CHEM2402/2912/2916 [Part 2]

Ligand-Field Theory and Reaction KineticsLigand-Field Theory and Reaction Kinetics

A/Prof Adam BridgemanA/Prof Adam BridgemanRoom: 222

E il A B id @ h d dEmail: A.Bridgeman@chem.usyd.edu.auwww.chem.usyd.edu.au/~bridge_a/chem2

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

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Other times by appointment or by chance

Where Are We Going…?

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

Jahn-Teller effectElectronic spectra of multi-electron atomsElectronic spectra of multi electron atomsNephelauxetic effect and the spectrochemical seriesSelection rulesCharge transfer transitions

• Week 5: Bonding in Complexes, Metalloproteins and Materialsπ-Donor and π-Acceptor Ligands Metal-metal bonding in clusters and solidsR l f li d fi ld ff t i l t h i tRole of ligand-field effects in electrochemistry

• Week 6: Kinetics and Ligand-Field EffectsDifferential rate laws

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Differential rate lawsLigand substiution reactions

By the end of week 4…

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

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

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

Slide 4/22

Schedule

L t 1 El t i b ti t• 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

L t 3 I t ti l t i t• Lecture 3: Interpreting electronic spectraSelection rules and charge transfer transitions

Slide 5/22

Revision – Ligand-Field Splitting

• In the absence of any ligands, the five d-orbitals of a Mn+ transition metal ion are degenerateo a e dege e ate

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

L

L

L

Mn+Mn+

LL

LMn+

Slide 6/22

L

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(eg)a e age

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

• The ligand-field splitting

(eg)

(t2g)(Δ t)The ligand-field splitting

eg

(Δoct)

Δoct

LLL

t2g

LL

LL

Mn+Mn+

LL

LL

Mn+

L

LL

Slide 7/22

L

L

L

LJKB – Lecture 3, S & A 19.1

L

L

Electronic Spectra of d1 Ions

• A d1 octahedral complex can undergo 1 electronic transition• The ground state (t2g)1 comprises three degenerate arrangementse g ou d state (t2g) co p ses t ee dege e ate a a ge e ts• The excited state (eg)1 comprises two degenerate arrangements• The electronic transition occurs at Δoct

eg eg

Δoct

Ti3+(aq)

t2g t2g

ground state excited state

Slide 8/22JKB – 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 arrangementse g ou d state (t2g) (eg) co p ses t o dege e ate a a ge e ts• The excited state (t2g)2(eg)2 comprises three degenerate arrangements• The electronic transition occurs at Δoct• No other transitions are possible without changing the spinNo other transitions are possible without changing the spin

eegeg

ΔCr2+(aq)

t t2

Δoct

t2g t2g

ground state excited state

Slide 9/22JKB – 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 Δocte e ect o c t a s t o occu s at oct• No other transitions are possible changing the spin

d6d6d9

ground state excited stateground state excited state

Fe2+(aq) Cu2+(aq)

Slide 10/22

Fe (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 t2 by lowering the energy of d and dIt also produces a much smaller splitting of t2g by lowering the energy of dxz and dyz

• Δoct >>> δ1 >> δ2 eg

δΔoct

δ1

δ2

t2g

LL L

LL

LL

Mn+

LL

LL

Mn+

LL

LL

Mn+

Slide 11/22

LL

JKB – Lecture 6, S & A p467

L

tetragonal elongation

Which Complexes Will Distort?

• Relative to average: t2g go down by 0.4Δoct in octahedral complex• Relative to average: eg go up by 0.6Δoct in octahedral complexg

• Relative to average dz2 is stablilized by ½δ1 and dx

2-y

2 is destablilized by ½δ1

• Relative to average dxz and dyz are stablilized by ⅔δ2 and dxy is destablilized by ⅓δ2

+½δ+½δ1eg δ1+0.6 Δoct

-½δΔoct

-½δ1

+⅔δ2δ2t2g -0.4 Δoct

2

-⅓δ2

Slide 12/22octahedron distorted octahedron

Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

Δoct >>> δ1 >> δ2

d configuration degeneracy LFSE stabilized? distortiont2g eg

1 1 3 -0.4Δoct - 0.33δ2 yes small

+½δ1+½δ1eg +0.6 Δoct

-½δ1½δ1

+⅔δ2

Slide 13/22

t2g -0.4 Δoct -⅓δ2

Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

Δoct >>> δ1 >> δ2

d configuration degeneracy LFSE stabilized? distortiont2g eg

1 1 3 -0.4Δoct - 0.33δ2 yes small234

2 3 -0.8Δoct - 0.67δ2 yes small3 1 -1.2Δoct no no3 1 2 0 6Δ 0 5δ yes large

+½δ1

45

3 1 2 -0.6Δoct - 0.5δ1 yes large3 2 1 0 no none

+½δ1eg +0.6 Δoct

-½δ1½δ1

+⅔δ2

Slide 14/22

t2g -0.4 Δoct -⅓δ2

Which Complexes Will Distort?Δoct >>> δ1 >> δ2

dn configuration degeneracy LFSE stabilized? distortiont2 et2g eg

12

1 3 -0.4Δoct - 0.33δ2 yes small2 3 -0.8Δoct - 0.67δ2 yes small

345

3 1 -1.2Δoct no no3 1 2 -0.6Δoct - 0.5δ1 yes large3 2 1 05

67

3 2 1 0 no none4 3 -0.4Δoct - 0.33δ2 yes small5 3 -0.8Δoct - 0.67δ2 yes small

22

89

0 8 oct 0 6 δ2 yes small6 1 -1.2Δoct no no6 2 -0.6Δoct - 0.5δ1 yes large

223

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Which Complexes Will Distort?Δoct >>> δ1 >> δ2

• Low spin:

dn configuration degeneracy LFSE stabilized? distortiont2g eg

p

2g g

4 45 56 67 6 1

+½δ+½δ1eg +0.6 Δoct

-½δ-½δ1

+⅔δ2

Slide 16/22

t2g -0.4 Δoct

2

-⅓δ2

Which Complexes Will Distort?

• Large distortions (always seen crystallographically):

high spin d4

l i d7

Cr2+

low spin d7

d9

Co2+

Cu2+

• Small distortions (often not seen crystallographically):

d1

d2

low spin d4

low spin d5

high spin d6

Slide 17/22

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

+Slide 18/22

+

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 octahedralo S de 5, a d co p e d sto t a d ot be octa ed a• 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(B) and (C) are not usually resolved but act to broaden the band

eg Ti3+(aq)

(B) (C)

t2g

(A) (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 complexesObtain the value of Δ from the spectrum of these ions• Obtain the value of Δoct 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 complexeshigh spin d , low spin d and d 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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• Effects of interelectron repulsion

Practice

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Practice

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