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

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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

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

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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