Help on using Tanabe-Sugano diagrams
To make use of the Tanabe-Sugano diagrams provided in textbooks, it would be
expected that they should at least be able to cope with typical spectra for
d3, d8 octahedral and d2, d7 tetrahedral
systems since these are predicted to be the most favoured from Crystal Field Stabilisation
calculations. This is not the case. All the diagrams presented are impractical,
being far too small and for chromium(III) actually stop before the region
of interest of many simple coordination complexes.
No textbooks give Tanabe-Sugano diagrams for tetrahedral systems.
The spectra of tetrahedral complexes are generally not considered or are treated by
the use of Orgel diagrams. If we ignore spin-forbidden transitions, where the energy
of the states depend on both the B and C Racah parameter, then
it should be possible to use the dn, d10-n relationship
between octahedral and tetrahedral for interpretation of the spin-allowed transitions.
This is because, for example, the d3 octahedral and d7 tetrahedral states
have the same energy dependencies on D/B.
When using the Tanabe-Sugano diagram in this way the major difference is that the size of
D tetrahedral is only roughly 4/9 times that of
D octahedral and so all complexes are high spin and
the area of interest is moved closer to the left hand side of the diagram.
Procedure
- Record the UV/Vis spectrum of your sample.
- Tabulate peak information in wavelength(nm) and convert to wavenumber(cm-1)
- (calculate extinction coefficients based on the concentration-
not critical for this exercise)
- calculate the experimental ratio of v2 / v1
- use the appropriate Tanabe-Sugano diagram
to locate where the ratio of the second to first peak matches
that of the experimental value above.
For d2 (oct), d8 (tet) and d3, d8 (oct) d2, d7 (tet)
JAVA applets and
spreadsheets are available which perform these calculations.
- Tabulate the values of v1 / B', v2 / B', v3 / B1 from the Y-intercepts and
D/B' from the X-intercept.
- Using your experimental values of v1 and v2 (v3 if seen), calculate
an average value of B' from these Y intercept values.
- Calculate D based on your value of
D/B'.
- Assign all the spin-allowed transitions you observed.
- Comment on the size of the experimental B' compared to the free-ion value.
- Do you observe any peaks that might be spin-forbidden transitions?
If so, can you assign them?
- Comment on the size of your calculated extinction coefficients (if measured).
The expected values should be compared to the following rough guide.
For M2+ complexes, expect D=
7500 - 12500 cm-1.
For M3+ complexes, expect D=
14000 - 25000 cm-1.
B for first-row transition metal free ions is around 1000 cm-1.
Depending on the position of the ligand in the nephelauxetic series, this
can be reduced to as low as 60% in the complex.
Extinction coefficients for octahedral complexes are expected to be around
50-100 times smaller than for tetrahedral complexes. For a typical spin-allowed
but Laporte (orbitally) forbidden transition in an octahedral complex, expect
e < 10 m2mol-1.
A set of UV/Vis spectra (in JCAMP-DX format) of some simple
coordination complexes are available.
Created and maintained by
Dr. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Feb 1999. Last modified 11th February 1999.
URL http://wwwchem.uwimona.edu.jm:1104/courses/Tanabe-Sugano/TShelp.html