1. The Electronic Spectra of Coordination Compounds

2. The UV/Vis spectra of transition metal complexes shows the transitions of the electrons. Analysis of these spectra can be quite complex.

3. Electron Spectra
The UV/Vis spectra are used to determine the value of ∆o for the complex. The spectra arise from electronic transitions between the t2g and eg sets of molecular orbitals. Electron-electron interactions can greatly complicate the spectra. Only in the case of a single electron is interpretation of the spectrum straightforward.

4. Obtaining ∆o
For a d1 configuration, only a single peak is seen. It results from the electron promotion from the t2g orbitals to the eg orbitals. The “toothed” appearance of the peak is due to a Jahn-Teller distortion of the excited state. The energy of the peak = ∆o.

5. General Observations
d1, d4, d6 and d9 usually have 1 absorption, though a side “hump” results from Jahn-Teller distortions.

6. General Observations
d2, d3, d7 and d8 usually have 3 absorptions, one is often obscured by a charge transfer band.

7. General Observations
d5 complexes consist of very weak, relatively sharp transitions which are spin-forbidden, and have a very low intensity.

8. Qualitative Explanation
Consider a Cr(III) complex such as [Cr(NH3)6]3+. The ground state configuration is:
____ ____ A transition from the
dz2 dx2-y2 dxy to the dx2-y2, or the
dyz or dxz to the dz2
____ ____ ____ orbitals involve a relatively
dxy dyz dxz minor change in environment.

9. Qualitative Explanation
The transition from the dxz orbitals to the dz2 orbitals involves a relatively minor change in the electronic environment.

10. Qualitative Explanation
Consider a Cr(III) complex such as [Cr(NH3)6]3+. The ground state configuration is:
____ ____ A transition from the
dz2 dx2-y2 dxy to the dz2, or the
dyz or dxz to the dx2-y2
____ ____ ____ orbitals involve a major
dxy dyz dxz change in environment.

11. Qualitative Explanation
The transition from orbitals in the xy plane to the dz2 orbitals involves a fairly major change in the electronic environment.

12. Qualitative Explanation
Since the promotion of an electron from the t2g set of orbitals to the eg set can involve differing changes in environment, several peaks will be seen in the spectrum.

13. 3d Multi-electron Complexes
For complexes with more than one electron in the 3d (and 4s) orbitals of the metal, electron interactions must be considered.
The electrons are not independent of each other, and the orbital angular momenta (ml values) and the spin angular momenta (ms values) interact.

14. 3d Multi-electron Complexes
The interaction is called Russel-Saunders or
L-S coupling. The interactions produce atomic states called microstates that are described by a new set of quantum numbers.
ML = total orbital angular momentum =Σml
MS = total spin angular momentum = Σms

15. Determining the Energy States of an Atom
A microstate table that contains all possible combinations of ml and ms is constructed.
Each microstate represents a possible electron configuration. Both ground state and excited states are considered.

16. Energy States
Microstates would have the same energy only if repulsion between electrons is negligible. In an octahedral or tetrahedral complex, microstates that correspond to different relative spatial distributions of the electrons will have different energies. As a result, distinguishable energy levels, called terms are seen.

17. Energy States
To obtain all of the terms for a given electron configuration, a microstate table is constructed. The table is a grid of all possible electronic arrangements. It lists all of the possible values of spin and orbital orientation. It includes both ground and excited states, and must obey the Pauli Exclusion Principle.

18. Atomic Quantum Numbers
Quantum numbers L and S describe collections of microstates, whereas ML and MS describe the individual microstates themselves.

19. Constructing a Microstate Table
The microstate table is a grid that includes all possible combinations of L, the total angular momentum quantum number, and S, the total spin angular momentum quantum number.
For two electrons,
L = l1+ l2, l1+ l2-1, l1+ l2-2,…│l1- l2│
S = s1+ s2, s1+ s2-1, s1+ s2-2,…│s1- s2│

20. Term Symbols
Each energy state or term is represented by a term symbol. The term symbol is a capitol letter that is related to the value of L.
L = 1 2 3 4
Term Symbol S P D F G

21. Term Symbols
The upper left corner of the term symbol contains a number called the multiplicity. The multiplicity is the number of unpaired electrons +1, or 2S+1.

22. Determining the Relative Energy of Term States
The rules for predicting the ground state always work, but they may fail in predicting the order of energies for excited states.

23. Energy States for a d2 Configuration
A microstate table for a d2 electron configuration will contain 45 microstates (ML = 4-4, and MS=1, 0 or -1) associated with the following terms:
1S, 1D, 1G, 3P, and 3F

24. Determining the Ground State Term
We only need to know the ground state term to interpret the spectra of transition metal complexes. This can be obtained without constructing a microstate table.
The ground state will
a) have the maximum multiplicity
b) have the maximum value of ML for the configuration obtained in part (a).

25. Energy States for a d2 Configuration
Problem: Determine the ground state of a free atom with a d2 electron configuration.

26. The Splitting of Terms
In an octahedral field, the free ion terms will split due to their differing spatial orientations.
Term # of States Terms in Oh Field
S 1 A1g
P 3 T1g
D 5 T2g + Eg
F 7 T1g + T2g + A2g
G 9 A1g + Eg+T1g+T2g

27. Correlation Diagrams
The diagrams show the free ion terms on the left, and the effect of a strong octahedral field on the right.
This diagram is for a d2 ion.

28. Correlation Diagrams
The terms converge on the right side of the diagram in three clusters. Each of these represents the possible electron configurations for a d2 ion in a strong octahedral field.

29. Correlation Diagrams
At the left, the free-ion terms (due to L-S coupling) predominate.
At the right, the electron configurations predominate.
The diagram shows the intermediate cases in which both factors need to be considered.

30. Correlation Diagrams
This correlation diagram shows the ground state and spin-allowed transitions in bold lines.

31. Selection Rules
There are several selection rules that govern the intensities of the absorption bands seen in transition metal complexes.
1. Transitions between states of the same parity (gg or uu) are forbidden. This is the Laporte Rule. This rule would forbid electronic transitions between d orbitals, since all d orbitals are gerade.

32. Selection Rules
1. Transitions between states of the same parity (gg or uu) are forbidden. This is the Laporte Rule.
This rule is relaxed due to vibrations of the complex that cause a loss of the center of symmetry. As a result, molar absorbitivities of L/mol-cm are observed.

33. Selection Rules
2. Transitions between states of different multiplicities are forbidden. This is called the spin selection rule.
This rule can be relaxed very slightly for the first row transition metals by spin-orbit coupling. Typical molar absorbitivities are less than 1 L/mol-cm, with very pale color observed.

34. Spin-Forbidden Transitions
Mn2+ (and Fe+3) usually have a high spin d5 configuration. As a result, all electronic transitions are spin-forbidden. Mn(II) compounds are sometimes a very pale pink, and Fe(III) compounds a very pale green due to relaxing of the selection rule.

35. Correlation Diagrams
This diagram shows the possible transitions that do not violate the spin selection rule A d2 complex should have 3 possible transitions.

36. Correlation Diagrams
A non-crossing rule is observed in correlation diagrams.
Terms or energy states of the same symmetry interact so that their energies never cross.

37. Tanabe-Sugano Diagrams
In order to accurately interpret the electronic spectra of transition metal complexes, a series of diagrams have been created.
These diagrams are used to assign transitions (initial energy state and final energy state) to peaks observed in the spectra, and to calculate the value of ∆o.

38. Tanabe-Sugano Diagrams
Tanabe-Sugano diagrams have the lowest energy state (the ground state) plotted along the horizontal axis. The energy of excited states can then be readily compared to the ground state.

39. Tanabe-Sugano Diagrams

40. Tanabe-Sugano Diagrams
Many tables eliminate, or use dotted lines for excited states that are spin-forbidden.

41. Tanabe-Sugano Diagrams
Also, since the d orbitals are all gerade, the g subscript is usually left off.

42. Tanabe-Sugano Diagrams
The vertical axis is E/B, where B is a Racah parameter. B is a measure of repulsion between terms of the same multiplicity.

43. Tanabe-Sugano Diagrams
The horizontal axis is ∆o/B. In order to determine ∆o, we need to determine the value of B, or mathematically eliminate it.

44. Tanabe-Sugano Diagrams
The diagrams for configurations d4-d7 have a vertical “break” in the middle of the diagram. This is due to the shift from a high spin (weak field) complex to a low spin (high field) complex.

45. Symmetry Labels and Electron Configurations
At the far right side of the diagrams, at an infinitely strong octahedral field, the symmetry labels correspond to the electron configuration of the complex.
T designates a triply degenerate asym-metrically occupied state. or

46. Symmetry Labels and Electron Configurations
An E label designates a doubly degenerate asymmetrically occupied state.
An A or B label designates a non-degenerate state. or or

47. Interpretation of Spectra – d1 & d9
There is only 1 spin-allowed transition, with the energy absorbed equal to the value of ∆o.

48. Interpretation of Spectra – d1 & d9
The d1 excited state exhibits a strong Jahn-Teller distortion, as seen in the UV/Vis spectrum.

49. Interpretation of Spectra – d1 & d9
The d9 ground state exhibits a strong Jahn-Teller distortion. The result is a “side peak” in the UV/Vis spectrum.

50. Interpretation of Spectra – d4 & d6 (high spin)
The Tanabe-Sugano diagrams show only one spin-allowed transition for either complex. The frequency of the absorption equals Δo.

51. Interpretation of Spectra – d4 & d6 (high spin)
The single peak shows distortion from octahedral geometry due to the Jahn-Teller effect.

52. Interpretation of Spectra – d4 & d6 (high spin)
The ground state of Cr2+ (d4) and the excited state of Fe2+ (d6) should exhibit strong Jahn-Teller distortions.

53. Interpretation of Spectra – d3 & d8
For a d3 ground state, the first transition, from 4A2g(F) to 4T2g(F) corresponds to ∆o.
LFSE = .6Δo- .8 Δo
= -.2 Δo ∆o
LFSE = -1.2 Δo

54. Interpretation of Spectra – d3 & d8
The frequency of the lowest energy transition provides the value of ∆o.
The third peak is obscured by a very intense charge transfer band. ∆o

55. Interpretation of Spectra – d3 & d8
The curvature of the 4T1 states is a result of the non-crossing rule. Since the terms won’t cross, they mix, and curve away from each other.

56. Interpretation of Spectra – d3 & d8
The Tanabe-Sugano diagram for d8 is the same as that for d3, except the multiplicity is different.
Three peaks are expected, with the lowest energy absorption equal to Δo.

57. Interpretation of Spectra – d3 & d8
The peaks are jagged due to distortions from octahedral geometry.

58. Interpretation of Spectra – d3 & d8
The Tanabe-Sugano diagram can be used to assign transitions to each absorption.

59. Interpretation of Spectra – d3 & d8ν1
The first peak is due to the 4A2g(F) 4T2g(F) transition. ν1

60. Interpretation of Spectra – d3 & d8ν1 ν2
The second peak is due to the 4A2g(F) 4T1g(F) transition. ν2

61. Interpretation of Spectra – d3 & d8ν3 ν1 ν2
The third peak is due to the 4A2g(F) 4T1g(P) transition. ν3

62. Interpretation of Spectra – d3 & d8ν1
The first peak [4A2g(F) 4T2g(F)] has an energy equal to ∆o. ν1

63. Interpretation of Spectra – d2 & d7
The interpretation of spectra from d2 or d7 (high spin) complexes is the most complicated due to curvature in the ground state of the Tanabe-Sugano diagrams.
Since the ground state and an excited state have the same symmetry (4T1g), they mix and curve away from each other.

64. Interpretation of Spectra – d2 & d7

65. Interpretation of Spectra – d2 & d7
The repulsion of like terms means that the energy of the ground state fluctuates with field strength. ∆o ∆o

66. Interpretation of Spectra – d2 & d7
If ν1 and ν3 are both seen in the spectrum, the difference between the two absorptions = ∆o. ∆o ∆o ν3 ν1

67. Interpretation of Spectra – d2 & d7
The transition corresponding to ν3 is often quite weak, as it involves the simultaneous excitation of two electrons, and is therefore less probable. ∆o ∆o ν3 ν1

68. Interpretation of Spectra – d2 & d7
It is not easy to assign the absorptions due to several complications:
1. Lines cross in the Tanabe-Sugano diagram, making assignment difficult
2. The second and third absorptions may overlap, making it difficult to determine the actual position of the peaks

69. Interpretation of Spectra – d2 & d7
An additional problem arises from the crossing of lines. Assignment of the absorptions is not obvious.

70. Interpretation of Spectra – d5 (high spin)
There are no spin allowed transitions for d5 high spin configurations. Extinction coefficients are very low, though the selection rule is relaxed by spin-orbit coupling.

71. Interpretation of Spectra – d5 (high spin)
Mn2+ compounds are white to pale pink in color.

72. Charge Transfer Spectra
Many transition metal complexes exhibit strong charge-transfer absorptions in the UV or visible range. These are much more intense than dd transitions, with extinction coefficients ≥ 50,000 L/mol-cm (as compared to 20 L/mol-cm for dd transitions).

73. Charge Transfer Spectra
Examples of these intense absorptions can be seen in the permanganate ion, MnO4-. They result from electron transfer between the metal and the ligands.

74. Charge Transfer Spectra
In charge transfer absorptions, electrons from molecular orbitals that reside primarily on the ligands are promoted to molecular orbitals that lie primarily on the metal. This is known as a charge transfer to metal (CTTM) or ligand to metal charge transfer (LMCT). The metal is reduced as a result of the transfer.

75. Charge Transfer Spectra
_ _ eg
d _ _ _ _ _ _ _ _ t2g
_ _ _ _ _ _
free metal octahedral complex ligand σ orbitals
Ligand to metal charge transfer

76. Ligand to Metal Charge Transfer
LMCT occurs in the permangate ion, MnO41-. Electrons from the filled p orbitals on the oxygens are promoted to empty orbitals on the manganese. The result is the intense purple color of the complex.

77. Ligand to Metal Charge Transfer
LMCT typically occurs in complexes with the metal in a fairly high oxidation state. It is the cause of the intense color of complexes in which the metal, at least formally, has no d electrons (CrO42-, MnO41-).

78. Metal to Ligand Charge Transfer
MLCT typically occurs in complexes with π acceptor ligands. The empty π* orbitals on the ligands accept electrons from the metal upon absorption of light. The result is oxidation of the metal.

79. Charge Transfer Spectra
_ _ _ _ _ _
_ _ _ _ _ _ π*
_ _ eg
d _ _ _ _ _
_ _ _ t2g
free metal octahedral complex ligand π* orbitals
Metal to ligand charge transfer

80. Metal to Ligand Charge Transfer
Examples of LMCT include iron(III) with acceptor ligands such as CN- or SCN1-. The complex absorbs light and oxidizes the iron to a +4 oxidation state.

81. Metal to Ligand Charge Transfer
The metal may be in a low oxidation state (0) with carbon monoxide as the ligand. Many of these complexes are brightly colored, and some appear to exhibit both types of electron transfer.