Understanding the Absorption Electronic Spectra ofCoordination Compounds using Crystal Field Theory

Chapter 20

Understanding the Absorption Electronic Spectra ofCoordination Compounds using Crystal Field Theory

Chapter 20

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Review of the Previous Lecture

1. Used Molecular Orbital Theory and Group Theory to evaluate the covalency in metal ligand interactions

2. Explained the origins of the spectrochemical series by considering both and π metal ligand interactions donor ligands π donor ligands π acceptor ligands

3. Provided context for the 18 electron rule

4. Defined metal-ligand π back bonding with π acceptor ligands like CO Trans influence

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1. Electronic spectroscopy pertaining to d-orbital e- (Introduction)

Typically occur in the visible range of light.

A. The Spectrochemical Series

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I- < Br - < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O < [NCS]- < NH3 < en < [CN]- ~ CO

Weak field ligands Strong field ligandsLigands increasing Δoct

Small Δ High spin π donors

Large Δ Low spin π acceptors

σ donor

Cobalt (III) Octahedral Complexes; d6

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Weak field vs Strong field extremities dictated by ligand coordination.

Small ΔoctWeak Field, High spin

S = 2

Big ΔoctStrong Field, Low spin

S = 0

Δoct

ΔoctE

t2g

eg

t2g

eg

d-d electronic transition and Δoct

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Δoct = E = hν = h c λ̶

Δoct , λ

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3+

I- < Br - < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O < [NCS]- < NH3 < en < [CN]- ~ CO

L = CN- L = NH3 L = H2O

Cobalt (III) Octahedral Complexes; d6

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B. Electronic transition selection rules

1. Spin rule: Transitions must not involve a change of spin.

ΔS = 0

2. Laporte rule: Transitions must have a change of parity.

Gerade Ungerade

No gerade gerade nor ungerade ungerade

Technically all d-d electronic transitions are Laporte forbidden

gerade gerade

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I. Spin forbidden d-d e- transitions

[Mn(H2O)6]2+ is high-spin d5 Mn(II):

S = 2.5 S = 1.5

t2g

eg

Extremely weak transitions; ε < 1 M-1cm-1

RECALL Beer’s Law: A = ε b c

ε is a property of the absorbance of a chemical species A measure of the intensity of an electronic transition

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II. Spin-allowed d-d e- transitions

A. For ideal octahedral complexes, d-d e- transitions that are spin-allowed but Laporte forbidden

Extremely weak transitions; ε = 10 - 200 M-1cm-1

i.e. [Co(H2O)6]3+ is high-spin d6 Co(III):

S = 2 S = 2

t2g

eg

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II. Spin-allowed d-d e- transitions

B. For ideal tetrahedral complexes, d-d e- transitions are spin-allowed but “less” Laporte forbidden than octahedral complexes because they are non-centrosymmetric complexes.

ε = 100 – 1,000 M-1cm-1

i.e. [NiCl4]2- is high-spin d8 Ni(II):

S = 1 S = 1e

t2 t2

e

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2. Charge transfer electronic transitionsCharge transfer e- transitions are Spin and Laporte allowed; ε > 1,000 M-1cm-1

Typically observed in the UV range of light whereas d-d electronic transitions are typically observed in the Visible range of light.

A. Ligand to metal charge transfer (LMCT)

Transfer of an electron from an orbital with primarily ligand character to one with primarily metal character

Can be observed with π donor ligands

M Le-

Revisit Molecular Orbital Diagram including π interaction with Weak Field Ligands

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[CoF6]3-

Co3+ ; d6

High spin, S = 2

Revisit Molecular Orbital Diagram including π interaction with Weak Field Ligands

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[CoF6]3-

Co3+ ; d6

High spin, S = 2

LMCT Bands

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2. Charge transfer electronic transitions

B. Metal to ligand charge transfer (MLCT)

Transfer of an electron from an orbital with primarily metal character to one with primarily ligand character

Can be observed with π acceptor ligands

M Le-

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Revisit Molecular Orbital Diagram including π interaction with Strong Field Ligands [Co(CO)6]3+

Co3+ ; d6

Low spin, S = 0

MLCT Bands

LMCT Bands

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3. Solution preparations to observe e- transitions

1. Concentrated solutions of coordination complexes need to be made to observe d-d e- transitions• Lower energy transitions• ε < 1,000 M-1cm-1

• mM concentrations

2. Dilute solutions of coordination complexes need to be made to observe charge transfere- transitions• Higher energy transitions• ε > 1,000 M-1cm-1

• μM concentrations