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Electronic spectra of transition metal complexes
Characteristics of electronic spectra
a) Wavelength
Energy of electronic transition
b) Shape.
Gaussian Band Shape - coupling of electronic and
vibrational states
c) Intensity.
Molar absorptivity,  (M1cm1) due to probability
of electronic transitions.
d) Number of bands
Transitions between States of given dn configuration.
Band intensity in electronic spectra ()
Electronic transitions are controlled by quantum mechanical selection rules
which determine the probability (intensity) of the transition.
Transition
Spin and Symmetry forbidden "d-d" bands
Spin allowed and Symmetry forbidden "d-d" bands
Spin and Symmetry allowed LMCT and MLCT bands
εmax (M1cm1)
0.02 - 1
(Oh) 1 - 10
(Td) 10 – 103
103 - 5 x 104
Spin Selection Rule: There must be no change in the spin multiplicity (2S + 1)
during the transition.
i.e. the spin of the electron must not change during the transition.
Symmetry (Laporte) Selection Rule: There must be a change in parity (g ↔ u)
during the transition
Since s and d orbitals are g (gerade) and p orbitals are u(ungerade), only
s ↔ p and p ↔ d transitions are allowed and d → d transition are formally
forbidden. [i.e. only transitions for which Δl = ± 1 are allowed].
d → d bands are allowed to the extent that the starting or terminal level of the
transition is not a pure d-orbital. (i.e. it is a molecular orbital of the complex
with both metal and ligand character).
States for dn configurations
Russel-Saunders Coupling
• Angular momentum of individual electrons couple to give total angular
momentum for dn configuration ML = ∑ml
• Spin momentum of individual electron spins couple together to give total
spin, S = ∑s
• Inter-electronic repulsions between the electrons in the d orbitals give rise to
ground state and excited states for dn configurations.
• States are labeled with Tern Symbols
• Electonic transitions between ground and excited states are summarized in
Orgel and Tanabe-Sugano diagrams .
• Term Symbols (labels for states) contain information about L and S for state
Hund’s Rules. i) Ground state has maximum spin, S
ii) For states of same spin, ground state has maximum L.
Number of d-d bands in electronic spectrum
Excitation from ground state to excited stated of dn configuration
Ground State
Triple degeneracy of a d2 ion’s 3T2g
ground state due to three possible sites
for hole in t2g level
Singly degenerate 3T2g ground state.
Only one possible arrangement for
three electrons in t2g level
Triple degenerate ground state for d7
Three possible sites for hole in t2g level
d2
eg
t2g
eg
d
3
t2g
eg
d
7
t2g
Singly degenerate 3T2g ground state.
Only one possible arrangement for
six t2g electrons.
eg
d
8
t2g
Excited States
Labeling of d-d bands in electronic spectrum.
•
Consider states of dn configuration
•
Determine free ion ground state Term Symbol (labels for
states)
•
Assign splitting of states in ligand field
•
Spectroscopic labeling of bands.
•
•
Orgel diagrams (high-spin)
Tanabe-Sugano diagrams (high-spin and low-spin)
Individual electron l = 2, ml = 2, 1, 0, -1, -2
Maximum ml = l
l =
0, 1, 2, 3,
Orbital: s, p, d , f
_______________________________
dn configuration, L = 0, 1, 2, 3, 4
Term Symbol
S, P, D, F, G
ML = Σ ml, maximum ML = L
Spin Multiplicity = 2 S +1
Free ion ground state Term Symbols for dn configurations
Term Symbols (labels for states) contain information about L and S for ground state
Hund’s Rules. i) Ground state has maximum spin, S
ii) For states of same spin, ground state has maximum L
M L = m l
L = M L( m ax)
l = 2,
ml =
2
1
S = s
0
-1
-2
Te rm
2 S + 1 s ym bo l
L
S
2
1/2
2
3
1
3
3
3 3/2
4
4
2
2
5
5
0
5/2
6
6
2
D
F
F
D
S
Splitting of the weak field dn ground state terms in an
octahedral ligand field
Ground state determined by inspection of degeneracy of terms for given dn
Orgel Diagrams
3
3
2
Eg
3
P
A2g
D
4
F
d1
2
T2g
Ti3+
3
T2g
o
d2
2
4
Eg
3
P
3
T1g
3
T1g
V2+
5
T1g
4
F
T2g
d3
3
5
T2g
D
5
T1g
o
T1g
T1g(P)
T2g
3
2
4
4
3
2
T1g(P)
4
o
A2g
Eg
d4 o
T2g
3
A2g
3
T1g(P)
Cr3+
Mn3+
The d-d bands of the d 2 ion [V(H2O) 6]3+
(a) [Ni(H20)6]2+
(b) [Ni(NH3)6]2+
Racah Inter-electronic Repulsion Parameters (B, C)
1
S
1
G
3
P
E(3P) = A+7B
1
D
E(1D) = A - 3B + 2C
3
F
E(3F) = A - 8B
d2
3
F
3
F
3
P
= 15B
1
D
= 5B + 2C
The Tanabe-Sugano diagram for the d2 ion
Evidence for covalent bonding in metal-ligand interactions
The Nephelauxetic Effect (“cloud expansion”)
Reduction in electron-electron repulsion upon complex formation
Racah Parameter, B: electron-elctronic repulsion parameter
Bo is the inter- electronic repulsion in the gaseous Mn+ ion.
B is the inter- electronic repulsion in the complexed MLxn+ ion.
The smaller values for B in the complex compared to free gaseous ion is
taken as evidence of smaller inter-electronic repulsion in the complex due to
a larger “molecular orbital” on account of overlap
of ligand and metal orbital, i.e. evidence of covalency (cloud expansion”).
Nephelauxetic Ratio, β = B
Bo
Nephelauxetic Effect
Nephelauxetic Ligand Series
I < Br < CN < Cl < NCS < C2O42- < en < NH3 < H2O < F
Small β
Covalent
Large β
Ionic
Nephelauxetic Metal Series
Pt4+ < Co3+ < Rh3+~Ir3+ < Fe3+ < Cr3+ < Ni2+ < V4+< Pt2+~ Mn2+
Small β
Large β
Large overlap
Small overlap
Covalent
Ionic
Empirical Racah parameters, h, k
β = 1– [h(ligand) x k(metal)]
Cr(NH3)63+
β = 1 –hk
β = 1 –(1.4)(0.21)
= 0.706
Cr(CN)63-
β = 1 –hk
β = 1 –(2.0)(0.21)
= 0.580
Bo - B = hligands x kmetal ion
Bo
Typical Δo and λmax values for octahedral (ML6) d-block metal complexes
__________________________________________________________________
Complex
Δo cm-1
~ λmax (nm)
Complex
Δo cm-1
λmax
(nm)
___________________________________________________________________________________
[Ti(H2O)6]3+
20,300
493
[Fe(H2O)6]2+
9,400
1064
3+
3+
[V(H2O)6]
20,300
493
[Fe(H2O)6]
13,700
730
[V(H2O)6]2+
12,400
806
[Fe(CN)6]335,000
286
[CrF6]315,000
667
[Fe(CN)6]433,800
296
3+
3[Co(H2O)6] , l.s. 20,700
483
[Fe(C2O4)3]
14,100
709
[Cr(H2O)6]2+
14,100
709
[Co(CN)6]3- l.s.
34,800
287
[Cr(H2O)6]3+
17,400
575
[Co(NH3)6]3+ l.s. 22,900
437
3+
2+
[Cr(NH3)6]
21,600
463
[Ni(H2O)6]
8,500
1176
[Cr(en)3]3+
21,900
457
[Ni(NH3)6]2+
10,800
926
[Cr(CN)6]326,600
376
[Ni(en)3]2+
11,500
870
___________________________________________________________________________________
1.
Assign the metal oxidation state in the following compounds.
a.
b.
c.
K2[PtCl6]
Na2[Fe(CO)4]
[Mn(CH3)(CO)5]
2.
Account for the following:
The manganous ion, [Mn(H2O)6]2+, reacts with CN- to form [Mn(CN)6]4- which has
m = 1.95 B.M., but with I- to give [MnI4]2- which has m = 5.93 B. M.
[Co(NH3)6]Cl3 is diamagnetic, whereas Na3[CoF6] is paramagnetic (m = 5.02 B.M).
[PtBr2Cl2]2 is diamagnetic and exists in two isomeric forms, whereas [NiBr2Cl2]2
has a magnetic moment, m = 3.95 B.M., and does not exhibit isomerism.
Copper(II) complexes are typically blue with one visible absorption band in their
electronic spectra whereas copper(I) complexes are generally colorless.
Assign a spectroscopic label to the Cu2+ transition.