December 1 Lecture Ligand Exchange Mechanisms of ......Square Pyramidal Octahedral Pentagonal Bipyramidal Let us invoke crystal field theory to rationalize mechanisms of ligand exchange

Ligand Exchange Mechanisms of Transition Metal ComplexesPart 2

Chapter 26

Ligand Exchange Mechanisms of Transition Metal ComplexesPart 2

Chapter 26

2

3. Substitution in octahedral complexesA. Water exchange

[M(OH2)6]n+ + H218O [M(OH2)5(18OH2)]n+ + H2O

k

Class k(s-1) MetalsI 108 Alkali, Alkaline earth

II 105 - 108 Mg2+

Transition Metals with 2+ ChargeLanthanide Metals with 3+ Charge

Low CFSEIII 1 – 104 Transition Metals with 3+ Charge

High CFSEIV 10-9 - 10-1 Cr3+, Ru3+, Pt2+, Co3+

Very High CFSE

3

Water ExchangeResidence time forH2O molecule infirst hydration shell

Kinetically LabileKinetically InertIIIIIIIV

4

3B. Mechanism: Dissociation vs Association

Dissociation Association

Square Pyramidal Octahedral Pentagonal Bipyramidal

Let us invoke crystal field theory to rationalize mechanisms of ligand exchangefor octahedral complexes.

5

3B. Mechanism: Dissociation vs AssociationDissociation Association

6


Class IV metals:i.e. Cr3+ d3

7



CFSE

C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 Δoct

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CFSE

C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 ΔoctC.N. = 5 = [-0.457*2 + -0.086*1)] Δoct = -1.0 Δoct

Gain 0.2 Δoct; Loss in stability

Dissociation

9



CFSE

C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 ΔoctC.N. = 7 [-0.528*2 + 0.282*1] Δoct = -0.7 Δoct


Association

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CFSE

C.N. = 6 (Oh) = (-0.4*3+ 0.6*0) Δoct = -1.2 Δoct

C.N. = 5 = [-0.457*2 + -0.086*1)] Δoct = -1.0 ΔoctC.N. = 7 [-0.528*2 + 0.282*1] Δoct = -0.7 Δoct

Ligand exchange is inert because bothmechanisms result in a high barrier.

DissociationAssociation

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Class II metals:i.e. Cu2+ d9

CFSE

C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 Δoct

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CFSE

C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 5 = [-0.457*4 + -0.086*2 + 0.086*2 + 0.914*1]Δoct = -0.91Δoct

Loss 0.31 Δoct; Gain in stability

Dissociation

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CFSE

C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 7 = [-0.528*4 + 0.282*4 + 0.493*1]Δoct = -0.491Δoct


Association

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CFSE

C.N. = 6 (Oh) = (-0.4*6 + 0.6*3) Δoct = -0.6 ΔoctC.N. = 5 = [-0.457*4 + -0.086*2 + 0.086*2 + 0.914*1]Δoct = -0.91Δoct

Loss 0.31 Δoct; Gain in stability Dissociation is preferred.

Also a Z-out Jahn Teller distortion occurs for these types ofmetals, which makes dissociation at axial position easier.

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3B. Mechanism: Dissociation vs Association (In general)Dissociation Association

A comparable analysis to that of crystal field theory can be done withmolecular orbital theory.

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3B. Mechanism: Dissociation vs Association (In general)

LGO electrons go into these M.O.s

E

a1g

t1u

eg

t2g

eg*

a1g*

t1u*

t2geg

a1g

t1u

p orb

s orb

d orb

t1uegSigma LGOs

a1g


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E

a1g

t1u

eg

t2g

eg*

a1g*

t1u*

t2geg

a1g

t1u

p orb

s orb

d orb

t1uegSigma LGOs

a1g


I. Metals are labile if

eg* is populated

(Dissociative mech)

t2gn has n < 3

(Associative mech)

Δoct

Metal d orbitalElectrons go into

these M.O.s

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E

a1g

t1u

eg

t2g

eg*

a1g*

t1u*

t2geg

a1g

t1u

p orb

s orb

d orb

t1uegSigma LGOs

a1g


Δoct


these M.O.s

II. Metals are inert if

eg* is not populated

t2gn has n = 3-6

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E

a1g

t1u

eg

t2g

eg*

a1g*

t1u*

t2geg

a1g

t1u

p orb

s orb

d orb

t1uegSigma LGOs

a1g


Δoct


these M.O.s

III. If no CFSE (i.e. d10 ;high spin d5)

then k 1Z2

r

As oxidation stateincreases, k decreases As ionic radius

decreases, k decreases

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3C. A closer look at ligand exchange of octahedral complexes

For most ligand substitutions in octahedral complexes, experimental evidence supportsdissociative pathways.

But

at high [Y], the rate of substitution is independent of Y; suggests dissociative mechanism

at low [Y], the rate depends on Y and ML5X ; suggests associative mechanism

Is this a contradiction?

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3C I. Eigen-Wilkins Mechanism

An encounter complex is first formed between substrate and entering ligand in a pre-equilibriumstep. This is followed by loss of the leaving ligand in the rate determining step.

ML5X + Y {ML5X---Y} ML5Y + Xk1

k-1 Encounter complex

k2

Slow Step

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3C I. Eigen-Wilkins Mechanism

An encounter complex is first formed between substrate and entering ligand in a pre-equilibriumstep. This is followed by loss of the leaving ligand in the rate determining step.

ML5X + Y {ML5X---Y} ML5Y + Xk1

k-1 Encounter complex

k2

Slow Step

Rate = k2[{ML5X---Y}]

Cannot be directly measured

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k-1

ML5X + Y {ML5X---Y} ML5Y + Xk1 k2

Slow Step

Rate = k2[{ML5X---Y}] (A)K1 = [{ML5X---Y}] (B)

[ML5X] [Y]

[{ML5X---Y}] = K1[ML5X] [Y] (C)

[ML5X]initial = [ML5X] + [{ML5X---Y}] (D)

= [ML5X] + K1[ML5X] [Y]

= [ML5X] (1 + K1[Y])

[ML5X] = [ML5X]initial (E)

1 + K1[Y]

Rate = k2 K1[ML5X] [Y] (F)

= k2 K1[ML5X]initial [Y]

1 + K1[Y]

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Rate = k2 K1[ML5X]initial [Y]

1 + K1[Y]

k-1


Slow Step

at low [Y], where K1[Y] <<1


Rate = kobs [ML5X]initial [Y]

K1 can be estimated theoretically

The rate depends on Y and ML5X ; suggests associative mechanism

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1 + K1[Y]

k-1


Slow Step

at high [Y], where K1[Y] >> 1

The rate is independent of Y ; suggests dissociative mechanism


K1[Y]Rate = k2 [ML5X]initial

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3C II. Evidence for Dissociative (or Id)

i.e. Co3+

1. The rate of ligand substitution depends on X

The rate correlates with the M-X bond strength; the stronger the bond, the slower the rate

Consistent with the rate determining step involving bond breaking in a dissociative step

2. ΔV╪ is positive (volume of activation)

3. Bulky Ys favor this reaction mechanism because require more space for metal binding

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3C III. Evidence for Associative (or Ia)

i.e. Cr3+, Rh3+, Ir3+, Ti3+

1. Strong dependence on Y due to strong M-Y bond

2. ΔV╪ is negative

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3D. Ligand exchange is not stereoretentive

We will only focus on the D/Id pathways.

The dissociation step is limiting and a 5-coordinate intermediate must be involved

The stereochemistry of the product is independent of the leaving group (X) and dependson the structure of the intermediate.

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3D. Ligand exchange is not always stereoretentive

We will only focus on the D/Id pathways.

The dissociation step is limiting and a 5-coordinate intermediate must be involved

The stereochemistry of the product is independent of the leaving group (X) and dependson the structure of the intermediate.

cis-isomer Square Pyramidal

intermediate

cis-isomer(retention of

stereochemistry)

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3D. Ligand exchange is not always stereoretentive

cis-isomer

Trigonal bipyramidal intermediate cis-isomers trans-isomers

Documents

December 1 Lecture Ligand Exchange Mechanisms of ......Square Pyramidal Octahedral Pentagonal Bipyramidal Let us invoke crystal field theory to rationalize mechanisms of ligand exchange