d8 Tanabe-Sugano diagram
A Tanabe-Sugano diagram of the spin-allowed and some forbidden transitions for octahedral d8 and tetrahedral d2 complexes is given below.
spin-allowed transitions
Solid lines
3T1g(P) ← 3A2g Green
3T1g(F) ← 3A2g Blue
3T2g(F) ← 3A2g Red

spin-forbidden transitions
dotted lines
1A1g(S) ← 3A2g Copper
1T2g(G) ← 3A2g Dk green
1Eg(G) ← 3A2g Lt green
1T1g(G) ← 3A2g Grey
1A1g(G) ← 3A2g Brown
1T2g(D) ← 3A2g Orange
1E g(D) ← 3A2g Teal

Select the region of interest (usually between 5-15 Δ /B) then click on the curve to get values. See the instructions for more information.

For the octahedral d8 case, the ground term is 3A2g which is plotted along the X-axis. The first spin-allowed transition is to the 3T2g level which is the red line. The second spin-allowed transition is to the 3T1g level (drawn in blue) and the third transition is to the 3T2g(P) level (drawn in green).
The low-energy spin-forbidden transitions arising from the 1D, 1G and 1S terms are shown as well (teal, orange, gold, gray, light green, dark green and copper).

For the tetrahedral d2 case, the transitions are expected at much smaller values of Δ/B since Δtet ~ 4/9 Δoct. (The subscript g's should be dropped as well.)

If the lowest spin-allowed energy Peak 1 is at cm-1 and the ratio of the energies (ν2/ν1) is then
to find where this ratio falls on the Tanabe-Sugano diagram and get the predicted value of B' and the energy of the transitions.
Initial output is:

From this a JCAMP-DX file can be created suitable for plotting with MDL CHIME
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Copyright © 2003-2011 by Robert John Lancashire, all rights reserved.

Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Dec 2003. Links checked and/or last modified 25th March 2011.
URL http://wwwchem.uwimona.edu.jm/courses/Tanabe-Sugano/TanSugd8.html