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CHEM2402/2912/2916 [Part 2]

A/Prof Adam BridgemanRoom: 222

Email: A.Bridgeman@chem.usyd.edu.au

www.chem.usyd.edu.au/~bridge_a/chem2

Office Hours: Monday 2-4pm, Wednesday 2-4pm

Other times by appointment or by chance

Ligand-Field Theory and Reaction Kinetics

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Where Are We Going…?

• Shriver & Atkins, ‘Inorganic Chemistry’, OUP, 4th Edition, 2006• Week 4: Electronic Spectroscopy

Jahn-Teller effect Electronic spectra of multi-electron atoms Nephelauxetic effect and the spectrochemical series Selection rules Charge transfer transitions

• Week 5: Bonding in Complexes, Metalloproteins and Materials π-Donor and π-Acceptor Ligands Metal-metal bonding in clusters and solids Role of ligand-field effects in electrochemistry

• Week 6: Kinetics and Ligand-Field Effects Differential rate laws Ligand substiution reactions

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By the end of week 4…

Ligand-field (‘d-d’) spectroscopy• be able to predict/explain number of bands for d1-d9 (high-spin)• be able to calculate oct for d1, d3, d4, d6, d7, d8 and d9

• be able to explain differences in band intensity (spin forbidden, orbitally forbidden, Laporte forbidden)

• be able to explain the appearance of charge transfer transitions• be able to explain and predict the occurance of the Jahn-Teller effect

and its consequences (structural, spectroscopic, reaction rates)

Resources• Slides for lectures 1-3• Shriver and Atkins “Inorganic Chemistry” Chapter 19 (4th Edition)

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Schedule

• Lecture 1: Electronic absorption spectroscopy Jahn-Teller effect and the spectra of d1, d4, d6 and d9 ions

• Lecture 2: Interpreting electronic spectraInterelectron repulsion and the nephelauxetic effect

• Lecture 3: Interpreting electronic spectraSelection rules and charge transfer transitions

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Revision – Ligand-Field Splitting

Mn+

• In the absence of any ligands, the five d-orbitals of a Mn+ transition metal ion are degenerate

• Repulsion between the d-electrons and ligand lone pairs raises the energy of each d-orbital

Mn+

LL

L

L

L

L

Mn+

JKB – Lecture 3, S & A 19.1

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LL

L

L

L

L

Mn+Mn+

LL

L

L

L

L

Revision – Ligand-Field Splitting

• Two of the d-orbitals point along x, y and z and are more affected than the average

• Three of the d-orbitals point between x, y and z and are affected less than the average

• The ligand-field splitting

JKB – Lecture 3, S & A 19.1

Mn+

LL

L

L

L

L

oct

eg

t2g

(eg)

(t2g)(oct)

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Electronic Spectra of d1 Ions

• A d1 octahedral complex can undergo 1 electronic transition• The ground state (t2g)1 comprises three degenerate arrangements• The excited state (eg)1 comprises two degenerate arrangements• The electronic transition occurs at oct

JKB – Lecture 3, S & A 19.3

eg

t2gt2g

ground state excited state

eg

oct

Ti3+(aq)

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Electronic Spectra of High Spin d4 Ions

• A high spin d4 octahedral complex can also undergo just 1 transition• The ground state (t2g)2(eg)1 comprises two degenerate arrangements• The excited state (t2g)2(eg)2 comprises three degenerate arrangements• The electronic transition occurs at oct

• No other transitions are possible without changing the spin

eg

t2gt2g

ground state excited state

eg

oct

Cr2+(aq)

JKB – Lecture 3, S & A 19.3

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Electronic Spectra of High Spin d6 and d9 Ions

• High spin d6 and d9 octahedral complexes can also undergo just 1 transition• The electronic transition occurs at oct

• No other transitions are possible changing the spin

ground state

d6

excited stateground state

d9

excited state

Fe2+(aq) Cu2+(aq)

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LL

L

L

L

L

Mn+

2

LL

L

L

L

L

Mn+

JKB – Lecture 6, S & A p467

oct

Effect of Distortion on the d-Orbitals

• Pulling the ligands away along z splits eg and lowers the energy of dz2

• It also produces a much smaller splitting of t2g by lowering the energy of dxz and dyz

• oct >>> 1 >> 2 eg

t2g

LL

L

L

L

L

Mn+

1

tetragonal elongation

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+½1

2

oct

Which Complexes Will Distort?

• Relative to average: t2g go down by 0.4oct in octahedral complex

• Relative to average: eg go up by 0.6oct in octahedral complex

• Relative to average dz2 is stablilized by ½1 and dx

2-y

2 is destablilized by ½1

• Relative to average dxz and dyz are stablilized by ⅔2 and dxy is destablilized by ⅓2

eg

t2g

1+0.6 oct

-0.4 oct

-½1

+⅔2

-⅓2

octahedron distorted octahedron

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+½1

Which Complexes Will Distort?

eg

t2g

+0.6 oct

-0.4 oct

-½1

+⅔2

-⅓2

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

oct >>> 1 >> 2

1 3 -0.4oct - 0.332 yes small

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+½1

Which Complexes Will Distort?

eg

t2g

+0.6 oct

-0.4 oct

-½1

+⅔2

-⅓2

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

2

3

4

5

oct >>> 1 >> 2

1 3 -0.4oct - 0.332 yes small

2 3 -0.8oct - 0.672 yes small

3 1 -1.2oct no no

3 1 2 -0.6oct - 0.51 yes large

3 2 1 0 no none

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Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

2

3

4

5

6

7

8

9

oct >>> 1 >> 2

1 3 -0.4oct - 0.332 yes small

2 3 -0.8oct - 0.672 yes small

3 1 -1.2oct no no

3 1 2 -0.6oct - 0.51 yes large

3 2 1 0 no none

4 3 -0.4oct - 0.332 yes small

5 3 -0.8oct - 0.672 yes small

6 1 -1.2oct no no

6 2 -0.6oct - 0.51 yes large

2

2

2

3

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Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

4 4

5 5

6 6

7 6 1

oct >>> 1 >> 2

• Low spin:

+½1

eg

t2g

+0.6 oct

-0.4 oct

-½1

+⅔2

-⅓2

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Which Complexes Will Distort?

• Large distortions (always seen crystallographically):

high spin d4

low spin d7

d9

• Small distortions (often not seen crystallographically):

d1

d2

low spin d4

low spin d5

high spin d6

high spin d7

Cr2+

Co2+

Cu2+

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Jahn-Teller Theorem

• This is a general result known as the Jahn-Teller theorem:

Any molecule with a degenerate ground state will distort

bonding

antibonding

+

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Effect on Spectroscopy

• From Slide 6, there is one d-d transition for an octahedral d1 ion

• From Slide 15, a d1 complex will distort and will not be octahedral

• There are now 3 possible transitions• (A) is in infrared region and is usually hidden under vibrations• (B) and (C) are not usually resolved but act to broaden the band

eg

t2g

Ti3+(aq)

(A) (B) (C)

(B) (C)

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Summary

By now you should be able to....• Show why there is a single band in the visible spectrum for d1, high spin d4, high spin d6

and d9 octahedral complexes• Obtain the value of oct from the spectrum of these ions• Show the electronic origin of the (Jahn-Teller) distortion for

high spin d4, low spin d7 and d9 octahedral complexes

• Predict whether any molecule will be susceptible to a Jahn-Teller distortion

• Explain how the Jahn-Teller effect leads to broadening of bands in the UV/Visible spectrum

Next lecture• Effects of interelectron repulsion

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Practice

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Practice