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Homework Help: Ligand Field Strength and UV absorbance

  1. Feb 16, 2017 #1
    • Moved from a technical forum, so homework template missing
    I have a lab report for an experiment in which we used ion exchange chromatography to separate three Chromium complexes, CrCl2(OH2)4+, CrCl(OH2)52+, and Cr(OH2)63+
    We used a UV vis to find the absorbance of each compound and used that to calculate the mole fraction of each ion in a mixture.

    One of the questions I need to address in my report is the wavelengths of absorption maxima for the three compounds and comment on the relative ligand field strengths of Cl- and H2O

    I don't know if field strength has anything to do with absorbance wavelengths but I think it has something to do with how as time goes by, OH replaces Cl in the complex as time goes on. Am I on to something here?

    Any help would be appreciated,
  2. jcsd
  3. Feb 17, 2017 #2


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    Gold Member

    No, field strength has to do a lot with absorbance wavelengths. Different ligand leads to different ligand field splitting of energies of d-orbitals of Cr3+. This shift in energy causes a shift in the state energy. Absorption corresponds to transition between ground and excited states, so absorption spectra should shift with different ligands. That is why you are using UV-Vis absorption spectroscopy in the first place.

    Cl- is a ligand with stronger ligand field than H2O. Consequently, for this Chromium(III) complex, which is a octahedral complex, the absorption spectra should red shift (energy gap between ground and excited state becomes smaller) with increasing number of H2O because Cr3+ is a d3 ion. Refer to Tanabe-Sugano Diagram that shows electronic state level energies with different ligand field strength.

    You can reverse this line of argument, and that gives you the answer to your report.
    Last edited: Feb 17, 2017
  4. Feb 17, 2017 #3
    Thanks! I took the lecture for this lab class last year so I forgot about that. That explains why the peak absorption shifts to lower wavelengths as more Cl is displaced by water.
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