1. CHEM310
INORGANIC CHEMISTRY
Part 3

2. ORGANOMETALLIC CHEMISTRY
Introduction (types and rationale)
Molecular orbital (bonding) of CO, arrangement “in space”
or ligand types (hapticity)
3. 16 and 18 electron rule (learning to count)
Synthesis, steric effects and reactivity - Wilkinsons catalyst (part 1)
Characterisation IR nmr etc.
Applications (oxidative addition b elimination)

3. 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:
(AlMe3)2

4. 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

5. Strong preference for s-donor groups but Cp is often p-bound (deceptively like with transition metals)
Electropositive metals: often 3c-2e or 4c-2e hydrides/alkyls
Cp2Mg
Cp2Fe
(MeLi)4
(Me3Al)2

6. As a Nucleophile
Addition to polar C=X bonds
(C=O, C=N, CºN)
Substitution at sp2 carbon
(often via addition)

7. 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)

8. As a base
More prominent in polar solvents think of free R- acting as base
Elimination
mechanism can be more complex than this
Metallation
chelate effect more important than inductive effect!

9. b-hydrogen transfer mainly for Al:
for more electropositive elements, deprotonation and nucleophilic attack are faster
for less electropositive elements, often no reaction

10. Chemistry: structures, bonding and properties of molecules.
Transition metal compounds

11. Metal hydride complexes, e.g.
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.

12. Exercise. Which of the following compounds is an organometallic compound?
In general, metals in organometallic compounds include:
main group metals
transition metals
f-block metals
In this course, transition metals are our main concern.

13. 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

14. 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
1900
Accidentally produced while trying to make other compounds
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

15. 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

16. Common organometallic ligands

17. 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

18. * Organometallic chemistry is related to material sciences.
e.g. Organometallic Polymers
Small organometallic compounds:
Precursors to films for coating (MOCVD)
(h3-C3H5)2Pd -----> Pd film
CH3CC-Au-CNMe -----> Au film
Luminescent materials

19. * Biological Science. Organometallic chemistry may help us to understand some enzyme-catalyzed reactions.
e.g. B12 catalyzed reactions.

20. b). From academic point of view:
* Organometallic compounds display many unexpected behaviors- discover new chemistry- new structures e.g.
New reactions, reagents, catalysts, e.g.
Ziegler-Natta catalyst, Wilkinson catalyst
Reppe reaction, Schwartz's reagent
Sharpless epoxidation, Tebbe's reagent

21. 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

22. 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

23. 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 rules
For transition metals, we get the 18-electron rule
Structures which have this preferred count are called electron-precise
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 complexes)
Addition of 2e- from 5th ligand converts complex to 5 CN 18e- , marginally more stable

24. Predicting reactivity
Most likely associative

25. Predicting reactivity
20 e (Sterics!)
Most likely dissociative

26. "bond" to the allyl fragment
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 first
Use analogies ("PPh3 is similar to NH3").
Rewrite the structure properly before you start counting.
1 e
2 e
3 e
Pd = 10 Cl¾ = 1
P® = 2
allyl = 3
+ ¾¾
e-count 16
covalent bond
dative bond
"bond" to the allyl fragment

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

29. 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

30. 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

31. 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.

32. 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)

33. 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 kcal/mol with 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
                              

34. 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

35. 2. Metal-centered nucleophiles (i. e
2. Metal-centered nucleophiles (i.e. using R+ as a reagent) Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example:
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

36. 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
bonding
Similar for alkenes, acetylenes, phosphines, and dihydrogen.

37. 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 metals

38. 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)

39. 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 +

40. N.B. From the carbonyl complex we can synthesize other derivatives

41. 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
EPR
Not used much: GC LC

42. IR spectra and metal-carbon bonds
The υCO stretching frequency of the coordinated CO is very informative
Recall that the stronger a bond gets, the higher its stretching frequency
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 cm-1
increasing M=C double
bonding
decreasing M=C double
bonding

43. 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: M
M-C≡O C=O
the C=O group
in a bridging
carbonyl is more
like the C=O in
a ketone, which
typically has
υC=O = 1750 cm-1
~ double bond
~ triple bond
terminal carbonyl bridging carbonyl
(~ cm-1) (~ cm-1)
Bridging CO between 1700 and 2200 cm-1

44. Bridging versus terminal carbonyls in [Fe2(CO)9]

45. Summary
1. As the CO bridges more metal centers its stretching
frequency drops – same for all p ligands
– More back donation

46. 2. As the metal center becomes increasingly electron rich the stretching frequency drops

47. 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 < ethylene

48. 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 donor
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)

49. 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 complexes.
                                                                                                               

50. 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
                                                                                                                                 

51. 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 meta?                                                                               

52. 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

53. 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 ordering:
                                                                                                              

55. Cone Angle (Tolman)
Steric hindrance:
A cone angle of 180 degrees -effectively protects (or covers) one half of the coordination sphere of the metal complex
                                                                        
Phosphine Ligand
Cone Angle
PH3
87o
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

56. 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
reactivity!

57. 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)
Catalytic processes by the complexes
Wilkinson, Monsanto
Carbon-carbon bond formation (Heck etc.)

58. 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

59. b) Oxidative Addition Basic reaction:
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

60. 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 (!)
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

61. Stepwise addition, with polar X-Y bonds
HX, R3SnX, acyl and allyl halides, ...
low-valent, electron-rich metal fragment (IrI, Pd(0), ...)
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

62. Cis or trans products depends on the mechanism

63. 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

64. Relative rates of reductive elimination
Most crowded is the fastest reaction

65. 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

66. Fischer carbenes act effectively as σ donors and p acceptors
The empty antibonding M=C p 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. State)
Middle to late T.M.
Fe(0), Mo(0) Cr(0)
Early T.M.
Ti(IV), Ta(V)
Substituents attached to carbene carbon
At least one highly electronegative heteroatom
H or alkyl
Typical other ligands
Good p acceptors
Good s and p donors
Electron count 18
10-18

67. 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

68. Modifications of the ligand
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
CO, RNC (isonitriles): 1,1-insertion
2. Olefins: 1,2-insertion, b-elimination
1,2
1,1

69. 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

70. In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate.
The metal-bound substrate atom increases its valence
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

71. Standard Cossee mechanism
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)

72. 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

73. 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

74. Catalysis (homogeneous)
Reduction of alkenes etc.

75. The size of the substrate has an effect on the rate of reaction

76. Same reaction different catalyst

77. Alternative starting material

79. 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:
Oxidative addition of methyl iodide to [Rh(CO)2I2]-
Coordination and insertion of CO - intermediate 18-electron acyl complex
Can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst

80. Catvia Process

81. Wacker process (identify the steps)

82. Identify the steps