Lecture-May 2-Ligand Exchange Mechanisms of Transition ......4 1A. Kinetics ≠ Thermodynamics A complex can be stable but either labile or inert to ligand exchange. A complex can

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 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╪

8


Energy

Reaction Coordinate

MLnX + Y

MLnY + X


I

TS1 TS2 Dissociative:

MLnX MLn + X

Intermediate

MLn + Y MLnY

∆G╪

9


Energy

Reaction Coordinate

MLnX + Y

MLnY + X


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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Energy

Reaction Coordinate

MLnX + Y

MLnY + X


TSInterchange (I) Mechanism:

MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X∆G╪ TS

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Energy

Reaction Coordinate

MLnX + Y

MLnY + X



MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X

Dissociative interchange (Id):

Bond breaking dominates over bond formation.

∆G╪

13


Energy

Reaction Coordinate

MLnX + Y

MLnY + X



MLnX + Y Y▪▪▪▪MLn▪▪▪▪X MLnY + X

Associative interchange (Ia):

Bond formation dominates over bond breaking.

∆G╪

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Energy

Reaction Coordinate

MLnX + Y

MLnY + X


TSHow to distinguish between associative anddissociative interchange?

∆G╪

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Energy

Reaction Coordinate

MLnX + Y

MLnY + X


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)

16


Energy

Reaction Coordinate

MLnX + Y

MLnY + X


TS

∆G╪

If ∆S╪ and ∆V╪ are positive, dissociative interchange

Y + MLnX

Y MLn▪▪▪▪▪▪▪▪X

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Energy

Reaction Coordinate

MLnX + Y

MLnY + X


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


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


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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24


SquarePyramid

SquarePyramid

TrigonalBipyramidal

Berry Pseudorotation

25


TrigonalBipyramidal

All three can engage in pi interaction

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Energy

Reaction Coordinate

C

A

B D

E

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Energy

Reaction Coordinate

C

To increase the rate of the reaction: Stabilize the transition state

A

B D

E

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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




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2D. Decrease Ea




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–


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

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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–


Strong trans effect = strong donor + strong π acceptor

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Lecture-May 2-Ligand Exchange Mechanisms of Transition ......4 1A. Kinetics ≠ Thermodynamics A complex can be stable but either labile or inert to ligand exchange. A complex can