LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION
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LECTURE 5: ORGANOMETALLIC REACTIONS ILIGAND SUBSTITUTION
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LIGAND SUBSTITUTION
H2O PdCl
Cl
H2O PdH
ClOH
H2O PdH
Cl O
Cl Pd ClCl
Cl 2--
Cl PdCl
Cl
H2O PdCl
OH
H2O PdCl
OH"-H elim"
- 2 e (CuCl2 CuCl)
C2H4 H2O
H2O PdCl
ClOH
-
ins
Cl-CH3CHO
H2O Pd ClH
Cl-
Pd(0) + H+ +H2O + 2 Cl-
OH-
- Cl-
-H elim
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Ligand Substitution 4
LIGAND SUBSTITUTION
Basic premise about metal-catalyzed reactions:
• Reactions happen in the coordination sphere of the metal
• Reactants (substrates) come in, react, and leave again
• Binding or dissociation of a ligand is oftenthe slow, rate-determining step
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LIGAND SUBSTITUTION
This premise is not always correct, but it appliesin the vast majority of cases.
Notable exceptions:
• Electron-transfer reactions
• Activation of a single substrate for external attack– peroxy-acids for olefin epoxidation– CO and olefins for nucleophilic attack
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2 MAIN MECHANISTIC PATHWAYS
ASSOCIATIVE (A):
LnML’ + L’’ slow LnML’L’’ fast LnML’’ + L’
DISSOCIATIVE (D):
L’’
LnML’ slow LnM fast LnML’’
+ L’
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Ligand Substitution 7
DISSOCIATIVE LIGAND SUBSTITUTION
Example:
Factors influencing ease of dissociation:• 1st row < 2nd row > 3rd row
• d8-ML5 > d10-ML4 > d6-ML6
• stable ligands (CO, olefins, Cl-) dissociate easily(as opposed to e.g. CH3, Cp).
LnM CO LnM LnM L'+ COL'
18 e 16 e 18 e
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Ligand Substitution 8
DISSOCIATIVE SUBSTITUTION at ML6
16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry.
The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques.
18-e
oct
16-e
SP distortedTBP
or
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Ligand Substitution 9
ASSOCIATIVE LIGAND SUBSTITUTION
Example:
Sometimes the solvent
is involved.Reactivity of cis-platin:
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e(NH3)2PtCl2 (NH3)2Pt(Cl)(Br)
(NH3)2Pt(Cl)(H2O)+
Br-
- Cl-
H2O - Cl- Br-- H2O
NucleoBase - H2O
(NH3)2Pt(Cl)(NB)+
slow
fast
fast- Cl-
slow
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trans influence and trans effect
• In square planar complexes, some ligands direct substitution to a position trans to themselves.
• When reaction is controlled by factors influencing the ground state energy of the complex – trans influence
• Reaction is controlled by factors affecting the transition state energy.
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trans influence
Ligands that form strong bonds or tend to weaken the metal ligand bond trans to the metal.
In the ground state this property is called the trans influence.
H- > PR3 > SCN- > I-, CH3-, CO, CN- > Br- > Cl- > NH3 >
OH-
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trans kinetic effect
• Tendency of certain ligands to direct incoming groups to trans position with reactions under kinetic control.
C2H4, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3> OH-
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overall trans effect
CO, CN-, NO C2H4 > PR3, H- > CH3-, S=C(NH2)2 > Ph-
NO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O
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Exercise 7.1
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Ligand Substitution 15
ASSOCIATIVE LIGAND SUBSTITUTION
Example:
Sometimes the solvent is involved.Reactivity of cis-platin:
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e
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rate
Rate = ks[ML4] = k1 [ML4][Y]
Mechanism:
Square pyramidal – trigonal bipyramid – with retention of configuration.
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Associative substitution with 18 e systems
• Can occur if the metal can delocalize a pair of electrons onto one of its ligands
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Ligand Substitution 18
LIGAND REARRANGEMENT
Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactionsof an apparently saturated complex:
M N
O
M N O
3-e 1-e
M M
3-e5-eMCO
RM
O
R
(1+2)-e 1-e
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Ligand Substitution 19
DISSOCIATIVE LIGAND SUBSTITUTION
Example:
Rate = k [ML6]
LnM CO LnM LnM L'+ COL'
18 e 16 e 18 e
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Rate of substitution of Ligands
Rate of substitutions of a particular ligand is a function of ligand type.
Ligands that are nuetral in their free state dissociate rather easily.
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Ligand Substitution 22
Redox-induced ligand substitutionUnlike 18-e complexes, 17-e and 19-e complexes are labile.
Oxidation and reduction can induce rapid ligand substitution.
• Reduction promotes dissociative substitution.
• Oxidation promotes associative substitution.
• In favourable cases, the product oxidizes/reducesthe starting material redox catalysis.
L'
LnM
LnM+ LnM L' +
LnM- Ln-1M- + L
- e-
+ e-18-e
17-e 19-e
19-e 17-e
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Ligand Substitution 23
Redox-induced ligand substitution
CO
L
Fe(CO)4L
Fe(CO)4L
Fe(CO)4
Fe(CO)5
Fe(CO)5
Initiation by added reductant.
Sometimes, radical abstractionproduces a 17-e species
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Ligand Substitution 24
Photochemical ligand substitution
Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (Ligand Field transition, LF).This often results in fast ligand dissociation.
Requirement: the complex must absorb, so it must have a colour!or use UV if the complex absorbs there
d
dh
M(CO)6