Organometallic Chemistry by Q4r3Y1


       Part 3

1. Introduction (types and rationale)

2. Molecular orbital (bonding) of CO, arrangement “in space”
or ligand types (hapticity)

3. 16 and 18 electron rule (learning to count)

4. Synthesis, steric effects and reactivity - Wilkinsons catalyst (part 1)

5. Characterisation IR nmr etc.

6. Applications (oxidative addition b elimination)
                 What is organometallic chemistry?

Chemistry: structures, bonding and properties of molecules.

Organometallic compounds: containing direct metal-carbon bonds.
Either s or p bonds can occur

Main group:

   Structures
      s bonds and 3c-2e (or even 4c-2e) bonds
      Chem 210

   Synthesis
      the first M-C bond

   Reactivity
      nucleophilic and basic
      auxiliaries in organic synthesis
      source of organic groups for transition metals
   Strong preference for s-donor groups but Cp is often p-bound
    (deceptively like with transition metals)

          Cp2Mg                                                Cp2Fe

   Electropositive metals: often 3c-2e or 4c-2e hydrides/alkyls

                              (Me3Al)2                                 (MeLi)4
As a Nucleophile

   Addition to polar C=X bonds
     (C=O, C=N, CºN)

                                 R M
                                        O                    OM
   Substitution at sp2 carbon
     (often via addition)

                          OR'      R    OR'          R
                     +                     - MOR'
       R M
                     O                 OM                O
    Substitution at sp3 carbon does occur
      but is far less easy and often has a multistep mechanism

    Substitution at other elements:
     often easy for polar M-X bonds
      (Si-Cl, B-OMe)

MeMgBr + B(OMe)3       BrMg     B(OMe)2     MeOMgBr + MeB(OMe)2   Me3B
As a base
 More prominent in polar solvents think of free R- acting as base

   Elimination


             R M                            + RH + MX

    mechanism can be more complex than this

   Metallation
                     H                        M
            R M                                            + RH
              Me2N                        Me2N
    chelate effect more important than inductive effect!
 b-hydrogen transfer

                     Al       H
                                        Al     H
                          O               O

   mainly for Al:
    for more electropositive elements, deprotonation
     and nucleophilic attack
     are faster
    for less electropositive elements, often no reaction
Chemistry: structures, bonding and properties of molecules.

Transition metal compounds
Some compounds do not contain metal-carbon bond, but they are usually
included in the field of organometallic chemistry. They include:

• Metal hydride complexes, e.g.
                                      • Phosphine complexes, e.g.

 • N2-complexes, e.g.
Exercise. Which of the following compounds is an organometallic compound?
 a)                                     b)               Cu
              Ti                                  H3 N           NH3
      CH3O           OCH3                                                In general, metals in
                                                                         compounds include:
                  Cl                                     O
c)                               CH2    d)                               • main group metals
        Cl        Pt                              O      Pt     O
                        CH2                                              • transition metals
                  Cl                                     O               • f-block metals

                                                      Ph                 In this course,
                       Li         Me
                                                          P    CO        transition metals are
                                             OC                 CO       our main concern.
        Me           Li                                            O
e)                                                            Co C
                   Me             Li   f) OC Co
                                                                 Co CO
                                           O C Co                   CO
             Li             Me              OC
                                              OC P
            A brief history of organometallic chemistry

1) Organometallic Chemistry
   has really been around for
   millions of years

    Naturally occurring
      Cobalimins contain
      Co—C bonds

    Vitamin B12
 2) Zeise’s Salt synthesized in 1827 = K[Pt(C2H4)Cl3] • H2O
     Confirmed to have H2C=CH2 as a ligand in 1868
     Structure not fully known until 1975

 3) Ni(CO)4 synthesized in 1890

 4) Grignard Reagents (XMgR) synthesized about
     Accidentally produced while trying to make other
     Utility to Organic Synthesis recognized early on

5) Ferrocene synthesized in 1951
     Modern Organometallic
      Chemistry begins with this
      discovery (Paulson and Miller)

     1952 Fischer and Wilkinson
Nobel -Prize Winners related to the area:

Victor Grignard and Paul Sabatier (1912)
Grignard reagent

K. Ziegler, G. Natta (1963)
Zieglar-Natta catalyst

E. O. Fisher, G. Wilkinson (1973)
Sandwich compounds

K. B. Sharpless, R. Noyori (2001)
Hydrogenation and oxidation

Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005) Metal-
catalyzed alkene metathesis
                 Common organometallic ligands

M       H    M    C      M       C
                                     C         M           M

    M       CO      M   CNR      M       N2
                                                       H           H
M       CS         M    NO       M       PR3       M           M
                                                       H           X

    M              M         M           M

                                                               M   C

                                                               M   C
               Why organometallic chemistry ?
a). From practical point of view:
  * OMC are useful for chemical synthesis, especially catalytic processes,
        e.g. In production of fine chemicals
                In production of chemicals in large-scale
                reactions could not be achieved traditionally
* Organometallic chemistry is related to material sciences.
  e.g. Organometallic Polymers

           PBu3                            PBu3
           Pt   C C     C C                Pt   C C           C C
                                 n                                  n
           PBu3                            PBu3

  Small organometallic compounds:
  Precursors to films for coating (MOCVD)

  (h3-C3H5)2Pd -----> Pd film

  CH3CC-Au-CNMe -----> Au film

  Luminescent materials
* Biological Science. Organometallic chemistry may help us to
  understand some enzyme-catalyzed reactions.

  e.g. B12 catalyzed reactions.

        H                         R

 b). From academic point of view:
     * Organometallic compounds display many unexpected behaviors-
discover new chemistry- new structures e.g.

                                                       H       H
   H3N:     M           M                          M       M
                               M          M            H       SiR3

                C   C         C    C
                M   C         M    C

      New reactions, reagents, catalysts, e.g.
      Ziegler-Natta catalyst, Wilkinson catalyst
      Reppe reaction, Schwartz's reagent
      Sharpless epoxidation, Tebbe's reagent
Types of bonds possible from Ligands

Language: All bonds are coordination or coordinative

Remember that all of these bonds are weaker than normal organic
bonds (they are dative bonds)

Simple ligands e.g. CH3-, Cl-, H2 give s bonds

 systems are different e.g. CO is a s donor and p acceptor

Bridging ligands can occur two metals

Metal-metal bonds occur and are called d bonds – they are weak
and are a result of d-d orbital overlap
             18 Electron Rule (Sidgwick, 1927)
• OM chemistry gives rise to many “stable” complexes - how can we
  tell by a simple method
• Every element has a certain number of valence orbitals:
   1 { 1s } for H
   4 { ns, 3´np } for main group elements
   9 { ns, 3´np, 5´(n-1)d } for transition metals

         s               px            py            pz

   dxy           dxz           dyz          dx2-y2        dz2
• Therefore, every element wants to be surrounded
  by 2/8/18 electrons
   – For main-group metals (8-e), this leads to the standard Lewis structure
   – For transition metals, we get the 18-electron rule
• Structures which have this preferred count are called
• Every orbital wants to be “used", i.e. contribute to binding an electron pair

The strength of the preference for electron-precise structures depends on the
  position of the element in the periodic table

• For early transition metals, 18-e is often unattainable for steric reasons - the
  required number of ligands would not fit
• For later transition metals, 16-e is often quite stable (square-planar d8
• Addition of 2e- from 5th ligand converts complex to 5 CN 18e- , marginally
  more stable
Predicting reactivity

                             14 e
           - C2H4                         CO
(C2H4)2PdCl2                                  (C2H4)(CO)PdCl2
    16 e   CO                                 - C2H4   16 e

                             18 e

                    Most likely associative
Predicting reactivity

                             16 e
               - CO                       MeCN
   18 e   Cr(CO)6                         Cr(CO)5(MeCN) 18 e
       MeCN                                  - CO
                        20 e (Sterics!)

                  Most likely dissociative
 N.B. How do you know a fragment forms a covalent or a dative bond?

 •   Chemists are "sloppy" in writing structures. A "line" can mean a
     covalent bond, a dative bond, recognise/understand the bonding
 •   Use analogies ("PPh3 is similar to NH3").
 •   Rewrite the structure properly before you start counting.

           Cl        PPh3                  Cl        PPh3         Pd =       10
                                                                  Cl¾ =       1
                Pd                    1e
                                                Pd           2e
                                                                  P® =        2
                             dative                               allyl =     3
                             bond                                         + ¾¾
                                                                  e-count    16

                     "bond" to the
                     allyl fragment
"Covalent" count: (ionic method also useful)
1. Number of valence electrons of central atom.
   • from periodic table
2. Correct for charge, if any
   • but only if the charge belongs to that atom!
3. Count 1 e for every covalent bond to another atom.
4. Count 2 e for every dative bond from another atom.
   • no electrons for dative bonds to another atom!
5. Delocalized carbon fragments: usually 1 e per C (hapticity)
6. Three- and four-center bonds need special treatment
7. Add everything

N.B. Covalent Model:
18 = (# metal electrons + # ligand electrons) - complex charge

The number of metal electrons equals it's row number (i.e., Ti = 4e, Cr = 6 e,
   Ni = 10 e)
Hapto (h) Number (hapticity)

For some molecules the molecular formula provides insufficient
information with which to classify the metal carbon interactions

The hapto number (h) gives the number of carbon (conjugated) atoms
bound to the metal

It normally, but not necessarily, gives the number of electrons contributed
by the ligand

We will describe to methods of counting electrons but we will
employ only one for the duration of this module
The two methods compared:
some examples

N.B. like oxidation state
assignments, electron
counting is a formalism and does
not necessarily reflect the
distribution of electrons in the
molecule – useful though

Some ligands donate the same
number of electrons

Number of d-electrons and
donation of the other ligands
can differ

Now we will look at practical
examples on the black board
            Does it look reasonable ?

   Remember when counting:

   Odd electron counts are rare

   In reactions you nearly always go from even to even (or
    odd to odd), and from n to n-2, n or n+2.

   Electrons don’t just “appear” or “disappear”

   The optimal count is 2/8/18 e. 16-e also occurs
    frequently, other counts are much more rare.
Exceptions to the 18 Electron Rule
ZrCl2(C5H5)2 Zr(4) + [2 x Cl(1)] + [2 x C5H5(5)] =16
TaCl2Me3 Ta(5) + [2+ x Cl(1)] + [3 x M(1)] =10
WMe6 W(6) + [6 x Me(1)] =12
Pt(PPh3)3 Pt(10) + [3 x PPh3(2)] =16
IrCl(CO)(PPh3)2 Ir(9) + Cl(1) + CO(2) + [2 x PPh3(2)] =16

What features do these complexes possess?
• Early transition metals (Zr, Ta, W)
• Several bulky ligands (PPh3)
• Square planar d8 e.g. Pt(II), Ir(I)
• σ-donor ligands (Me)
Alkyl ligands:

Transition metal alkyl complexes important for catalysts e.g. olefin
polymerization and hydroformylation thermodynamic

Problem is their weak kinetic stability
(Thermally fine: M-C bond dissociation energies are typically 40-60
kcal/mol with 20-70 kcal/mol)

Simple alkyls are sigma donors, that can be considered to donate one or two
electrons to the metal center depending on which electron counting formalism
you use
Synthesis of Metal Alkyl Complexes
1. Metathetical exchange using a carbon nucleophile (R-). Common
reagents are RLi, RMgX (or R2Mg), ZnR2, AlR3, BR3, and PbR4. Much
of this alkylation chemistry can be understood with Pearson's "hard-
soft" principles
2. Metal-centered nucleophiles (i.e. using R+ as a reagent) Typical
examples are a metal anion and alkyl halide (or pseudohalogen). for

NaFp + RX             Fp-R + NaX      [Fp = Cp(CO)2Fe]

3. Oxidative Addition. This requires a covalently unsaturated, low-
valent complex (16 e- or less). A classic example:

4. Insertion- To form an alkyl, this usually involves an olefin
insertion. The simplest generic example is the insertion of ethylene
into an M-X bond, i.e.

        M-X + CH2CH2               M-CH2CH2-X
Carbonyl Complexes
Bonding of CO
Electron donation of the lone pair
on carbon s This electron
donation makes the metal more
electron rich - compensate for this
increased electron density, a filled
metal d-orbital may interact with
the empty p* orbital on the
carbonyl ligand
p-backbonding or p-
backdonation or synergistic

Similar for alkenes, acetylenes,
phosphines, and dihydrogen.
What stabilizes CO complexes is M→C π–bonding

The lower the formal charge on the metal ion the more willing it is to
donate electrons to the π–orbitals of the CO

Thus, metal ions with higher formal charges, e.g. Fe(II) form CO
complexes with much greater difficulty than do zero-valent metal ions

For example Cr(O) and Ni(O), or negatively charged metal ions such
as V(-I)

In general to get a feeling for stability examine the charges on the
Syntheses of metal carbonyls

Metal carbonyls can be made in a variety of ways.

For Ni and Fe, the homoleptic or binary metal carbonyls can be made by the
direct interaction with the metal (Equation 1).

In other cases, a reduction of a metal precursor in the presence of CO (or using
CO as the reductant) is used (Equations 2-3).

Carbon monoxide also reacts with various metal complexes, most typically filling
a vacant coordination site (Equation 4) or performing a ligand substitution
reactions (Equation 5)

Occasionally, CO ligands are derived from the reaction of a coordinated ligand
through a deinsertion reaction (Equation 6)
Synthesis of carbonyl complexes

Direct reaction of the metal

– Not practical for all metals due to need for harsh
conditions (high P and T)

– Ni + 4CO Ni(CO)4
– Fe + 5CO Fe(CO)5
Reductive carbonylation
– Useful when very aggressive conditions would be
required for direct reaction of metal and CO

» Wide variety of reducing agents can be used
– CrCl3+ Al + 6CO  AlCl3 + Cr(CO)6
– 3Ru(acac)3 + H2 + 12CO  Ru3(CO)12 +
N.B. From the carbonyl complex we can synthesize other derivatives
Main characterization methods:

• Xray diffraction Þ (static) structure Þ bonding
• NMR Þ structure en dynamic behaviour
• EA Þ assessment of purity
• (calculations)
Useful on occasion:
• IR
• MS
Not used much:
• GC
• LC
              IR spectra and metal-carbon bonds
The υCO stretching frequency of the coordinated CO is very
Recall that the stronger a bond gets, the higher its stretching
M=C=O (C=O is a double bond) canonical structure
Lower the υCO stretching frequency as compared to the M-C≡O
  structure (triple bond)
Note: υCO for free CO is 2041 cm-1)

      [Ti(CO)6]2- [V(CO)6]- [Cr(CO)6] [Mn(CO)6]+ [Fe(CO)6]2+

υCO    1748      1858      1984      2094     2204 cm-1

      increasing M=C double            decreasing M=C double
              bonding                         bonding
          Bridging versus terminal carbonyls

Bridging CO groups can be regarded as having a double bond
C=O group, as compared to a terminal C≡O, which is more like a
triple bond:
                             ~ double bond     the C=O group
  ~ triple bond                                in a bridging
                                               carbonyl is more
                       M                       like the C=O in
    M-C≡O                   C=O                a ketone, which
                                               typically has
                                               υC=O = 1750 cm-1

terminal carbonyl        bridging carbonyl
(~ 1850-2125 cm-1)     (~1700-1860 cm-1)

   Bridging CO between 1700 and 2200 cm-1
Bridging versus terminal carbonyls in [Fe2(CO)9]
                         OC    Fe
                                    C O
                              OC             CO


        terminal                        bridging
        carbonyls                       carbonyls

1. As the CO bridges more metal centers its stretching
frequency drops – same for all p ligands
– More back donation
2. As the metal center becomes increasingly electron rich the stretching
frequency drops
Alkene ligands

Dewar-Chatt-Duncanson model

The greater the electron density
back-donated into the p* orbital on
the alkene, the greater the reduction
in the C=C bond order

Stability of alkene complexes also
depends on steric factors as well

An empirical ordering of relative
stability would be:
tetrasubstituted < trisubstituted <
trans-disubstituted < cis-
disubstituted < monosubstituted <
Alkyne ligands:

Similar to alkenes
Alkynes tend to be more
electropositive-bind more
tightly to a transition metal
than alkenes -alkynes will
often displace alkenes

Difference is 2 or 4 electron
sigma-type fashion (A) as
we did for alkenes,
including a pi-backbond (B)

The orthogonal set can also
bind in a pi-type fashion
using an orthogonal metal
d-orbital (C)
The back-donation to the antibonding orbital (D) is a delta-bond-the
degree of overlap is quite small - contribution of D to the bonding of
alkynes is minimal
 The net effect p-donation - alkynes are usually non-linear
 in TM complexes

 Resonance depict the bonding of an alkyne.
I is the metallacyclopropene resonance form

Support for this versus a simple two electron donor, II,
can be inferred from the C-C bond distance as well the R-
C-C-R angles

III generally does not contribute to the bonding of alkyne
Ally ligands:

Allyl ligands are ambidentate ligands that can bind in both a
monohapto and trihapto form The trihapto form can be expressed as
a number of difference resonance forms as shown here for an
unsubstituted allyl ligand: Important applications
Dihydrogen Ligands:

Metal is more electropositive than hydrogen

Hydrogen acts as a two electron sigma donor to the metal center.

The complex is an arrested intermediate in the oxidative addition of dihydrogen

How does this affect the oxidation state of the
Dihydrogen complexes Bonding is “simple” a 3C-2electron bond.

H2 - neutral two electron sigma donor

One could also describe a back-donation of electrons from a filled metal
orbital to the sigma-* orbital on the dihydrogen
Electronic Attributes of Phosphines

Like that of carbonyls

As electron-withdrawing sigma-donating capacity decreases

At the same time, the energy of the p-acceptor (sigma-*) on phosphorous is
lowered in energy, providing an increase in backbonding ability.

Therefore, range of each capabilities –tuning rough ordering -CO stretching
frequency indicator- low CO stretching frequency- greater backbonding to M

Experiments such as this permit us to come up with the following empirical
Cone Angle (Tolman)
                                       Phosphine     Cone
Steric hindrance:
                                       Ligand        Angle
A cone angle of 180 degrees -
effectively protects (or covers) one
half of the coordination sphere of     PH3           87o
the metal complex                      PF3           104o
                                       P(OMe)3       107o
                                       PMe3          118o
                                       PMe2Ph        122o
                                       PEt3          132o
                                       PPh3          145o
                                       PCy3          170o
                                       P(t-Bu)3      182o
                                       P(mesityl)3   212o
You would expect a dissociation event
to occur first before any other reaction
-steric bulk (rate is first order
-increasing size)

This will also have an effect on
activity for catalysts

N.B. “flat” can slide past each other

For example Wilkinson's catalyst
(more later)

Has a profound effect on the
Reaction chemistry of complexes
Three general forms:
1. Reactions involving the gain and loss of ligands
      a. Ligand Dissoc. and Assoc. (Bala)
      b. Oxidative Addition
      c. Reductive Elimination
      d. Nucleophillic displacement
2. Reactions involving modifications of the ligand
      a. Insertion
      b. Carbonyl insertion (alkyl migration)
      c. Hydride elimination (equilibrium)
3. Catalytic processes by the complexes
      Wilkinson, Monsanto
      Carbon-carbon bond formation (Heck etc.)
a) Ligand dissociation/association (Bala)

 • Electron count changes by -/+ 2

 • No change in oxidation state

 • Dissociation easiest if ligand stable on its own
    (CO, olefin, phosphine, Cl-, ...)

 • Steric factors important

                  M                            M      + Br-
b) Oxidative Addition
Basic reaction:

                          X                X
                  LnM +             LnM
                          Y                Y

• Electron count changes by +/- 2
  (assuming the reactant was not yet coordinated)
• Oxidation state changes by +/- 2
• Mechanism may be complicated The new M-X and M-Y bonds are
  formed using:
• the electron pair of the X-Y bond
• one metal-centered lone pair
One reaction multiple mechanisms

Concerted addition, mostly with non-polar X-Y bonds
   H2, silanes, alkanes, O2, ...

    Arene C-H bonds more reactive than alkane C-H bonds (!)

                     X                    X                 X
   LnM           +              LnM                   LnM
                     Y                    Y                 Y
Intermediate A is a s-complex

Reaction may stop here if metal-centered lone pairs
are not readily available

Final product expected to have cis X,Y groups
Stepwise addition, with polar X-Y bonds
    – HX, R3SnX, acyl and allyl halides, ...

    – low-valent, electron-rich metal fragment (IrI, Pd(0), ...)

      LnM         X Y             LnM X Y                  LnM

Metal initially acts as nucleophile

    – Coordinative unsaturation less important

Ionic intermediate (B)

Final geometry (cis or trans) not easy to predict

Radical mechanism is also possible
Cis or trans products depends on the mechanism
                                                 OC     Ir     H          Ir(III)
                                                 Et3P           cis

                 OC     Ir Cl                    OC     Ir      I         Ir(III)
                        Ir(I)                           Cl          cis

                                      CH3Br      OC     Ir      Cl        Ir(III)

                                                        Br      trans
c) Reductive elimination
This is the reverse of oxidative addition - Expect cis elimination
Rate depends strongly on types of groups to be eliminated.

Usually easy for:
• H + alkyl / aryl / acyl
   – H 1s orbital shape, c.f. insertion

• alkyl + acyl

   – participation of acyl p-system
• SiR3 + alkyl etc

Often slow for:
• alkoxide + alkyl
• halide + alkyl
    – thermodynamic reasons?

     We will do a number of examples of this reaction
Relative rates of reductive elimination
L         CH3                                 L           CH3
    Pd             + solv                         Pd                     LPd(solv) + CH3   CH3
L         CH3                              solv           CH3

     Complex                        Rate Constant (s-1)          T(oC)

     Ph3P             CH3
               Pd                       1.04 x 10-3              60
     Ph3P             CH3

     MePh2P              CH3
                    Pd                  9.62 x 10-5              60
     MePh2P              CH3

     Ph       Ph
          P          CH3
               Pd                       4.78 x 10-7              80
          P          CH3
     Ph       Ph

                         Most crowded is the fastest reaction
  Special case:
  Nucleophilic Attack on a Coordinated CO acyl anion

                                                             Fisher carbene

This is Fischer carbene It has a metal carbon double bond
Such species can be made for relatively electronegative
metal centers N.B. mid to late TMs
Fischer carbenes are susceptible to nucleophilic attack at
the carbon
Fischer carbenes act effectively as σ donors and p acceptors
The empty antibonding M=C  orbital is primarily on the carbon making it
susceptible to attack by nucleophiles

Other type is called a Shrock carbene (alkylidene)

Characteristic           Fischer-type            Schrock-type
Typical metal (Ox.       Middle to late T.M.     Early T.M.
State)                   Fe(0), Mo(0) Cr(0)      Ti(IV), Ta(V)
Substituents             At least one highly     H or alkyl
attached to carbene      electronegative
carbon                   heteroatom
Typical other            Good p acceptors        Good s and p
ligands                                          donors
Electron count           18                      10-18
Nucleophilic displacement
Ligand displacement can be described as nucleophilic substitutions
O.M. complexes with negative charges can behave as nucleophiles
in displacement reactions Iron tetracarbonyl (anion) is very useful
                                                             R       R'
                                               R'X                            O
                                                             O2           R       OH
     [Fe(CO)4]2-       RX         [ R   Fe(CO)4]-

                                                        X2               O
                                              H+                  R          X
                            CO                     R H

                              O                              R
                        [ R       Fe(CO)4]-        H+                O
Modifications of the ligand
a) Insertion reactions
Migratory insertion!
The ligands involved must be cis - Electron count changes by -/+ 2
No change in oxidation state
If at a metal centre you have a s-bound group (hydride, alkyl, aryl)
a ligand containing a p-system (olefin, alkyne, CO) the s-bound
group can migrate to the p-system
1. CO, RNC (isonitriles): 1,1-insertion
2. Olefins: 1,2-insertion, b-elimination

     R                                            R
 M                  M                         M                 M
     CO                       R
             1,1       O                                  1,2
1,1 Insertion

The s-bound group migrates to the p-system
if you only see the result, it looks like the p-system has inserted into the M-X
bond, hence the name insertion

To emphasize that it is actually (mostly) the X group that moves, we use the
term migratory insertion (Both possible tutorial)
The reverse of insertion is called elimination
Insertion reduces the electron count, elimination increases it
Neither insertion nor elimination causes a change in oxidation state
a- elimination can release the “new” substrate or compound
In a 1,1-insertion, metal and X group "move" to the same atom of the inserting
The metal-bound substrate atom increases its valence

               Me         M                      Me         M
                                             M                    S Me
               CO           O                    SO2         O O

CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion

1,2 insertion (olefins)

Insertion of an olefin in a metal-alkyl bond produces a new alkyl

Thus, the reaction leads to oligomers or polymers of the olefin

• polyethene (polythene)
• polypropene
Standard Cossee mechanism

  M               M               M                M
      R               R               R

Why do olefins polymerise?
Driving force: conversion of a p-bond into a s-bond
    One C=C bond: 150 kcal/mol
    Two C-C bonds: 2´85 = 170 kcal/mol
    Energy release: about 20 kcal per mole of monomer
    (independent of mechanism)

Many polymerization mechanisms
   Radical (ethene, dienes, styrene, acrylates)
   Cationic (styrene, isobutene)
   Anionic (styrene, dienes, acrylates)
   Transition-metal catalyzed (a-olefins, dienes, styrene)
b Hydride elimination (usually by b hydrogens)
Many transition metal alkyls are unstable (the reverse of insertion)
the metal carbon bond is weak compared to a metal hydrogen
Bond Alkyl groups with β hydrogen tend to undergo β elimination

                 M -CH2-CH3  M - H + CH2=CH2

Two examples
A four-center transition state in which the hydride is transferred to the metal
An important prerequisite for beta-hydride elimination is the presence of an
open coordination site on the metal complex - no open site is available - displace
a ligand metal complex will usually have less than 18 electrons, otherwise a 20
electron olefin-hydride would be the immediate product.

 To prevent beta-elimination from taking place, one can use alkyls that:
 Do not contain beta-hydrogens
 Are oriented so that the beta position can not access the metal center
 Would give an unstable alkene as the product
Catalysis (homogeneous)
Reduction of alkenes etc.
The size of the substrate has an effect on the rate of reaction
Same reaction different catalyst
Alternative starting material
The Monsanto acetic acid process

Methanol - reacted with carbon monoxide in the presence of a catalyst to afford
acetic acid

Insertion of carbon monoxide into the C-O bond of methanol

The catalyst system - iodide and rhodium

Iodide promotes the conversion of methanol to methyl iodide,
Methyl iodide - the catalytic cycle begins:

1. Oxidative addition of methyl iodide to [Rh(CO)2I2]-
2. Coordination and insertion of CO - intermediate 18-electron acyl complex
3. Can then undergo reductive elimination to yield acetyl iodide and regenerate
   our catalyst
Catvia Process
Wacker process (identify the steps)
Identify the steps

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