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Ligand Exchange Mechanisms of Transition Metal ComplexesPart 1
Chapter 26
Ligand Exchange Mechanisms of Transition Metal ComplexesPart 1
Chapter 26
2
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
3
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
4
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.
5
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
6
1A. Kinetics ≠ ThermodynamicsResidence time forH2O molecule infirst hydration shell
Kinetically LabileKinetically Inert
7
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.
8
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╪
9
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╪
10
1B. Types of substitution mechanismsII. Involving no intermediate formation
Energy
Reaction Coordinate
MLnX + Y
MLnY + X
TS: Transition State
TS
∆G╪
11
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
12
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╪
13
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╪
14
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╪
15
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)
16
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.
17
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.
18
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
19
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
20
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]
21
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
22
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.
23
2C. Stereoretentive reaction
24
2C. Stereoretentive reaction
SquarePyramid
SquarePyramid
TrigonalBipyramidal
Berry Pseudorotation
25
2C. Stereoretentive reaction
TrigonalBipyramidal
All three can engage in pi interaction
26
2C. Stereoretentive reaction
Energy
Reaction Coordinate
C
A
B D
E
27
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
28
2C. Stereoretentive reaction
Energy
Reaction Coordinate
C
To increase the rate of the reaction: Stabilize the transition state
A
B D
E
29
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
30
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)
31
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)
32
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-
33
2D. Decrease Ea
II. Stabilize the transition state/intermediate
Energy
Reaction Coordinate
C
A
B D
E
34
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
35
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