1. Electronic spectra of transition metal complexes

2. Characteristics of electronic spectra
Wavelength Energy of electronic transition
Shape. Gaussian Band Shape - coupling of electronic and vibrational states
Intensity. Molar absorptivity,  (M1cm1) due to probability of electronic transitions.
d) Number of bands Transitions between States of given dn configuration.

4. Band intensity in electronic spectra (e)
Electronic transitions are controlled by quantum mechanical selection rules which determine the probability (intensity) of the transition.
Transition εmax (M1cm1)
Spin and Symmetry forbidden "d-d" bands
Spin allowed and Symmetry forbidden "d-d" bands (Oh)
(Td) 10 – 103
Spin and Symmetry allowed LMCT and MLCT bands x 104

5. 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).

6. 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.

7. Number of d-d bands in electronic spectrum
Excitation from ground state to excited stated of dn configuration
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
Singly degenerate 3T2g ground state. Only one possible arrangement for
six t2g electrons.

8. 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)

9. Individual electron l = 2, ml = 2, 1, 0, -1, -2
Maximum ml = l
l = , 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

10. 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

13. 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

15. Orgel Diagrams
Mn3+
Ti3+
Cr3+
V2+

17. (a) [Ni(H20)6]2+ (b) [Ni(NH3)6]2+

19. The Tanabe-Sugano diagram for the d2 ion

20. 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

21. Nephelauxetic Effect Nephelauxetic Ligand Series
I < Br < CN < Cl < NCS < C2O42- < en < NH3 < H2O < F
Small β Large β
Covalent Ionic
Nephelauxetic Metal Series
Pt4+ < Co3+ < Rh3+~Ir3+ < Fe3+ < Cr3+ < Ni2+ < V4+< Pt2+~ Mn2+
Small β Large β
Large overlap Small overlap
Covalent Ionic

22. Empirical Racah parameters, h, k β = 1– [h(ligand) x k(metal)]
Cr(NH3) β = 1 –hk
β = 1 –(1.4)(0.21) =
Cr(CN)63- β = 1 –hk
β = 1 –(2.0)(0.21) =
Bo - B = hligands x kmetal ion Bo

24. __________________________________________________________________
Typical Δo and λmax values for octahedral (ML6) d-block metal complexes
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Complex Δo cm-1 ~ λmax (nm) Complex Δo cm-1 λmax (nm)
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[Ti(H2O)6] , [Fe(H2O)6] ,
[V(H2O)6] , [Fe(H2O)6] ,
[V(H2O)6] , [Fe(CN)6] ,
[CrF6] , [Fe(CN)6] ,
[Co(H2O)6]3+, l.s. 20, [Fe(C2O4)3] ,
[Cr(H2O)6] , [Co(CN)6]3- l.s ,
[Cr(H2O)6] , [Co(NH3)6]3+ l.s ,
[Cr(NH3)6] , [Ni(H2O)6] ,
[Cr(en)3] , [Ni(NH3)6] ,
[Cr(CN)6] , [Ni(en)3] ,

25. 1. Assign the metal oxidation state in the following compounds.
a. K2[PtCl6]
b. Na2[Fe(CO)4]
c. [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 ( = B.M).
[PtBr2Cl2]2 is diamagnetic and exists in two isomeric forms, whereas [NiBr2Cl2]2
has a magnetic moment,  = 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.