1. Transition Metals, Compounds and Complexes
Dr. E.R. Schofield
Lecture 4: More Orgel Diagrams, d5 complexes and Selection Rules
Orgel diagram for d2, d3, d7, d8
Orgel diagram for d5 ions
Spin and Laporte Selection Rules
Demo: Co oct to tet and Mn(II)

2. Ligand field strength (Dq)
Orgel diagram for d2, d3, d7, d8 ions
Energy
A2 or A2g
T1 or T1g
T2 or T2g
T1 or T1g P F
d2, d7 tetrahedral d2, d7 octahedral
d3, d8 octahedral d3, d8 tetrahedral
Ligand field strength (Dq)

3. Energy diagram for oct d3, d8, tet d2, d7
15 B'
15 B
15 B > 15 B'
10 Dq
2 Dq
6 Dq
A2(g)
T1(g)
T2(g) x P F
T1(g)
T2(g)
A2(g)
D = 10 Dq
10 Dq
2 Dq
6 Dq

4. d7 tetrahedral complex Calculating B' and x 4T1 n1 n2 n3
[CoCl4]2-
4T1 n1 n2 n3
n1 = cm-1 IR region
n2 = cm-1 visible
n3 = cm-1 visible
10 Dq
2 Dq
6 Dq x
15 B' A
4T1
25 000
20 000
15 000
10 000
5 000
4T2
v / cm-1
4A2

5. Racah Parameters Free ion [Co2+]: B = 971 cm-1 d7 octahedral complex
[Co(H2O)6]2+
[CoCl4]2-
d7 octahedral complex
15 B' = cm-1
B' = 920 cm-1
d7 tetrahedral complex
15 B' = cm-1
B' = 727 cm-1
B' = 0.95 B
B' = 0.75 B
Nephelauxetic ratio, b
b is a measure of the decrease in electron-electron repulsion on complexation

6. The Nephelauxetic Effect
cloud expanding
some covalency in M-L bonds – M and L share electrons
effective size of metal orbitals increases
electron-electron repulsion decreases
Nephelauxetic series of ligands
F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < I-
Nephelauxetic series of metal ions
Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

7. Orgel Diagram, d5 oct and tet
4E(g)
4T2(g)
4E(g), 4A1(g)
4T1(g)
6A1(g)
4A2(g)
4E(g)
4T2(g)
4E(g), 4A1(g)
4T1(g)
6A1(g)
4A2(g)
50 000 4G 4P 4D 4F
40 000
Energy (cm-1)
30 000
20 000
10 000 6S
500
1000
Ligand Field Strength, Dq (cm-1)

8. e Multiple absorption bands Very weak intensity d5 octahedral complex
[Mn(H2O)6]2+
Transitions are forbidden e
4Eg (G)
4A1g (G)
0.01
0.02
0.03
4T2g (D)
4Eg (D)
4T1g(G)
4T2g (G)
v / cm-1
20 000
25 000
30 000

9. Spin Selection Rule
DS = 0
There must be no change in spin multiplicity during an electronic transition
Laporte Selection Rule
D l = ± 1
There must be a change in parity during an electronic transition
g  u
Selection rules determine the intensity of electronic transitions

10. e Spin allowed - Laporte forbidden Transition between d orbitals 2Eg E
0.01
0.02
0.03
[Ti(OH2)6]3+, d1, Oh field
Spin allowed
n / cm-1 -
Laporte forbidden
Transition between d orbitals
10 000
20 000
30 000
2Eg E 2D
2T2g
Doct

11. e e Spin allowed; Laporte forbidden 4A2g 4T1g 4T2g 3T1 3T2 3A2
[V(H2O)6]3+, d2 Oh
3T1
3T2
3A2
[CoCl4]2-, d7 Td
600 10
4T1g
4T2g
4A2g
400 5
200
v / cm-1
n / cm-1 -
25 000
20 000
15 000
10 000
5 000
30 000
20 000
10 000
Spin allowed; Laporte forbidden F P Dq
A2g
T2g
T1g T1 T2 A2
d7 tetrahedral d2 octahedral

12. Relaxation of the Laporte Selection Rule for Tetrahedral Complexes
Octahedral complex
Centrosymmetric
Laporte rule applies
Tetrahedral complex
Non-centrosymmetric
Laporte rule relaxed
inversion
centre
Oh complex d  eg and t2g p  t1u
Td complex d  e and t2 p  t2
Orbital mixing:
In tet complexes, d-orbitals have some p character

13. Intenstity of transitions in d5 complexes Laporte forbidden
10 000
20 000
30 000
40 000
50 000 4G 4P 4D 4F
Dq (cm-1)
500
1000
Energy (cm-1)
4E(g)
4T2(g)
4E(g), 4A1(g)
4T1(g)
6A1(g)
4A2(g)
Spin forbidden
Weak transitions occur due to: Unsymmetrical Vibrations (vibronic transitions)
Spin-orbit Coupling