Chapter 4 Complexes of pbound Ligands

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Chapter 4
Complexes of p-bound Ligands
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Outline

• 1. Alkene complexes
• 2. Butadiene complexes
• 3. Alkyne complexes
• 4. Enyl Complexes
• 5. Complexes of the cyclic p-perimeters CnHn
• Metal p-complexes of Unsaturated ligands with
  heteroatoms
• 7. Characterization of organometallic compounds

• References and suggested readings
• The Organometallic Chemistry of the Transition
  Metals, Robert H. Crabtree, 3rd Edition, 2001,
  Chapters 5, 10.
• Organometallic Chemistry, G. O. Spessard, G. L.
  G. L. Miessler, Prentice-Hall New Jersey, 1997,
  Chapter 5.
• Organometallics, C. Elschenbrouch, A. Salzer,
  1989, Chapter 15.
• Organotransition Metal Chemistry, Akio Yamamoto,
  1986. Chapters 3.4-3.9, 6.1d.

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1. Alkene complexes
A. Structure and bonding of monoolefin complexes
Non-planar
Examples
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X2-Type
L-Type
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Bonding

• Q1. Which interaction is dominant?
• e- poor systems.
• e- rich systems

Factors favoring X2-type binding - strong donor
ligands - a net negative charge on the complex -
low-oxidation state metals
X2-Type
L-Type
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Q2. What are the effects of complexation?
d(C-C) u(CC) e- density on
C
Q3. Do you expect that the complexed olefins be
more reactive or less reactive towards
nucleophiles?
(a) (b) (c)
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Q4. Consider the complexes A below. Which of the
following olefins would you expect to have the
strongest Pt- olefin bond?
(a) CH2CH2 (b) CH3-CHCH2 (c) CH2CHCN
For low valent metals, e.g. Pt(0), Ni(0), Pd(0),
backbonding is important. Electron-deficient
olefin forms the more stable olefin complexes.
Q5. Which of the following olefin complexes have
a stronger Pt- olefin bond?
Usual binding strength of olefins
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Q6. Which of the following olefin complexes have
a stronger Pt- olefin bond?
Q7. Which of the following olefin complexes is
more stable?
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B. Preparation of olefin complexes
1. By addition to unsaturated complexes
2. From Substitution reactions LnM-L' olefin
----gt L' LnM-(olefin) Examples
precipitation
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3. Metal salt olefin reducing agents. e.g.
Mechanism ?
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Another example
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4. From alkyl and related species
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C. Reactivity of olefin complexes
1). Insertion reactions
Examples
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Examples
monomer
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3). Nucleophilic attack on coordinated olefins.
Examples
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Summary Reactions of olefin complexes.
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2. Butadiene complexes
A. Most common structures of butadiene complexes
1,3-Dienes can act as L2-type or LX2-type ligands
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Dienes have a relatively low-energy LUMO (p
orbital) and are better p-acceptors than
conventional alkene ligands. Metal?ligand
back-bonding is particularly strong in complexes
of early 4d and 5d transition metals which prefer
high oxidation states in organometallic compounds.
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Bonding?
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B. Synthesis of butadiene complexes
1. By substitution reactions. e.g.
2. Reaction of coordinated ligands. e.g.
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Other methods
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C. Reactions of butadiene complexes
Examples
18e
18e
16e
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18e
18e
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3. Alkyne complexes
A. Structure and bonding of alkyne complexes
R-CC-R can be simple h2-ligand or bridging
ligand. As simple h2-ligand, they can be 2e or
4e donors. e.g.
18e
18e
Q1. How can they be 2e or 4e donors?
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In terms of bonding
Alkynes are more electronegative, therefore are
better p-acceptors
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As bridging ligands
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Examples, structures of Alkyne complexes
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Q2. For each pair of the following complexes,
which one is more stable?
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B. Preparation of alkyne complexes similar to
olefin complexes
1. By substitution reactions e.g.
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Additional examples
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2. By other methods e.g.
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Other methods
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C. Reactivity of alkyne complexes
Similar to olefins complexes, e.g.
Q1. Give the product for the following reaction.
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4. Enyl Complexes
Enyl Organic ligands with odd number of carbon
chains. Examples
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A. Allyl complexes
1. Structure of allyl complexes
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2. Preparation of allyl complexes
a. Metal salt main group organometallic
reagents.
Examples
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Examples
b). Low valent metal complexes allylhalide. E.g.
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Exercise 1.
c) Metal salts olefin base.
A previously mentioned example
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Question. What is the product for the following
reaction?
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Examples
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3. Reactivity of allyl complexes
a). With Nucleophiles
b). With electrophile (for h1-allyls)
c). Insertion reactions.
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Exercise. Predict the product of following
reactions.
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B. Examples of other enyl complexes and their
preparation
They can be prepared similar to allyl complexes.
Examples a). Metal salt main group
organometallic reagents.
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b). Transfer of H or H-
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5. Complexes of the cyclic p-perimeters CnHn
Common complexes of cyclic p-perimeters CnHn
Common structural types
a). Sandwich complexes, e.g.
Cr Mn Fe Co Ni
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c) Multidecker sandwich, e.g.
b) Half-sandwich complexes, e.g.
d). Complexes with tilted sandwich structures.
E.g.
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A. C3R3 as ligands
Common starting materials for preparing h3-C3R3
complexes?
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Examples and preparation.
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Examples and preparation.
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B. C4R4 as ligands
Q1. What are the common starting materials for
preparing M(h4-C4R4) complexes?
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Example and preparation.
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Example and preparation.
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Important chemical properties
a. Show aromatic properties. e.g.
b. Can be used as a source of C4R4.
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C. C5R5 as ligands
Common C5R5
Other important Cp-type ligands
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Binding mode of C5R5
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Common Types of Cp complexes
Examples Metallocenes
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Examples Bent Metallocenes
Metals of group 4 and the 2nd and 3rd rows of
groups 5-7 do not form stable Cp2M complexes.
However they do form stable metallocenes with
additional ligands attached to the metal center.
Examples Half-Sandwich Complexes
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A).How to prepare CpM complexes?
Common starting materials
Examples. 1). From a source of Cp - most common
route Cp- reagents NaCp, CpMgBr, TlCp, CpSnMe3
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2). From C5H6 or related ligands. e.g.
c). From a source of Cp. e.g.
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B. What are the typical chemical properties of
h5-Cp ligand?
--- h5-Cp ligand is usually unreactive.
--- The following reactions may occur to h5-Cp.
Electrophilic substitution-like benzene
Metallation reactions
Nucleophilic attack on Cp
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Examples.
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D. Arene complexes
1). Common structures -----

• As simple h6, h4, h2 ligands.

h6 - most common
18e
18e
16e
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As bridging ligands
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2). Common synthetic routes
a) From benzene
By substitution reactions
Benzene metal salts reducing agents

b) From reactions of coordinated ligands
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Exercise. Provide the reagents or product for the
following reactions.
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Additional examples
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3). Common Chemical reactions
Effect of complexation
Less reactive toward E, more reactive toward
Nu-.
Expected reactivity
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Examples
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E. C7H7 and C8H8 as ligands
C7H7 can be a h1, h3, h5 and h7 ligand. e.g.
18e
18e
18e
18e
C7H7 complexes can be made from
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C8H8 can be h2, h4, h6 and h8 ligands. e.g.
COT complexes can be made from
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Exercise Suggest the products or reagents for
the following reactions.
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6. Metal p-complexes of unsaturated ligands with
heteroatoms.
Many unsaturated ligands with heteroatoms can
form complexes. Examples
18e
18e
18e
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5e donor
6e donor
4e donor
3e donor
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6e
6e
6e
5e
4e
Donor
Donor
6e
6e
6e
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Synthesis similar to organic ones. e.g.
Give the reagents or products for the following
preparations.
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7. Characterization of Organometallic Compounds.
Common Organometallic Functionalities.
How to characterize them?
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Common spectroscopic
methods Chemical analysis Composition MS Comp
osition and functional groups IR Functional
groups NMR Functional groups and
stereochemistry ESR Paramagnetic
compounds X-ray 3-D structures
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1). Characteristic IR frequencies of some common
organometallic groups.
2). NMR (1H, 13C) in characterization of
organometallics.
Chemical shifts gt functional groups. e.g.
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Spin-spin coupling gt stereochemistry
The signal of C(or H) may be split by X or M if
they are also NMR active. e,g. when X is P, M is
Rh or Pt. J(C-X)cis lt J(C-X)trans J(H-X)cis lt
J(H-X)trans
Exercise. Based on the spectra data, propose
structures for the following reactions.
IR(cm-1) 2023, 1950, 1940 1H NMR (d) 0.4
(d, 3H), 2.5(m, 1H) 3.1(t, 2H), 4.6(t, 2H),
5.6(tt, 1H)
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IR(cm-1) 1965, 1915, 1630 1H NMR (d) 2.6
(br, 3H), 4.52 (s, 6H)
IR(cm-1) 1975, 1930 1H NMR (d) 0.17 (s, 3H),
4.40 (s, 6H)