Lecture-November 24-Ligand Exchange Mechanisms of ......7 1B. Types of substitution mechanisms I....
Transcript of Lecture-November 24-Ligand Exchange Mechanisms of ......7 1B. Types of substitution mechanisms I....
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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 spectroscopy corresponding to d-d electron transitions Considered the atomic state of multielectron systems
3. Explained the use of Orgel and Tanabe Sugano Diagrams
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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╪
This component of the reaction coordinate plotconcerns the kinetics of ligand exchange. There is atleast one activation barrier that a metal complex mustovercome to be transformed into a different metalcomplex.
This component of the reaction coordinate plotconcerns the thermodynamics of ligand exchange.The driving force for the change of a metal complexinto another has to do with the new compoundhaving a lower potential energy than the startingcompound.
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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 Constant = 1.380649 x 10-23 JK-1
h : Planck’s Constant = 6.62607015 x 10-34 JsR: Universal Gas Constant = 8.3145 J mol-1K-1
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
Bond breaking dominates over bond formation.
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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
Bond formation dominates over bond breakage.
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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
k1k2
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 = kobs [ML3X]
Rate = (k1[Y] + k2) [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
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2C. Stereoretentive reaction
SquarePyramid
SquarePyramid
TrigonalBipyramidal
Berry Pseudorotation
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2C. Stereoretentive reaction
TrigonalBipyramidal
All three can engage in pi interaction
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2C. Stereoretentive reaction
Energy
Reaction Coordinate
C
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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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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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
M
TX
Y
If T is a π acceptor ligand (i.e. CO) then increase the electrophilicity of the metal center. Themetal center will accept electron density that the incoming nucleophilic ligand (Y) donatesto it.
e- e-π backbonding
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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)
Strong trans effect = strong donor + strong π acceptor