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Ligand Exchange Mechanisms of Transition Metal ComplexesPart 1
Chapter 26
Ligand Exchange Mechanisms of Transition Metal ComplexesPart 1
Chapter 26
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Review of the Previous Lecture
1. Discussed Ligand Field Theory
2. Reevaluated electronic spectroscopic that correspond with d-d electron transitionsconsidering the atomic state of multielectron system
3. Explained the use of Orgel and Tanabe Sugano Diagrams
4. Revisited charge transfer electron transitions by discussing them in the context ofmolecular orbital diagrams for coordination compounds
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1. Substitution Reactions
If ligand exchange occurs with t1/2 ≤ 1 min
• MLnX is kinetically labile; reacts rapidly
If ligand exchange occurs with t1/2 > 1 min
• MLnX is kinetically inert; reacts slowly
MLnX + Y MLnY + Xk
Leaving Group
Entering Group
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1A. Kinetics ≠ Thermodynamics
A complex can be stable but either labile or inert to ligand exchange.
A complex can be unstable but either labile or inert to ligand exchange.
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1A. Kinetics ≠ Thermodynamics A complex can be stable but either labile or inert to ligand exchange.
A complex can be unstable but either labile or inert to ligand exchange.
Water exchange rates typically used to dictate metal lability or inertness.
[M(OH2)x]n+ + H218O [M(OH2)x-1(18OH2)]n+ + H2O
k
Rate of water exchange = k[M(OH2)x]n+]
Forward Reaction
k (s-1) as a gauge of lability
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1A. Kinetics ≠ ThermodynamicsResidence time forH2O molecule infirst hydration shell
Kinetically LabileKinetically Inert
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1B. Types of substitution mechanismsI. Involving intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
I: IntermediateTS: Transition State
I
TS1 TS2
∆G╪
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1B. Types of substitution mechanismsI. Involving intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
I: IntermediateTS: Transition State
I
TS1 TS2 Dissociative:
MLnX MLn + X
Intermediate
MLn + Y MLnY
∆G╪
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1B. Types of substitution mechanismsI. Involving intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
I: IntermediateTS: Transition State
I
TS1 TS2 Associative:
MLnX + Y MLnXY
Intermediate
MLnXY MLnY + X
∆G╪
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TS
∆G╪
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TSInterchange (I) Mechanism:
MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X∆G╪ TS
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TSInterchange (I) Mechanism:
MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X
Dissociative interchange (Id):
Bond breaking dominates over bond formation.
∆G╪
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TSInterchange (I) Mechanism:
MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X
Associative interchange (Ia):
Bond formation dominates over bond breaking.
∆G╪
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TSHow to distinguish between associative anddissociative interchange?
∆G╪
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TS
∆G╪
Eyring Equation:-∆G╪
RTk = k’T e
h
k’ : Boltzmann Constanth : Planck’s Constant
Recall: ∆G╪ = ∆H╪ - T∆S╪
d(ln k) = - ∆V╪
dP RT
Can determine ∆H╪, ∆S╪, and ∆V╪ (Volume of activation)
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TS
∆G╪
If ∆S╪ and ∆V╪ are positive, dissociative interchange
Y + MLnX
Y MLn▪▪▪▪▪▪▪▪X
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1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TS
∆G╪
If ∆S╪ and ∆V╪ are negative, associative interchange
Y + MLnX
Y▪▪MLn▪▪X
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2. Substitution in square planar complexesA. A metal that is typically in a square planar orientation is Pt(II), d8
B. Substitution reactions for these complexes often proceed by associative mechanisms Typically a combination of normal associative and solvent-assisted associative
Associative:
ML3X + Y ML3XY
ML3XY ML3Y + X
Solvent-Assisted Associative:
ML3X + S ML3S + X
ML3S + Y ML3SY
ML3SY ML3Y + S
k1 k2
fast fast
fast
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Associative:
ML3X + Y ML3XY
ML3XY ML3Y + X
Solvent-Assisted Associative:
ML3X + S ML3S + X
ML3S + Y ML3SY
ML3SY ML3Y + S
k1 k2
fast fast
fast
Rate = -d[ML3X] = k1[ML3X][Y] + k2[ML3X]dt
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Associative:
ML3X + Y ML3XY
ML3XY ML3Y + X
Solvent-Assisted Associative:
ML3X + S ML3S + X
ML3S + Y ML3SY
ML3SY ML3Y + S
k1 k2
fast fast
fast
Rate = -d[ML3X] = k1[ML3X][Y] + k2[ML3X]dt
Under pseudofirst order conditions, Y large excess:
Rate = (k1[Y] + k2) [ML3X]Rate = kobs [ML3X]
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kobs = k1[Y] + k2
kobs
[Y]
Ya Yb Yc
y-intercept is k2 Not Y dependent
Slope is k1 Value is Y dependent Depends on nucleophilicity of Y Nucleophilicity, k1
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2C. Stereoretentive reaction
Mechanism of nucleophilic substitution (SN) in square planar complexes:
Point Group: D4h Considering only sigma interactions: a1g (s)
eu (px , py)b1g (dx2-y2 )
The entering ligand can interact with the empty metal pz orbital.
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2C. Stereoretentive reaction
SquarePyramid
SquarePyramid
TrigonalBipyramidal
Berry Pseudorotation
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2C. Stereoretentive reaction
Energy
Reaction Coordinate
C
To increase the rate of the reaction: Stabilize the transition state
A
B D
E
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2C. Stereoretentive reaction
Energy
Reaction Coordinate
C
To increase the rate of the reaction: Destabilize the ground state
A
B D
E
New ground
state
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2D. Decrease Ea
Energy
C
A
D
E
New ground
state
I. Destabilize the ground state
Trans Effect (Chernyaey, 1926): A labilization ofa ligand by another ligand trans to it
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2D. Decrease Ea
Trans Effect Series:
Ligands to the right of the series have an increasingly stronger trans labilizing effect.
(weak) F–, HO–, H2O <NH3 < py < Cl– < Br– < I–, SCN–, NO2–, SC(NH2)2, Ph–
< SO32– < PR3 < AsR3, SR2, H3C– < H–, NO, CO, CN–, C2H4 (strong)
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2D. Decrease Ea
Trans Effect Series:
(weak) F–, HO–, H2O <NH3 < py < Cl– < Br– < I–, SCN–, NO2–, SC(NH2)2, Ph–
< SO32– < PR3 < AsR3, SR2, H3C– < H–, NO, CO, CN–, C2H4 (strong)
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2D. Decrease Ea
Trans Effect Series:
(weak) F–, HO–, H2O <NH3 < py < Cl– < Br– < I–, SCN–, NO2–, SC(NH2)2, Ph–
< SO32– < PR3 < AsR3, SR2, H3C– < H–, NO, CO, CN–, C2H4 (strong)
Good donors have a stronger trans effect because they lower the electron density in thebond between the metal and the leaving group (X).
donor
e- e-
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2D. Decrease Ea
II. Stabilize the transition state/intermediate
Energy
Reaction Coordinate
C
A
B D
E
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2D. Decrease Ea
Trans Effect Series:(weak) F–, HO–, H2O <NH3 < py < Cl– < Br– < I–, SCN–, NO2
–, SC(NH2)2, Ph–
< SO32– < PR3 < AsR3, SR2, H3C– < H–, NO, CO, CN–, C2H4 (strong)
II. Stabilize the transition state/intermediate
1
2 M
TX
Y
If T is a π acceptor ligand (i.e. CO) then it will accept electron density that the incomingligand (Y) donates to the metal center.
e- e-
e-
π backbonding