An 18e- species with firmly bound ligand which will not
dissociate to generate a vacant site.
18e- 18e-
 Some d0 alkyls.
3/27/2020

𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠 𝑎𝑛𝑑 𝑂𝑟𝑔𝑎𝑛𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑖𝑐
 All the organometallic reaction (oxidative addition, reductive elimination
,insertion etc.) have very importance in catalysis especially homogenous
catalysis. .
 The homogenous catalysis use these reaction in their catalytic steps.
 Some examples-
Heck Oxo Wilkinson’s Catalyst Monsanto
Coupling Rex Hydroformylation Hydrogenation Acetic Acid Process
𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑣𝑒
𝑎𝑑𝑑𝑖𝑡𝑖𝑜𝑛
𝛽 ℎ𝑦𝑑𝑟𝑖𝑑𝑒
𝑒𝑙𝑖𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛
𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑣𝑒

𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠 𝑎𝑛𝑑 𝑂𝑟𝑔𝑎𝑛𝑜𝑚𝑒𝑡𝑎𝑙𝑙𝑖𝑐
 The organometallic reaction represent the various steps of catalytic cycle of
homogenous catalyst.
 Mainly oxidative addition is first step of the catalytic cycle and reductive
elimination is last.
 Also, the insertion plays important role in various catalytic cycles.
 To batter understand how these reaction are key steps of catalytic cycle we
have to see some mechanism.
 So some mechanism are represented in next few slides (Paly with animation
over slide show for batter understanding).

3/27/2020
𝒐𝒙𝒊𝒅𝒂𝒕𝒊𝒗𝒆
𝒂𝒅𝒅𝒊𝒕𝒊𝒐𝒏
𝒄𝒂𝒓𝒃𝒐𝒎𝒆𝒕𝒂𝒍𝒍𝒂𝒕𝒊𝒐𝒏
𝜷 𝒉𝒚𝒅𝒓𝒊𝒅𝒆
𝒆𝒍𝒊𝒎𝒊𝒏𝒂𝒕𝒊𝒐𝒏
𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒗𝒆
Product
Starting
materials
𝐻𝑒𝑐𝑘 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
𝟏, 𝟐 𝒊𝒏𝒔𝒆𝒓𝒕𝒊𝒐𝒏
Catalyst

3/27/2020
𝟏, 𝟐 𝒊𝒏𝒔𝒆𝒓𝒕𝒊𝒐𝒏
𝟏, 𝟏 𝒊𝒏𝒔𝒆𝒓𝒕𝒊𝒐𝒏
𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒗𝒆
𝝅 Complex formation
CO Complexation
Product
Starting
materials
𝑂𝑥𝑜 𝑝𝑟𝑜𝑐𝑒𝑠𝑠 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
Catalyst

3/27/2020
1,2 𝑖𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒗𝒆
𝑳𝒊𝒈𝒂𝒏𝒅 𝒅𝒊𝒔𝒔𝒐𝒄𝒊𝒂𝒕𝒊𝒐𝒏
Product Starting
materials
𝑊𝑖𝑙𝑘𝑖𝑛𝑠𝑜𝑛’𝑠 𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑡 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑠𝑚
Catalyst

𝑅𝑒𝑓𝑟𝑒𝑛𝑐𝑒 𝐵𝑜𝑜𝑘𝑠 𝑢𝑠𝑒𝑑 𝑡𝑜
𝑚𝑎𝑘𝑒 𝑡ℎ𝑖𝑠 𝑝𝑟𝑒𝑠𝑒𝑛𝑡𝑎𝑡𝑖𝑜𝑛
 The Organometallic Chemistry of the Transition Metals (Fourth Edition) by
Robert H. Crabtree. A JOHN WILEY & SONS, INC., PUBLICATION.
 Inorganic Chemistry (Fifth Edition) by Gary L. Miessler and Donald A. Tarr.
PEARSON.
 Inorganic Chemistry :: Principles of Structure and Reactivity (Fourth Edition)
by James E. Huheey. PEARSON.
 Organic Chemistry (Second Edition) By Jonathan Clayden, Nick Greeves and
Stuart Warren, OXFORD.
 Basic Organometallic Chemistry (Second Edition) by B.D. Gupta and Anil
Elias, UNIVERSITIES PRESS.

Organometallic Reactions and Catalysis

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