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

F

P

Ligand field strength (Dq)

Energy

A2 or A2g

T1 or T1g

T2 or T2g

A2 or A2g

T2 or T2g

T1 or T1g

T1 or T1g

T1 or T1g

Orgel diagram for d2, d3, d7, d8 ions

d2, d7 tetrahedral d2, d7 octahedral

d3, d8 octahedral d3, d8 tetrahedral

0

10 Dq

2 Dq

6 Dq

T1(g)

T1(g)

T2(g)

A2(g)

P

F

x

x15 B'15 B

15 B > 15 B'

Energy diagram for oct d3, d8, tet d2, d7

10 Dq

2 Dq

6 Dq

A2(g)

T1(g)

T2(g)

T1(g)

= 10 Dq

Calculating B' and x d7 tetrahedral complex

A

25 000 20 000

[CoCl4]2-

15 000 10 000 5 000

12

3

1 = 3 300 cm-1 IR region

2 = 5 800 cm-1 visible

3 = 15 000 cm-1 visible

v / cm-1

4A2

4T1

4T2

4T1

10 Dq

2 Dq

6 Dq

x

x

15 B'

Racah Parameters

d7 tetrahedral complex

15 B' = 10 900 cm-1

B' = 727 cm-1

[CoCl4]2-[Co(H2O)6]2+

d7 octahedral complex

15 B' = 13 800 cm-1

B' = 920 cm-1

Free ion [Co2+]: B = 971 cm-1

B' = 0.95B

B' = 0.75B

Nephelauxetic ratio,

is a measure of the decrease in electron-electron repulsion on complexation

- 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)

cloud expandingThe Nephelauxetic Effect

10 000

20 000

30 000

40 000

50 000

Orgel Diagram, d5 oct and tet

Ligand Field Strength, Dq (cm-1)

500 1000

Energy (cm-1)

4E(g)4T2(g)

4E(g), 4A1(g)

4T2(g)

4T1(g)

6A1(g)

4T2(g)

4T1(g)

4A2(g)

4T1(g)

6S

4G

4P

4D

4F

4E(g)4T2(g)

4E(g), 4A1(g)

4T2(g)

4T1(g)

6A1(g)

4T2(g)

4T1(g)

4A2(g)

4T1(g)

d5 octahedral complex

[Mn(H2O)6]2+

v / cm-1

20 000 25 000 30 000

Multiple absorption bands

Very weak intensity

4T2g (D)

4Eg (D)4T1g(G)

4Eg (G)

4A1g (G)

4T2g (G)0.01

0.02

0.03

Transitions are forbidden

Spin Selection Rule

S = 0

There must be no change in spin multiplicity during an electronic transition

Laporte Selection Rule

l = ± 1

There must be a change in parity during an electronic transition

Selection rules determine the intensity of electronic transitions

g u

/ cm-1-

2Eg

2T2g

2D

E

oct

[Ti(OH2)6]3+, d1, Oh field

10 000 20 000 30 000

0.01

0.02

0.03

Spin allowed

Laporte forbiddenTransition between d orbitals

Spin allowed; Laporte forbidden

F

P

Dq

A2g

T2g

T1g

T1

T2

A2

T1 T1g

d7 tetrahedral d2 octahedral0

10 00030 000cm-1

[V(H2O)6]3+, d2 Oh10

20 000

5

4T1g

4T2g

4T1g

4A2g

25 000 20 000 15 000 10 000 5 000v / cm-1

[CoCl4]2-, d7 Td

3T1

3T2

3A2

3T1

600

400

200

Relaxation of the Laporte Selection Rule for Tetrahedral Complexes

Octahedral complex

Centrosymmetric

Laporte rule applies

Tetrahedral complex

Non-centrosymmetric

Laporte rule relaxed

inversioncentre

Orbital mixing:Oh complex d eg and t2g p t1u

Td complex d e and t2 p t2

In tet complexes, d-orbitals have some p character

Intenstity of transitions in d5 complexes

6S

10 000

20 000

30 000

40 000

50 000

4G4P

4D

4F

Dq (cm-1)

500 1000

Energy (cm-1)

4E(g)4T2(g)4E(g),

4A1(g)

4T2(g)

4T1(g)

6A1(g)

4T2(g)

4T1(g)

4A2(g)

4T1(g)

Laporte forbidden

Spin forbidden

Weak transitions occur due to: Unsymmetrical Vibrations (vibronic transitions)

Spin-orbit Coupling