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Transition Metals, Compounds and Complexesor
Electronic Spectroscopy of Transition Metal Complexes
Dr. E.R. Schofield
Lecture 6: Tanabe-Sugano diagrams and Charge Transfer Transitions
Tanabe-Sugano diagrams
Calculating , E and B
d0 and d10 ions
Charge Transfer transitions
Tanabe-Sugano diagram for d3 ions
E/B
/B
[Cr(H2O)6]3+: Three spin allowed transitions1 = 17 400 cm-1 visible
2 = 24 500 cm-1 visible
3 = obscured by CT transition
24 500 = 1.41
17 400
/B = 24
3 = 2.11 = 2.1 x 17 400
3 = 36 500 cm-1
= 24
Calculating 3
E/B
/B
1 = 17 400 cm-1
2 = 24 500 cm-1
= 24
E/B = 34 cm-1
E/B = 24 cm-1
When 1 = E =17 400 cm-1
E/B = 24
so B = 725 cm-1
When 2 = E =24 500 cm-1
E/B = 34
so B = 725 cm-1
If /B = 24
= 24 x 725 = 17 400 cm-1
4A2g
4T1g
4T2g
4T1g
10 Dq
2 Dq
6 Dq
x
x
15 B'
For Oh d3, o = 1 = 17 400 cm-1
o / B = 24
B = 725 cm-1
Energy diagram for octahedral d3 complex 1 = 17 400 cm-1 visible
2 = 24 500 cm-1 visible
3 = obscured by CT transition
TiF4 d0 ion
TiCl4 d0 ion
TiBr4 d0 ion
TiI4 d0 ion
d0 and d10 ion have no d-d transitions
[MnO4]- Mn(VII) d0 ion
[Cr2O7]- Cr(VI) d0 ion
[Cu(MeCN)4]+ Cu(I) d10 ion
[Cu(phen)2]+ Cu(I) d10 ion
Zn2+ d10 ion
extremely purple
bright orange
d0 and d10 ions
white
white
orange
dark brown
colourless
dark orange
white
Charge Transfer Transitions
Charge Transfer Transitions
Ligand-to-metal charge transfer
LMCT transitions
Metal-to-ligand charge transfer
MLCT transitions
MdL
L
L
t2g*
eg*
d-d transitions
Selection rules
CT transitions are spin allowed and Laporte allowed
Transitions occur from a
singlet GS to a singlet ES
S = 0
Transitions occur between
metal based orbitals with d-
character and ligand based
orbitals with p-character
l = ± 1
CT transitions are therefore much more intense than d-d transitions
LMCT Transitions
[MnO4]-, dark purple
e- poor metal (electropositive), high charge
Cr(III), d3 ion, Mn(VII), d0 ion
LMCT = ligand to metal charge transfer
e- rich ligand
O2-, Cl-, Br-, I-
spin-allowed; Laporte allowed
O
Mn
O OO
-
nd
(n+1)p
a1 ,t2
t2
a1
e, t2
t1 ,t2
M 4LML4
MO diagram of MnO4-
t
L(t1) M(e) 17 700 cm-1
L(t1) M(t2*) 29 500 cm-1
L(t2) M(e) 30 300 cm-1
L(t2) M(t2*) 44 400 cm-1
(n+1)s
a1
t2
t2
t1
e
t2*
a1*
t*
LMCT Transitions spin-allowed; Laporte allowed
Cr
NH3
NH3H3N
H3N
H3N
2+
Cllo
g(/
L m
ol-1 c
m-1)
nm600(17 000 cm-1)
3
4
1
2
200(50 000 cm-1)
400(25 000 cm-1)
LMCT
d-d d-d
Identifying charge transfer transitions
Intensity
Solvatochromism - variation in absorption wavelength with solvent
[CrCl(NH3)5]2+, Cr(III), d3
Charge-Transfer Transitions: MLCT
[Cu(phen)2]+, dark orange
e- rich metal, low charge, lower OS
Cu(I), d10 ion
MLCT = metal to ligand charge transfer
-acceptor ligand with low-lying * orbitals
1,10-phenanthroline
nm400 500 600
300
400
500
100
200
max = 458 nm
spin-allowed; Laporte allowed
N
N
N
NCu
+
Charge-Transfer Transitions: MLCT
[Ru(bpy)3]2+, bright orange
e- rich metal, low charge, lower OS
Ru(II), d6 ion, low spin
MLCT = metal to ligand charge transfer
-acceptor ligand with low-lying * orbitals
2,2'-bipyridine
nm300 500
max = 452 nmMLCT
spin-allowed; Laporte allowed
200 400
-*
Ru
N
NNN
N
N
2+
• Interelectron repulsion Limits of LFT – existence of electronic states
• Russel-Saunders coupling Collecting microstates into terms
Using Hund's rules GS and ES with same multiplicity
• Effect of LF on free ion terms Orgel diagram for d1, d4, d6, d9 ions [Ti(OH2)6]3+
• Orgel diagram for d2, d3, d7, d8 ions [Ni(OH2)6]2+
Calculating , x and B' [Co(OH2)6]2+, [CoCl4]2-
Racah parameters in free ions and complexes
The Nephelauxetic effect
Orgel diagram for d5 ions [Mn(OH2)6]2+
• Spin and Laporte Selection Rules Selection rules for Oh and Td complexes
Lifting selection rules
Origin of band broadening [Ni(OH2)6]2+, [Mn(OH2)6]2+, [Ti(OH2)6]3+
•Tanabe-Sugano diagrams Low spin complexes [Mn(CN)6]4-
Calculating E, B' and [Cr(OH2)6]3+, [V(OH2)6]3+
• CT transitions MLCT, LMCT [Cu(phen)2]+, [Ru(bpy)3]2+, [MnO4]-