Ligand Field Strength and UV absorbance

[ReidMerrill]

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,
Thanks!

 

[HAYAO]

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.

Spoiler: Qualitative Answer

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.

 

[ReidMerrill]

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.

Spoiler: Qualitative Answer

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.

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.