This document presents information on the Tanabe-Sugano diagram, which is used in coordination chemistry to predict absorptions in the UV-visible and IR spectra of coordination compounds. It was developed by Yukito Tanabe and Satoru Sugano in 1954 to explain the absorption spectra of octahedral complex ions. The diagram plots orbital energy as a function of the Racah parameter B versus the ligand field splitting parameter Δo/B. It can be used to determine the ordering of electronic states and predict possible electronic transitions based on parameters like Δo, Racah parameters B and C, symmetry rules, and term symbols of electronic configurations. The diagram has advantages over earlier Orgel diagrams in that it can be applied to
3. Introduction
Background:
• Yukito Tanabe and satoru sugano- “ On the Absorption spectra of
complex ions”-1954.
• Explain the absorption spectra of octahedral complex ions.
Definition:
The Tanabe-Sugano diagram is a plot of orbital energy as a function of the
Racah parameter B vs ∆ₒ/B.
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4. Key points:
• used in coordination chemistry to predict absorptions in the
UV, visible and IR electromagnetic spectrum of coordination
compounds.
• can be used for both high spin and low spin complexes.
• All the terms are shown on diagram , spin forbidden
transitions are also shown.
• Ground state is taken as constant reference and energies of
all other terms are plotted with respect to ground state.
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5. Parameters
The exact ordering of electronic state is determined by the
following parameters:
1. The magnitude of ∆ₒ.
2. Racah parameters
3.Symmetry rules
4. Term symbol of configurations
5. splitting of term in octahedral field
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6. 2. Racah’s parameters
Describes various aspects of inter electronic repulsion.
Three Racah’s parameters.
• A- is an average total inter electronic repulsion. ( not
necessary in Tanabe-Sugano diagram).
• B & C- corresponds with individual d electron repulsions.
B is the most important Racah’s parameter to explain the
Tanabe-Sugano parameters.
• Typically for free ions , B= about 1000cm-1
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7. 3.Symmetry Rules
Symmetry Rules:
LaPorte: allowed transitions occur between orbitals of opposite
symmetry WRT inversion (gerade (even) and ungerade (odd) in
character tables)
Spin Multiplicity: allowed transition occur when spin multiplicity
is unchanged.
• d0 metal cations: charge-transfer transitions
LaPorte allowed; ligand ∏* to metal d orbital.
• d1 to d9 metal cations:
d-d transitions LaPorte forbidden; same orbital type.
• d10 metal cations: no d-d transitions because the orbitals are
filled.
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9. 5.Term splitting in octahedral field
L Term Term splitting in octahedral field
0 S A1g
1 P T1g
2 D Eg+ T2g
3 F A2g + T1g+ T2g
4 G A1g+ Eg+ T1g+ T2g
5 H Eg+ 2T1g+ T2g
6 I A1g + A2g+ Eg+ T1g+ T2g + T2g
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10. Russell-Saunders Coupling
Determination of ground state term of Transition Metal cations:
Steps:
Step 1:Draw d-orbitals and fill with # electrons for desired ion
Step 2: Calculation of Spin Multiplicity = no. of unpaired electrons +1 = S
Step 3:Find maximum ML (ml = -2, -1, 0, 1, 2 for d orbitals) = L
Step 4: Ground state term: SL
Example: Cr(II); d4
Orbital diagram:
4+1 = 5=S
2+1+0+(-1) = 2 =D=L
5D is the ground state term.
Spin-allowed transitions will be pentet to pentet.
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11. Tanabe-Sugano diagram for d2 octahedral complexes
[V(H2O)6]3+:
Figure: Tanabe-Sugano diagram for d2
octahedral complexes
Three spin allowed transitions:
v1= 17400 cm-1
v2= 25600 cm-1
v3=38000 cm-1
Lowest energy transition occurs
at E/B= 25.5
E/B= 17400 cm-1 /B = 25.5
So B= 680 cm-1
Again, ∆ₒ can be calculated,
∆ₒ/B= 27
So, ∆ₒ= 1860 cm-1
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13. Advantages over Orgel diagrams
• Tanabe–Sugano diagrams can be used for both high spin and
low spin complexes, unlike Orgel diagrams which apply only
to high spin complexes.
• In a Tanabe–Sugano diagram, the ground state is used as a
constant reference, in contrast to Orgel diagrams.
• However, if the ligand field splitting energy, 10Dq, is greater
than the electron-repulsion energy, then Orgel diagrams fail
in determining electron placement whereas Tanabe–Sugano
diagrams can be applied to situations when 10Dq is
significantly greater than electron repulsion.
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14. Applications as a qualitative tool
• Shows the energy splitting of the spectral terms with
the increase of the ligand field strength.
• It is even easier to predict the possible transitions and
their relative intensity.
• Used to predict the position of electronic transition.
• Tanabe–Sugano diagrams are very useful tools for
analyzing UV-vis spectra:
they are used to assign bands
and calculate Dq values for ligand field splitting.
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