Imperial CollegeLondon
Dr. Ed Marshall, M220, RCS [email protected]
Additional materials available on:www.ch.ic.ac.uk/marshall/3I3.html
Lecture notes also available on Blackboard
3I3 Slide 1
3I3 Advanced Organometallics
Lectures 1 - 4
Imperial CollegeLondonA question for you
What properties do you think are desirable for a catalyst?
• Cheap, robust and long-lived
• Low toxicity
• Lewis acidic metal centre – electronic unsaturation
• At least one vacant coordination site – coordinative unsaturation
• Variable oxidation states?
• Flexible metal-based frontier orbitals (energy, direction)
L MX
Large ligands (L) are often used to give coordinative (and electronic) unsaturation. If L bonds to M using a flexible mixture of orbitals, then M may also use a mixture of orbitals to bind to a substrate.
3I3-2
Imperial CollegeLondonThe next four lectures
Alkene and polyene ligands
Metal-carbon multiple bonds
Bonding, synthesis & reactivity
Alkene polymerisation
Olefin metathesis
3I3-3
Imperial CollegeLondonLearning objectives
1. Use simple MO theory to explain how a carbon-carbon p-cloud bonds to a metal.
2. To list methods used to synthesise metal complexes of alkenes and polyenes, and metal-carbon multiple bonds.
3. To describe typical reactions of these complexes.
4. To appreciate how polyene ligands may respond to the electronic needs of a metal, and how such a property is useful for catalysis.
5. To describe how cyclopentadienyl-based catalysts can be used to polymerise alkenes.
6. To outline the most important applications of olefin metathesis.
By the end of lecture 4, you should be able...
3I3-4
In order to get the most out of this course, it is worth making sure that you understand the following concepts…
• Crystal field theory versus molecular orbital theory
• LX ligand classifications
• How to count electrons and the 18 electron rule
• Metal-alkene bonding
Assumed Knowledge Imperial CollegeLondonAssumed knowledge
3I3-5
Section 1: Metal-alkene complexes
Imperial CollegeLondon3A Advanced Organometallics3I1 Advanced Organometallics
3I3-6
Note the similarity to CO ligands...
s-component: donation of C lone pair
p-component: backbonding into CO p*
s-component:C-C p → empty metal orbital
p-component:occupied metal d → empty C-C p*
The Dewar-Chatt-Duncanson Model of Metal-Alkene Bonding Imperial CollegeLondonDewar-Chatt-Duncanson model for metal-alkene bonding
3I3-7
C-C = 1.37 Å C-C = 1.43 Å
C-C = 1.49 Å C-C = 1.62 Å
C-C bond distance in ethene = 1.34 Å
Best Described as Metal-Alkenes or Metallacyclopropanes? Imperial CollegeLondon
H atoms nolonger planar
with the C-C bond
Metal-alkenes versus metallacyclopropanes
3I3-8
[Pt(C2H4)Cl3]2- versus [Pt(C2Cl4)(PPh3)2]
« Chem3D Embed » « Chem3D Embed »
No backbonding: “metal-alkene"
sp2 carbons
With backbonding: “metallacyclopropane"
sp3 carbons
The Concept Of Umpolung - Reversal Of Polarity Imperial CollegeLondon
2. Backbonding reduces d+ charge and reduces reactivity to nucleophiles
1. Free alkenes undergo electrophilic additions, but coordinated alkene ligands are susceptible
to nucleophilic attack
d+
The impact of metal coordination and backbonding on reactivity
Why sp3? Backbonding occurs to the p* antibonding orbital, therefore reducing the C-C bond order
3I3-10
Appendix: Synthesis & Reactivity of Polyene LIgands Imperial CollegeLondon
Two common methods:
1. Addition to electron poor metal centres / displacement of other L-ligands
2. Reduction of a metal complex in the presence of the neutral -ene ligand
16e- 18e-
Oxidation state: N Oxidation state: N-2
Synthesis of metal-alkene complexes
3I3-11
Synthesis Of Metal-Alkene Complexes Imperial CollegeLondon
1 (a) Addition to 16 electron species:
e.g. [Ir(CO)Cl(PPh3)2] + C60
[Ir(CO)Cl(PPh3)2C60]
16 e-
18 e-
1 (b) Displacement of other L-type ligands:
e.g. (h5-C5H5)2Zr(PMe3)2 + C2H4
(h5-C5H5)2Zr(C2H4)(PMe3)
18 e- 18 e-
Synthesis of metal-alkene complexes: examples
3I3-12
2. Reduction of a metal in the presence of an alkene
nbd = norbornadiene
Rh(III)
Synthesis Of Metal-Alkene Complexes Imperial CollegeLondon
e.g. (h5-C5H5)2TiCl2 + 2NaC2H4
Ti(IV) Ti(II)
(h5-C5H5)2Ti(C2H4)
Synthesis of metal-alkene complexes
e.g. RhCl3 + CH3CH2OH + CH3CHO
Rh(I)[(nbd)Rh(m-Cl)]2
3I3-13
Imperial CollegeLondonReactivity of metal-alkene complexes
Alkene ligands are often susceptible to nucleophilic attack
3I3-14
Imperial CollegeLondonSummary of section 1
3I3-15
1. Catalysis at a metal centre often requires a responsive metal (and therefore a responsive ligand set)
2. Binding an alkene to a metal often increases its susceptibility to nucleophilic attack
Most useful ligands are often those that can use different MOs to bind to a metal
Binding any organic fragment to a metal may activate it towards chemical modification
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