1. Electronic Spectra of Coordination Compounds
We know how to determine Free Ion Terms; d-electron configurations given below
Determining Lowest Energy Free Ion Term
From Table above: highest multiplicity, largest L if more than one (d3 = 4F)
From Metal ion and coordination geometry: Cr3+ Oh hs
Sketch d-orbitals with electrons
Spin Multiplicity = # unpaired e- + 1 (3 + 1 = 4)
Max Value of ML (sum of ml values) = = 3 (L = 3 = F) Example: ls Mn3+ Oh

2. Selection Rules
Laporte Selection Rule: transitions between states of same parity (g/u) are forbidden
gg and uu forbidden (all dd transitions); gu allowed (dp allowed)
Spin Selection Rule: transitions between different spin multiplicities are forbidden 4A24T1 are allowed; 4A22A2 are forbidden
Seem to rule out almost all transitions in metal complexes: Why Colored?
Mechanisms that “relax” the selection rules
Vibronic Coupling: vibrations of bonds cause loss of the center of symmetry
Relaxes Laporte Selection Rule and allows 5-50 M-1 cm-1 metal transitions
Tetrahedral complexes strongly absorb
No center of symmetry, so no Laporte forbideness
sp3/sd3 hybridization mixes p orbitals (u) with d orbitals (g) to relax Laporte
Spin-Orbit Coupling relaxes Spin Selection Rule in some case (<1 M-1cm-1)
Most important for 2nd and 3rd row transition metals
4. [V(H2O)6]3+ d2 Example

3. Correlation Diagrams
Free Ion (no ligand field) Term Symbols (Previous Lecture) shown on left
Strong Ligand Field Term Symbols shown on the right: overcomes LS coupling
Possible d2 electron configurations (t2g2 is ground state lowest energy)
Actual complexes lie in between
these weak and strong field limits
Free ion terms can be reduced to
constituent irreducible representations

4. Do = ligand field splitting ch 10 B = Racah parameter = repulsion
Each Free Ion Term irreducible representation Correlates with one from the strong-field limit
Transitions from Ground State must match same Spin Multiplicity (Bold Lines)
Non-Crossing Rule: lines connecting states of same symmetry do not cross
6. Tanabe-Sugano Diagrams
Special correlation diagrams useful for interpreting electronic spectra
Lowest energy state plotted along horizontal axis
Vertical distance above is correlated to energy of transition
Horizontal Axis: Do /B
Do = ligand field splitting ch 10
B = Racah parameter = repulsion
between terms of same multiplicity
Vertical Axis: E/B
E = energy of excited state above
ground state
n n n3

5. More on Tanabe-Sugano Diagrams
a. Simplified Tanabe-Sugano Diagrams show allowed transitions for Octahedral Geom
d2 and d8 opposite order
d3 and d7 opposite order
d4 and d6 opposite order

6. d4-d7 configurations have High Spin and Low Spin possibilities
d4 example
S = 2 (mult = 5) S = 1 (mult = 3)
Weak Field limit defined as Do /B = 27
Weak Field (High Spin): ground state d4 = 5Eg
Strong Field (Low Spin): ground state d4 = 3T1g
Weak Field Aqua Complex Spectra for 1st-Row Transition Metal Ions: compare number of peaks to Tanabe-Sugano Diagrams

7. Jahn-Teller Distortions and Electronic Spectra
d1 and d9 electron configurations are Jahn-Teller distorted
Cu2+ and Ti3+ spectra show some splitting of single expected absorption
Jahn-Teller distortion break degenerate electronic states (strongest if eg like d9)
Electron Configurations can be symmetry labelled based on degeneracy
Two absorbances
in Cu2+ spectrum.
B1gA1g too low E
to be observed.

8. e. Ti3+ d1 spectrum also shows splitting, but t2g Jahn-Teller effects are small?
Excited state (with eg Jahn-Teller distortion) is responsible for splitting
Distortion from pure Octahedral Geometries are common, so Electronic Spectra are often more complex than the Tanabe-Sugano Diagrams indicate
Example: [Fe(H2O)6]2+ has split peak at 1000nm. Use Tanabe Sugano Diagram to account for transition and explain splitting
Transition is 5T2g5Eg
Splitting due to Jahn-Teller
Distortion of Doubly
Degenerate eg

9. Determining Do from Electronic Spectra
Not always possible, as overlapping bands and complex math limits utility
Often possible to calculate Do and/or B directly from spectrum
d1, d4(hs), d6(hs), d9 complexes give only one band = Do

10. d3, d8 have ground state F terms
d3, d8 have ground state F terms. Do is found from lowest energy transition
d2, d7(hs)—Complicated by splitting into many states. See text for complex explanation of estimate of Do, but we will not cover these.
d5(hs), d4-d7(ls)
d5(hs) has no excited states with the
correct multiplicity, so all transitions are
Spin Forbidden. Very weak absorptions
due to Spin-Orbit relaxation only.
d4-d7(ls) have many excited states,
making analysis difficult

11. Tetrahedral Complexes
More intense absorptions due to no center of symmetry making Laporte Selection Rule moot
d-orbitals are split in opposite direction than octahedral
Hole Formalism: d1 Oh treated similarly as d9 tetrahedral
Use Correlation Diagram for the d10-n Oh configuration to describe dn Td
Example for d2 Td case, use d8 Oh diagram
Orgel Diagrams (L. E. Orgel, J. Chem. Phys., 1955, 1004.)
dn (Oh) and dn±5 have the same diagram
dn (Td) and dn±5 have the same diagram
dn and dn±5 (Oh) is the reverse of dn and dn±5 (Td)
dn (Oh) is the reverse of d10-n (Oh)
dn (Td) is the reverse of d10-n (Td)
High Spin Only, No Energies Listed, No Multiplicities Listed, add g for Oh

12. Orgel Diagrams
Simple tool (can memorize them) to predict # of bands and Term Symbols
Charge Transfer Spectra
a. Exchange of an electron from one Ligand to Metal or Metal to Ligand
LMCT
MLCT

13. Often in UV, but can be in visible region
Much more intense (50,000 M-1cm-1) than dd bands because Laporte Allowed
LMCT: MnO4- dark purple color, O ligand orbitals to empty Mn+7 d-orbitals
MLCT: Usually with p-acceptor ligands (CO, CN-, SCN-)
Both can occur in same complex: Cr(CO)6
Not always possible to determine LMCT vs. MLCT
May cover up dd bands
Often have to collect spectra at two different concentrations
Low concentration to get CT bands on scale
High concentration to get dd bands on scale
Intraligand Bands
Some ligands absorb light themselves (sp* or pp*)
Aromatic and/or conjugated ligands (dyes for example) can be highly colored
Coordination of metal ion may significantly change those bands