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CHEM2402/2912/2916 [Part 2]

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Ligand-Field Theory and Reaction Kinetics

A/Prof Adam Bridgeman

Room: 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

- 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

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 Doct 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)

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

Revision – Ligand-Field Splitting

- 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

JKB – Lecture 3, S & A 19.1

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

(eg)

(t2g)

(Doct)

eg

Doct

t2g

JKB – Lecture 3, S & A 19.1

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 Doct

eg

eg

Ti3+(aq)

Doct

t2g

t2g

excited state

ground state

JKB – Lecture 3, S & A 19.3

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 Doct
- No other transitions are possible without changing the spin

eg

eg

Cr2+(aq)

Doct

t2g

t2g

ground state

excited state

JKB – Lecture 3, S & A 19.3

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 Doct
- No other transitions are possible changing the spin

d6

d9

excited state

ground state

excited state

ground state

Cu2+(aq)

Fe2+(aq)

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
- Doct >>> d1 >> d2

eg

d1

Doct

t2g

d2

tetragonal elongation

JKB – Lecture 6, S & A p467

Which Complexes Will Distort?

- Relative to average: t2g go down by 0.4Doct in octahedral complex
- Relative to average: eg go up by 0.6Doct in octahedral complex
- Relative to averagedz2 is stablilized by ½d1 and dx2-y2 is destablilized by ½d1
- Relative to averagedxz and dyz are stablilized by ⅔d2 and dxy is destablilized by ⅓d2

+½d1

d1

eg

+0.6 Doct

-½d1

Doct

+⅔d2

d2

t2g

-0.4 Doct

-⅓d2

distorted octahedron

octahedron

Which Complexes Will Distort?

Doct >>> d1 >> d2

3

1

-0.4Doct - 0.33d2

yes

small

+½d1

eg

+0.6 Doct

-½d1

+⅔d2

t2g

-0.4 Doct

-⅓d2

Which Complexes Will Distort?

Doct >>> d1 >> d2

3

1

-0.4Doct - 0.33d2

yes

small

2

3

-0.8Doct - 0.67d2

yes

small

1

-1.2Doct

3

no

no

2

-0.6Doct - 0.5d1

3

yes

large

1

1

3

0

no

none

2

+½d1

eg

+0.6 Doct

-½d1

+⅔d2

t2g

-0.4 Doct

-⅓d2

Which Complexes Will Distort?

Doct >>> d1 >> d2

3

1

-0.4Doct - 0.33d2

yes

small

2

3

-0.8Doct - 0.67d2

yes

small

1

-1.2Doct

3

no

no

2

-0.6Doct - 0.5d1

3

yes

large

1

1

3

0

no

none

2

3

4

2

-0.4Doct - 0.33d2

yes

small

5

3

-0.8Doct - 0.67d2

2

yes

small

1

-1.2Doct

6

no

no

2

3

2

-0.6Doct - 0.5d1

6

yes

large

Which Complexes Will Distort?

Doct >>> d1 >> d2

- Low spin:

+½d1

eg

+0.6 Doct

-½d1

+⅔d2

t2g

-0.4 Doct

-⅓d2

Which Complexes Will Distort?

- Large distortions (always seen crystallographically)
- :
- high spin d4
- low spin d7
- d9

Cr2+

Co2+

Cu2+

- Small distortions (often not seen crystallographically):
- d1
- d2
- low spin d4
- low spin d5
- high spin d6
- high spin d7

Jahn-Teller Theorem

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

Any molecule with a degenerate ground state will distort

antibonding

bonding

+

Effect on Spectroscopy

- From Slide 6, there is one d-d transition for an octahedral d1ion
- From Slide 15, a d1complex 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

Ti3+(aq)

eg

(B)

(C)

t2g

(C)

(A)

(B)

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 d6and d9 octahedral complexes
- Obtain the value of Doct from the spectrum of these ions
- Show the electronic origin of the (Jahn-Teller) distortion for high spin d4, low spin d7 and d9octahedral 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

Practice

Practice