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Transition Metal Complexes

Electronic Spectra 2

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Electronic Spectra of Transition Metal

Complexes

Cr[(NH3)6]3+ d3 complex

Molecular Term SymbolsQuartet states

Doublet state

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Different Ways of

Transitions

a) dz2 dxy

Creates more repulsion

b) dz2 dxz

Creates less repulsion

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Correlation of Terms of Free Ion and

Oh Complexes

A1g + Eg + T1g + T2g9G

T1g + T2g + A2g7F

T2g + Eg5D

T1g (no splitting)3P

A1g (no splitting)1S

Terms in OhSymmetry

Number of

States

Atomic

Term

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Correlation of Terms of Free Ion and

Oh d1 and d2 Complexes

-0.80

0.20

1.20

Orgel Diagrams

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Tanabe-Sugano Diagram of d2

Configuration

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Tanabe-Sugano Diagrams

For a given C/B value

A plot of energy E (in terms of B) vs. ligand fieldsplitting o (in terms of B)

E = energy relative to the ground-state term (i.e.

ground state term energy = zero) As o increases, electrons tend to pair up, if possible

(i.e. change in spin multiplicity)

Electronic transition occurs from the ground state tothe next excited states with the same multiplicity(spinselection rule)

Help on Tanabe-Sugano diagramshttp://wwwchem.uwimona.edu.jm:1104/courses/Tanabe-Sugano/

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Non-crossing Rule

As the strength of the

interaction changes, statesof the same spin

degeneracy (multiplicity)

and symmetry CANNOT

cross.

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Determine the o and B using Tanabe-Sugano Diagram

28500/21500 ~ 1.32 at0 /B ~ 32.8

32.8B = 21550 B = 657 cm-1

0 /B = 32.8 0 = 21550 cm-1

28500 21550

32.8

Ratio = 1.32

32.8

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Nephelauxetic Effect Nephelauxetic : cloud expanding

B is a measure of electronic repulsionB(complex) < B(free ion)

B(complex)/B(free ion) < 1

Example: B for [Cr(NH3)

6]3+ = 657 cm-1

B for Cr3+ free ion ~ 1027 cm-1

Electronic repulsion decreases as molecular orbitals aredelocalized over the ligands away from the metal

Nephelauxetic Series

= B(complex)/B(free ion)small : large nephelauxetic effect, large delocalization, highcovalent character (soft ligands)

For a given metal center, ligands can be arranged in decreasingorder of

: F- > H2O > NH

3> CN-, Cl- > Br-

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Intensities of Transitions

Electronic Transition:

interaction of electric field component E ofelectromagnetic radiation with electron

Beers Law: absorbance A = log Io/I= bcc = concentration, M b = path length, cm

= molar extinction coefficient, M-1

cm-1

Probability of Transition transition moment fifi = f* i d

f : final state i : initial state: - erelectric dipole moment operator

Intensity of absorption fi2Allowed Transition fi 0Forbidden Transition fi = 0

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Spin Selection Rule

The electromagnetic field of the incident radiationcannot change the relative orientation of the spins of

electrons in a complex

S = 0, spin-allowed transitions

transition between states ofsame spin multiplicity

S 0, spin-forbidden transitions

transition between states ofdifferent spin multiplicity

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Laporte Selection Rule

In a centrosymmetric molecule or ion (with symmetry

element i ), the only allowed transitions are those

accompanied by a change in parity (u g, g u)

Laporte (Symmetry) Allowed gu, ug

Laporte (Symmetry) Forbidden gxg , uxu

d orbitals have g character in Oh

all d-d transitions are Laporte forbidden

= - er : u function

d orbital : g function

fi = f* i d= g x u x g = u = 0

In Td, no i. Laporte rule is silent.

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Intensities of Spectroscopic

Bands in 3d Complexes

Transition max (M-1cm-1)

Spin-forbidden (and Laporte forbidden) < 1

Laporte-forbidden (spin allowed) 20 - 100

Laporte-allowed ~ 500

Symmetry allowed (charge transfer) 1000 - 50000

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Relaxation of Laporte

Selection Rules

Depart from perfect symmetry

Ligand

Geometric Distortion

Vibronic coupling

Mixing of asymmetric vibrations

More intense absorption bands than

normal Laporte forbidden transitions

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Move of electrons

between metal and

ligand orbitals

Very high intensity

LMCT: ligand to metal

MLCT: metal to ligand

Charge Transfer (CT) Transitions

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Ligand to Metal Charge

Transfer (LMCT)

d(M)p (L) transitions are both spinand symmetry allowed and therefore

are usually have much higher intensity

than d-d transitions.

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d(M)p (L) LMCT of [Cr(NH3)5X]2+ X- weaker field ligand than NH3

0 reduced Symmetry reduced, Oh C4v

energy level splitted

LMCT energy : MCl > MBr > MI

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Comparison of

[Cr(NH3)6]3+ and

[Cr(NH3)5X]2+

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d0 Oxo Ions [MOx]y-

d(M) p(O) Charge Transfer LMCT energy

[MnO4]- (purple) < [TcO4]

- < [ReO4]- (white)

[CrO4]2- (yellow) < [MoO4]

2- < [WO4]2- (white)

[WS4]2- (red) < [WO4]2- (white)

d(1st row T.M.) lower than d(3rd row T.M.) in samegroup

p(E) higher down the same groupp(O) lower than p(S)

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Effect of M and L on LMCT

d

1st row T.M.

3rd row T.M.

2nd row T.M.

p

L

dM

p

S

O

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Optical Electrnegativities

Optical Electrnegativitiesvariation in position of LMCT bands

= | ligand metal | 00 = 3.0 X 104 cm-1

3.3NH32.1Mo(V)

3.5H2O2.3Rh(III) l.s.

3.02.5I-1.8 - 1.9Co(II)

3.32.8Br-2.0 - 2.1Ni(II)

3.43.0Cl-2.3Co(III) l.s.

4.43.9F-1.8 - 1.9Cr(III)LigandTdOhMetal

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Metal to Ligand Charge

Transfer (MLCT)

For metal ions in low oxidation state (dlow in energy)

For ligands with low-lying * orbitals,

especially aromatic ligands (e.g. di-imine ligands such as bipy and phen)

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Move of electrons

between metal and

ligand orbitals

Very high intensity

LMCT: ligand to metal

MLCT: metal to ligand

Charge Transfer (CT) Transitions

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Luminescence

Phosphorescence

S 0

Fluorescence

S =0Ruby:Cr3+ in alumina

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Phosphorescence of [Ru(bipy)3]2+

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Spectra of f-block Complexes

Free-ion limit

f-orbitals are deep inside atoms.Ligand show little effects

Sharp transitions

8

Tb3+9

Dy3+10

Ho3+11

Er3+12

Tm3+13

Yb3+14

Lu3+# of f

color-less

PinkyellowpinkredGreencolor-less

color-less

color

7

Gd3+6

Eu3+5

Sm3+4

Pm3+3

Nd3+2

Pr3+1

Ce3+0

La3+# of f

Pr3+(aq), f2

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Circular Dichroism Spectra

CD spectra can be observed for chrial

complexes, it can be used to infer the absolute

configuration of enantiomers

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