chem2402/2912/2916 [part 2]
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CHEM2402/2912/2916 [Part 2]. Ligand-Field Theory and Reaction Kinetics. A/Prof Adam Bridgeman Room: 222 Email: [email protected] www.chem.usyd.edu.au/~bridge_a/chem2 Office Hours: Monday 2-4pm, Wednesday 2-4pm Other times by appointment or by chance. Where Are We Going…?. - PowerPoint PPT PresentationTRANSCRIPT
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CHEM2402/2912/2916 [Part 2]
A/Prof Adam BridgemanRoom: 222
Email: [email protected]
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
Slide 3/22
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
Slide 4/22
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)
Slide 5/22
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
Slide 6/22
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
Slide 7/22
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)
Slide 8/22
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)
Slide 9/22
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
Slide 10/22
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)
Slide 11/22
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
Slide 12/22
+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
Slide 13/22
+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
Slide 14/22
+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
Slide 15/22
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
Slide 16/22
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
Slide 17/22
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+
Slide 18/22
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
+
Slide 19/22
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 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)
Slide 20/22
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