Ligand Reorganization and Electron Transfer Mechanisms of Transition Metal Complexes

Chapter 26

Ligand Reorganization and Electron Transfer Mechanisms of Transition Metal Complexes

Chapter 26

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Review of the Previous Lecture1. Discussed the difference between ligand exchange lability versus inertness

Categorized metal lability/inertness based on water exchange

2. Discussed ligand exchange general mechanisms involving intermediates and nointermediates

Association versus dissociation

Interchange processes

3. Explored the mechanisms of ligand exchange for square planar and octahedral complexes

Explained the importance of the Trans Effect for increasing the rate of ligand exchangefor inert d8 metal ions in a square planar geometry

Used CFSE arguments to explain ligand exchange lability and inertness

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1. Racemization

Right-handed Left-handedΛ

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1. Racemization

A. Intramolecular pathway (No bond breaking)

I. Bailar Twist

Right-handed Left-handed

Preferred by ligandswith small bite angles.

Transition StateTrigonal Prism

Λ

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1. Racemization

II. Rhombic Twist

Right-handed Left-handed

Transition StateTrigonal Prism

Λ

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1. Racemization

B. Partial Ligand Dissociation

Right-handed

Transition State

Trigonal bipyramidal or Square Planar

ΛLeft-handed

2. Electron-transfer processes

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Aox + e- → Ared Eo = x

Bred → Box + e- Eo = y

Eo = x + y

ΔGo = -nFEo

Taube’s two mechanisms: Outer and inner sphere electron transfer

2A. Outer sphere electron transfer

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No bonds are made or broken

Electron(s) transferred through space

2A. Outer sphere electron transfer

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Fe(H2O)63+ + Fe(H2O)62+ → Fe(H2O)62+ + Fe(H2O)63+

e-

ΔG = 0 because there is no driving force. Reactants and products are the same.

ΔG╪ = 33 kJ/mol There is an activation barrier.

Think about the bond lengths:

Fe-OH2 bond length is shorter than Fe-OH2

For the electron transfer to take place, there has to be both a bond length elongation and bondlength contraction. A vibration of the Fe-O and Fe-O is believed to occur to transiently make thelengths the same.

2A. Outer sphere electron transfer

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I. Process

i. Reactants come together creating an encounter complex

{ L-M -----M-L }

ii. Electron transfer process occurs

Franck-Condon approximation states that electronic transitions are far faster thanatomic/molecular motion.

Restriction: The energy for both reactants must become similar via the M-L bond lengthsbecoming similar for the electron transfer to occur.

n+ n+ - x e-

2A I. The process

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i. Reactants come together creating an encounter complex

{ L-M -----M-L }

ii. Electron transfer process occurs

Vibrational and electronic coupling

The rate of e- transfer will depend on the vibrational energy for the bond length changes

n+ n+ - x e-

2A I. The process

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{ L-M -----M-L }n+ n+ - x e-

Marcus-Hush Theory:

ΔG╪ = ΔGw╪ + ΔGo╪ + ΔGs╪ + RT ln k’ThZ

k’ = Boltzmann constant = 1.380649 x 10-23 JK-1

h = Planck constant = 6.6261 x 10-34 Js

Z = Effective collision frequency in solution ~ 1011 dm3 mol-1 s-1

Barrier for the encounter complex

Barrier for bond length changes

Solvent rearrangement

2A II. Requirements for outer sphere electron transfer

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i. Metals are inert to ligand exchange

ii. Bound ligands typically have no additional lone pairs.

The more lone pairs available on a ligand, the more it is capable of serving as a bridging ligand and facilitating an inner sphere electron transfer.

Example of a fast outer sphere electron transfer

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Ru(NH3)63+ + Ru(NH3)62+ Ru(NH3)62+ + Ru(NH3)63+Low spin Low spin

Ru3+-N Bond length: 2.104 Å Ru2+-N Bond length: 2.144 Å

∆ (Ru-N) = 0.040 Å; k = 820 M-1s-1 ; Relatively fast due to minimal bond length reorganization energy

k

Example of a slow outer sphere electron transfer

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Co(NH3)63+ + Co(NH3)62+ Co(NH3)62+ + Co(NH3)63+Low spin High spin

Co3+-N Bond length: 1.936 ÅCo2+-N Bond length: 2.114 Å

∆ (Co-N) = 0.178 Å; k = 10-6 M-1s-1

Very slow electron transfer due in part to bond length reorganization energy but, moreimportantly, due to changes in spin state that the metal ions have to undergo.

k

2B. Inner sphere electron transfer

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This process requires that a ligand bridge two metals and then facilitates electron transfer.

Requirement: The bridging ligand has multiple lone pairs and there is a labile metal ion.

2B I. Taube’s classic experiment in the 1950s

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a. The chloro ligand bridges the two metalsresulting in loss of an aqua ligand

[Co(NH3)5Cl]2+ + [Cr(H2O)6]2+3+ 2+

Inert Labile

[Co(NH3)5H2O]2+ + [Cr(H2O)5Cl]2+2+ 3+

Labile Inert

b. Electron transfer and ligand transfer

[Co(H2O)6]2+ + [Cr(H2O)5Cl]2+2+ 3+

c. Ligand exchange of the Co(II)

Taube won the Nobel Prize in 1983for his groundbreaking work onstudying the mechanism of electrontransfer

2B II. Rate of electron transfer depends on the bridging ligand

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Rate = kobs[oxidant][reductant]

Ligand kobs (M‐1s‐1) ObservationNH3 10‐5 One lone pairH2O 0.1 Two lone pairs

I‐ 106 Four lone pairs; “big ligand”

Oxalate (C2O42‐)

Even higherConjugated organic anion; 

coordinates each metal through two oxygen atoms

2B III. Remote vs adjacent attack

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Certain bridging ligands can bridge in different modalities, some of which are superior for electrontransfer.

i.e. SCN-

1 2

Better for conjugated ligands

Remote Adjacent

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