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]-