I have been studying about the UV-Vis spectroscopy. Sometimes we are given a graph which shows the absorbance and the wavelength,the appropriate Tanabe-Sugano diagram and we are asked to assign the transitions to the bands. I know that the bands can be d-d transitions, Ligand To Metal Charge Transfer or Metal To Ligand Charge Transfer or π→π* etc.Out of the things that I have studied, d-d transitions have low intensity whereas the LMCT phenomenon is extremely intense.

The part which gets confusing for me is I have seen some exercises for the complex [Co(H20)6]2+ for example, and there was a question that said why there is a small intensity band at ~640nm and the answer was because there is a two electron transition. Also, in another example of the complex [Ni(H2O)6]2+ where there was a question about why there is a shoulder, the answer was that it is spin forbidden and it has to do with borrowing energy from a close band.

Is there a way in order to understand if a small band is because of d-d transition or borrowing energy from another band or of a two electron transition etc?

  • $\begingroup$ 'is there a way to understand', yes but its not so simple: you will need to understand some symmetry and quantum stuff. Have a look at a good text such as 'Molecular Symmetry and Group Theory' by R. Carter (publ Wiley), there is a whole chapter of about 60 pages on your topics. There are also sections of chapters on bonding in metal complexes etc. $\endgroup$ – porphyrin May 1 '18 at 8:03

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