December 1 Lecture Ligand Exchange Mechanisms of ......Square Pyramidal Octahedral Pentagonal...
Transcript of December 1 Lecture Ligand Exchange Mechanisms of ......Square Pyramidal Octahedral Pentagonal...
Ligand Exchange Mechanisms of Transition Metal ComplexesPart 2
Chapter 26
Ligand Exchange Mechanisms of Transition Metal ComplexesPart 2
Chapter 26
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3. Substitution in octahedral complexesA. Water exchange
[M(OH2)6]n+ + H218O [M(OH2)5(18OH2)]n+ + H2O
k
Class k(s-1) MetalsI 108 Alkali, Alkaline earth
II 105 - 108 Mg2+
Transition Metals with 2+ ChargeLanthanide Metals with 3+ Charge
Low CFSEIII 1 – 104 Transition Metals with 3+ Charge
High CFSEIV 10-9 - 10-1 Cr3+, Ru3+, Pt2+, Co3+
Very High CFSE
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Water ExchangeResidence time forH2O molecule infirst hydration shell
Kinetically LabileKinetically InertIIIIIIIV
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3B. Mechanism: Dissociation vs Association
Dissociation Association
Square Pyramidal Octahedral Pentagonal Bipyramidal
Let us invoke crystal field theory to rationalize mechanisms of ligand exchangefor octahedral complexes.
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3B. Mechanism: Dissociation vs AssociationDissociation Association
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class IV metals:i.e. Cr3+ d3
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class IV metals:i.e. Cr3+ d3
CFSE
C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 Δoct
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class IV metals:i.e. Cr3+ d3
CFSE
C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 ΔoctC.N. = 5 = [-0.457*2 + -0.086*1)] Δoct = -1.0 Δoct
Gain 0.2 Δoct; Loss in stability
Dissociation
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class IV metals:i.e. Cr3+ d3
CFSE
C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 ΔoctC.N. = 7 [-0.528*2 + 0.282*1] Δoct = -0.7 Δoct
Gain 0.5 Δoct; Loss in stability
Association
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class IV metals:i.e. Cr3+ d3
CFSE
C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 Δoct
C.N. = 5 = [-0.457*2 + -0.086*1)] Δoct = -1.0 ΔoctC.N. = 7 [-0.528*2 + 0.282*1] Δoct = -0.7 Δoct
Ligand exchange is inert because bothmechanisms result in a high barrier.
DissociationAssociation
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class II metals:i.e. Cu2+ d9
CFSE
C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 Δoct
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class II metals:i.e. Cu2+ d9
CFSE
C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 5 = [-0.457*4 + -0.086*2 + 0.086*2 + 0.914*1]Δoct = -0.91Δoct
Loss 0.31 Δoct; Gain in stability
Dissociation
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class II metals:i.e. Cu2+ d9
CFSE
C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 7 = [-0.528*4 + 0.282*4 + 0.493*1]Δoct = -0.491Δoct
Gain 0.11 Δoct; Loss in stability
Association
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3B. Mechanism: Dissociation vs AssociationDissociation Association
Class II metals:i.e. Cu2+ d9
CFSE
C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 5 = [-0.457*4 + -0.086*2 + 0.086*2 + 0.914*1]Δoct = -0.91Δoct
Loss 0.31 Δoct; Gain in stability Dissociation is preferred.
Also a Z-out Jahn Teller distortion occurs for these types ofmetals, which makes dissociation at axial position easier.
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3B. Mechanism: Dissociation vs Association (In general)Dissociation Association
A comparable analysis to that of crystal field theory can be done withmolecular orbital theory.
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3B. Mechanism: Dissociation vs Association (In general)
LGO electrons go into these M.O.s
E
a1g
t1u
eg
t2g
eg*
a1g*
t1u*
t2geg
a1g
t1u
p orb
s orb
d orb
t1uegSigma LGOs
a1g
Dissociation Association
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3B. Mechanism: Dissociation vs Association (In general)
LGO electrons go into these M.O.s
E
a1g
t1u
eg
t2g
eg*
a1g*
t1u*
t2geg
a1g
t1u
p orb
s orb
d orb
t1uegSigma LGOs
a1g
Dissociation Association
I. Metals are labile if
eg* is populated
(Dissociative mech)
t2gn has n < 3
(Associative mech)
Δoct
Metal d orbitalElectrons go into
these M.O.s
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3B. Mechanism: Dissociation vs Association (In general)
LGO electrons go into these M.O.s
E
a1g
t1u
eg
t2g
eg*
a1g*
t1u*
t2geg
a1g
t1u
p orb
s orb
d orb
t1uegSigma LGOs
a1g
Dissociation Association
Δoct
Metal d orbitalElectrons go into
these M.O.s
II. Metals are inert if
eg* is not populated
t2gn has n = 3-6
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3B. Mechanism: Dissociation vs Association (In general)
LGO electrons go into these M.O.s
E
a1g
t1u
eg
t2g
eg*
a1g*
t1u*
t2geg
a1g
t1u
p orb
s orb
d orb
t1uegSigma LGOs
a1g
Dissociation Association
Δoct
Metal d orbitalElectrons go into
these M.O.s
III. If no CFSE (i.e. d10 ;high spin d5)
then k 1Z2
r
As oxidation stateincreases, k decreases As ionic radius
decreases, k decreases
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3B. Mechanism: Dissociation vs Association (In general)
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3C. A closer look at ligand exchange of octahedral complexes
For most ligand substitutions in octahedral complexes, experimental evidence supportsdissociative pathways.
But
at high [Y], the rate of substitution is independent of Y; suggests dissociative mechanism
at low [Y], the rate depends on Y and ML5X ; suggests associative mechanism
Is this a contradiction?
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3C I. Eigen-Wilkins Mechanism
An encounter complex is first formed between substrate and entering ligand in a pre-equilibriumstep. This is followed by loss of the leaving ligand in the rate determining step.
ML5X + Y {ML5X---Y} ML5Y + Xk1
k-1 Encounter complex
k2
Slow Step
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3C I. Eigen-Wilkins Mechanism
An encounter complex is first formed between substrate and entering ligand in a pre-equilibriumstep. This is followed by loss of the leaving ligand in the rate determining step.
ML5X + Y {ML5X---Y} ML5Y + Xk1
k-1 Encounter complex
k2
Slow Step
Rate = k2[{ML5X---Y}]
Cannot be directly measured
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k-1
ML5X + Y {ML5X---Y} ML5Y + Xk1 k2
Slow Step
Rate = k2[{ML5X---Y}] (A)K1 = [{ML5X---Y}] (B)
[ML5X] [Y]
[{ML5X---Y}] = K1[ML5X] [Y] (C)
[ML5X]initial = [ML5X] + [{ML5X---Y}] (D)
= [ML5X] + K1[ML5X] [Y]
= [ML5X] (1 + K1[Y])
[ML5X] = [ML5X]initial (E)
1 + K1[Y]
Rate = k2 K1[ML5X] [Y] (F)
= k2 K1[ML5X]initial [Y]
1 + K1[Y]
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Rate = k2 K1[ML5X]initial [Y]
1 + K1[Y]
k-1
ML5X + Y {ML5X---Y} ML5Y + Xk1 k2
Slow Step
at low [Y], where K1[Y] <<1
Rate = k2 K1[ML5X]initial [Y]
Rate = kobs [ML5X]initial [Y]
K1 can be estimated theoretically
The rate depends on Y and ML5X ; suggests associative mechanism
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Rate = k2 K1[ML5X]initial [Y]
1 + K1[Y]
k-1
ML5X + Y {ML5X---Y} ML5Y + Xk1 k2
Slow Step
at high [Y], where K1[Y] >> 1
The rate is independent of Y ; suggests dissociative mechanism
Rate = k2 K1[ML5X]initial [Y]
K1[Y]Rate = k2 [ML5X]initial
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3C II. Evidence for Dissociative (or Id)
i.e. Co3+
1. The rate of ligand substitution depends on X
The rate correlates with the M-X bond strength; the stronger the bond, the slower the rate
Consistent with the rate determining step involving bond breaking in a dissociative step
2. ΔV╪ is positive (volume of activation)
3. Bulky Ys favor this reaction mechanism because require more space for metal binding
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3C III. Evidence for Associative (or Ia)
i.e. Cr3+, Rh3+, Ir3+, Ti3+
1. Strong dependence on Y due to strong M-Y bond
2. ΔV╪ is negative
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3D. Ligand exchange is not stereoretentive
We will only focus on the D/Id pathways.
The dissociation step is limiting and a 5-coordinate intermediate must be involved
The stereochemistry of the product is independent of the leaving group (X) and dependson the structure of the intermediate.
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3D. Ligand exchange is not always stereoretentive
We will only focus on the D/Id pathways.
The dissociation step is limiting and a 5-coordinate intermediate must be involved
The stereochemistry of the product is independent of the leaving group (X) and dependson the structure of the intermediate.
cis-isomer Square Pyramidal
intermediate
cis-isomer(retention of
stereochemistry)
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3D. Ligand exchange is not always stereoretentive
cis-isomer
Trigonal bipyramidal intermediate cis-isomers trans-isomers