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Author Topic: UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl  (Read 4399 times)

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harami

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UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl
« on: November 25, 2006, 07:14:05 PM »

Whats a good way to rationalize the difference in intensity, concerning the much higher intensity in the cis isomer. Both have d-d transitions, because they are octahedral right? I know the trans isomer is more symmetrical, but what does this mean in terms of selection rules and transition intensity? This whole thing has really confused me because I haven't covered it in class but I need to know this stuff for a project.

Thanks
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Dan

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Re: UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl
« Reply #1 on: November 26, 2006, 01:51:29 AM »

Do a little research into the "Laporte rule". Centrosymmetric complexes are subject to this selection rule that says that the parity must change for the transition to be allowed.

ie. the gerade-gerade d-d transition is parity forbidden.

This rule doesn't apply to complexes that have no centre of inversion, as the symmetry labels gerade and ungerage do not apply.

There is a pretty good chapter on the electronic spectra of complexes in Shriver & Atkins' Inorganic chemistry, I'd suggest starting there.
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harami

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Re: UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl
« Reply #2 on: November 26, 2006, 06:34:28 AM »

Thanks, I unfortunately can't get a hold of the text because its been taken out. After some research: so does this mean that the trans isomer has forbidden Laporte transitions because it has a centre of symmetry, i? Whereas the cis isomer doesnt have that quality so it will have a higher molar extinction coefficient. Does that sound right?
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Dan

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Re: UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl
« Reply #3 on: November 26, 2006, 07:05:40 AM »

the trans isomer has forbidden Laporte transitions because it has a centre of symmetry, i? Whereas the cis isomer doesnt have that quality so it will have a higher molar extinction coefficient. Does that sound right?

on the right track, but watch how you word this. I don't like "forbidden Laporte transition", I'd go with "Laporte forbidden transition" meaning a transition that violates the Laporte rule.

I'd say that the d-d transitions in the trans isomer violate the Laporte selection rule (or are Laporte forbidden), which applies because the complex is centrosymmetric.
The corresponding transitions in the cis isomer are not subject to the Laporte rule, due to the lack of an inversion centre in the cis complex.

This stuff should be in any heavy inorganic textbook, or perhaps you can even find a book on inorganic spectroscopy.
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chiralic

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Re: UV-Vis spectra of cis and trans [Co(NH3)4Cl2]Cl
« Reply #4 on: November 26, 2006, 03:28:52 PM »

Hello harami:

Check on Google Tanabe-Sugano Diagrams too, because with this diagram you can predict absoptions in the UV-Vis of EM Spectrum for Coordinations Compounds.

Regards,

Chiralic
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