Transcript Document

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
Slide 2/22
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 3/22
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 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)
Slide 4/22
Schedule
• Lecture 1: Electronic absorption spectroscopy
Jahn-Teller effect and the spectra of d1, d4, d6 and d9 ions
• Lecture 2: Interpreting electronic spectra
Interelectron repulsion and the nephelauxetic effect
• Lecture 3: Interpreting electronic spectra
Selection 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 degenerate
Repulsion between the d-electrons and ligand lone pairs raises the energy
of each d-orbital
Mn+
JKB – Lecture 3, S & A 19.1
Mn+
Slide 6/22
Revision – Ligand-Field Splitting
•
•
•
Two of the d-orbitals point along x, y and z and are more affected than the
average (eg)
Three of the d-orbitals point between x, y and z and are affected less than
the average (t2g)
The ligand-field splitting (Doct)
eg
Doct
t2g
JKB – Lecture 3, S & A 19.1
Slide 7/22
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 Doct
eg
eg
Ti3+(aq)
Doct
t2g
t2g
ground state
JKB – Lecture 3, S & A 19.3
excited state
Slide 8/22
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 Doct
No other transitions are possible without changing the spin
eg
eg
Cr2+(aq)
Doct
t2g
t2g
ground state
JKB – Lecture 3, S & A 19.3
excited state
Slide 9/22
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 Doct
No other transitions are possible changing the spin
d6
ground state
d9
excited state
Fe2+(aq)
ground state
excited state
Cu2+(aq)
Slide 10/22
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
Doct
t2g
JKB – Lecture 6, S & A p467
d1
d2
tetragonal elongation
Slide 11/22
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 average dz2 is stablilized by ½d1 and dx2-y2 is destablilized by ½d1
Relative to average dxz and dyz are stablilized by ⅔d2 and dxy is destablilized by ⅓d2
+½d1
+0.6 Doct
eg
Doct
t2g
octahedron
-0.4 Doct
d1
-½d1
d2
+⅔d2
-⅓d2
distorted octahedron
Slide 12/22
Which Complexes Will Distort?
Doct >>> d1 >> d2
dn configuration
t2g
1
degeneracy
LFSE
stabilized?
distortion
3
-0.4Doct - 0.33d2
yes
small
eg
1
+½d1
eg
+0.6 Doct
-½d1
+⅔d2
t2g
-0.4 Doct
-⅓d2
Slide 13/22
Which Complexes Will Distort?
Doct >>> d1 >> d2
dn configuration
t2g
degeneracy
LFSE
stabilized?
distortion
eg
1
1
3
-0.4Doct - 0.33d2
yes
small
2
2
3
-0.8Doct - 0.67d2
yes
small
3
3
1
-1.2Doct
no
no
4
3
1
2
-0.6Doct - 0.5d1
yes
large
5
3
2
1
0
no
none
+½d1
eg
+0.6 Doct
-½d1
+⅔d2
t2g
-0.4 Doct
-⅓d2
Slide 14/22
Which Complexes Will Distort?
Doct >>> d1 >> d2
dn configuration
t2g
degeneracy
LFSE
stabilized?
distortion
eg
1
1
3
-0.4Doct - 0.33d2
yes
small
2
2
3
-0.8Doct - 0.67d2
yes
small
3
3
1
-1.2Doct
no
no
4
3
1
2
-0.6Doct - 0.5d1
yes
large
5
3
2
1
0
no
none
6
4
2
3
-0.4Doct - 0.33d2
yes
small
7
5
2
3
-0.8Doct - 0.67d2
yes
small
8
6
1
-1.2Doct
no
no
9
6
2
3
2
-0.6Doct - 0.5d1
yes
large
Slide 15/22
Which Complexes Will Distort?
Doct >>> d1 >> d2
•
Low spin:
dn configuration
t2g
4
4
5
5
6
6
7
6
degeneracy
LFSE
stabilized?
distortion
eg
1
+½d1
eg
+0.6 Doct
-½d1
+⅔d2
t2g
-0.4 Doct
-⅓d2
Slide 16/22
Which Complexes Will Distort?
•
:
Large distortions (always seen crystallographically)
 high spin d4
Cr2+
 low spin d7
Co2+
 d9
•
Cu2+
Small distortions (often not seen crystallographically):
 d1
 d2
 low spin d4
 low spin d5
 high spin d6
 high spin d7
Slide 17/22
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 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
(A) (B) (C)
Slide 19/22
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 Doct 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 JahnTeller distortion
• Explain how the Jahn-Teller effect leads to broadening of
bands in the UV/Visible spectrum
Next lecture
• Effects of interelectron repulsion
Slide 20/22
Practice
Slide 21/22
Practice
Slide 22/22