Transition Metal Complexes
Electronic Spectra 2
Electronic Spectra of Transition Metal
Complexes
•Cr[(NH3)6]3+
d3 complex
Molecular Term Symbols
Quartet states
Doublet state
Different Ways of
Transitions
a) dz2
dxy
Creates more repulsion
b) dz2
dxz
Creates less repulsion
Correlation of Terms of Free Ion and
Oh Complexes
Atomic Number of Terms in Oh
Term
States
Symmetry
S
1
A1g (no splitting)
P
3
T1g (no splitting)
D
5
T2g + Eg
F
7
T1g + T2g + A2g
G
9
A1g + Eg + T1g + T2g
Correlation of Terms of Free Ion and
Oh d1 and d2 Complexes
Orgel Diagrams
1.20
0.20
-0.80
Tanabe-Sugano Diagram of d2
Configuration
Tanabe-Sugano Diagrams
For a given C/B value
• A plot of energy E (in terms of B) vs. ligand field
splitting 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 to
the next excited states with the same multiplicity (spin
selection rule)
• Help on Tanabe-Sugano diagrams
http://wwwchem.uwimona.edu.jm:1104/courses/Tanabe-Sugano/
Non-crossing Rule
• As the strength of the
interaction changes, states
of the same spin
degeneracy (multiplicity)
and symmetry CANNOT
cross.
Determine the o and B using TanabeSugano Diagram
28500/21500 ~ 1.32 at
0 /B ~ 32.8
32.8B = 21550 B = 657 cm-1
0 /B = 32.8 0 = 21550 cm-1
32.8
28500
21550
32.8
Ratio = 1.32
Nephelauxetic Effect
• Nephelauxetic : cloud expanding
• B is a measure of electronic repulsion
B(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 are
delocalized over the ligands away from the metal
• Nephelauxetic Series
= B(complex)/B(free ion)
small : large nephelauxetic effect, large delocalization, high
covalent character (soft ligands)
For a given metal center, ligands can be arranged in decreasing
order of 
: F- > H2O > NH3 > CN-, Cl- > Br-
Intensities of Transitions
•
Electronic Transition:
interaction of electric field component E of
electromagnetic radiation with electron
•
Beer’
s Law: absorbance A = log Io/I = 
bc
c = concentration, M
b = path length, cm
= molar extinction coefficient, M-1cm-1
•
Probability of Transition transition moment µfi
µfi = f* µ i d
f : final state i : initial state
µ : - er electric dipole moment operator
•
Intensity of absorption µfi2
Allowed Transition
µfi 0
Forbidden Transition
µfi = 0
Spin Selection Rule
• The electromagnetic field of the incident radiation
cannot change the relative orientation of the spins of
electrons in a complex
S = 0, spin-allowed transitions
transition between states of same spin multiplicity
S 0, spin-forbidden transitions
transition between states of different spin multiplicity
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 gu, ug
Laporte (Symmetry) Forbidden gxg , uxu
• d orbitals have g character in Oh
all d-d transitions are Laporte forbidden
• µ = - er : u function
d orbital : g function
µfi = f* µ i d
= g x u x g = u = 0
• In Td, no i. Laporte rule is silent.
Intensities of Spectroscopic
Bands in 3d Complexes
Transition
-1cm-1)

(M
max
Spin-forbidden (and Laporte forbidden)
Laporte-forbidden (spin allowed)
Laporte-allowed
Symmetry allowed (charge transfer)
<1
20 - 100
~ 500
1000 - 50000
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
Charge Transfer (CT) Transitions
Move of electrons
between metal and
ligand orbitals
Very high intensity
LMCT: ligand to metal
MLCT: metal to ligand
Ligand to Metal Charge
Transfer (LMCT)
•d(M)p(L) transitions are both spin
and symmetry allowed and therefore
are usually have much higher intensity
than d-d transitions.
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 : M–Cl > M–Br > M–I
Comparison of
[Cr(NH3)6]3+ and
[Cr(NH3)5X]2+
d0 Oxo Ions [MOx]yd(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 same
group
p(E) higher down the same group
p(O) lower than p(S)
Effect of M and L on LMCT
d
d
M
3rd row T.M.
2nd row T.M.
1st row T.M.
S
L
p
O
p
Optical Electrnegativities
• Optical Electrnegativities
variation in position of LMCT bands
= | ligand –metal | 0
0 = 3.0 X 104 cm-1
Ligand


F-
3.9
4.4
Cl-
3.0
3.4
Ni(II)
2.0 - 2.1 Br-
2.8
3.3
Co(II)
1.8 - 1.9 I-
2.5
3.0
Metal
Oh
Cr(III)
1.8 - 1.9
Co(III) l.s. 2.3
Td
Rh(III) l.s. 2.3
H2O
3.5
Mo(V)
NH3
3.3
2.1
Metal to Ligand Charge
Transfer (MLCT)
•For metal ions in low oxidation state (d
low in energy)
•For ligands with low-lying * orbitals,
especially aromatic ligands (e.g. diimine ligands such as bipy and phen)
Charge Transfer (CT) Transitions
Move of electrons
between metal and
ligand orbitals
Very high intensity
LMCT: ligand to metal
MLCT: metal to ligand
Luminescence
Ruby:
Cr3+ in alumina
Fluorescence
S =0
Phosphorescence
S 0
Phosphorescence of [Ru(bipy)3]2+
Spectra of f-block Complexes
• Free-ion limit
• f-orbitals are deep inside atoms.
Ligand show little effects
• Sharp transitions
# of f
0
La3+
1
Ce3+
2
Pr3+
color
colorless
colorless
# of f
14
Lu3+
13
Yb3+
3
Nd3+
Pr3+(aq), f2
4
Pm3+
5
Sm3+
Green red
pink
yellow Pink
12
Tm3+
10
Ho3+
9
Dy3+
11
Er3+
6
Eu3+
8
Tb3+
7
Gd3+
colorless
Circular Dichroism Spectra
•CD spectra can be observed for chrial
complexes, it can be used to infer the absolute
configuration of enantiomers