[dkmsoxfan88]

How can I tell if a complex [Co2(MIMT)4(NO3)2] is tetrahedral or octahedral from knowing only the UV absorbance (545 nm) and the fact that the complex [Co2(MIMT)2(NO3)2] is tetrahedral (UV absorbance 525 nm)?

[Rabn]

How many ligands does each have, I don't know what MIMT is but it seems to be a chelating agent.  Here's is my take on this, I am likely incorrect but I want to reason through this.  First off both wavelengths are in the visible spectrum not U.V. spectrum( uv goes from ~ 200nm - 400 nm).  So, more ligands equals weaker bond strengths and longer bond lengths so it makes sense that the absorbance should occur at a higher wavelength with 6 ligands...octahedral.  The higher wavelength  means there is a more extensive conjugation network in the molecule with cobalt acting as a kind of electron switchboard.  There must be a way to relate wavelength to conjugation networks, not knowing what MIMT is doesn't help....maybe what I've written will spark an idea for you.

[shelanachium]

Look up MIMT on Web. Appears to be an imidazole-derived thiourea and not a chelator; may co-ordinate through S or N but not both at the same time.

[Rabn]

Now that I have an idea what mimt is it may be that that each cobalt coordinates to an O from NO3 and an N from the mimt...the challenge then is figure out how the molecule is arranged so that the electrons have a continuous path to travel for the conjugation.  Have you ever used chemdraw? or some other drawing program?  You really need to draw this out in order to understand how it be coordinated.

[Alpha-Omega]

This question is going to require some work.  I am attaching a few places that will assist you in devising an approach to solving such a problem.


How UV-Vis is used to distinguish geometries:

http://www.chm.uri.edu/courses/CHM402Spring2007.pdf

Basics for Transition Metal Complexes:

http://web.chemistry.gatech.edu/~wilkinson/Class_notes/CHEM_3111_6170/Introduction_to_transition_metal_complexes.pdf

Basics CFT:

http://artemis.austincollege.edu/acad/chem/kmccain/Chem%20112/ch25.pdf

CFT Predictng Geometries:

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch12/complex.php


http://dcbwww.unibe.ch/groups/ward/Chapter%204.html


http://wwwchem.uwimona.edu.jm/courses/CFT.html


Applications of UV-Vis Molecular Spectroscopy:

http://www.uccs.edu/~rbertran/c418s00/Lec24.ppt#257,2,Slide%202


d-Metal Complexes:

http://faculty.uml.edu/ndeluca/84.334/topics/topic6.htm

OH BTW:

This identical question has also been posted on physicsforums.com on 03/30/2008-you might want to monitor to see if an answer becomes available-I personally do not know too many physicists who could answer this.  It is a Physical Inorganic Chemistry question.  In fact, my friend (a member on that forum), a Physicist sent me the link and asked if I could give some input.

LINK:  http://www.physicsforums.com/showthread.php?p=1670401#post1670401

[Rabn]

I really think the key to this is drawing the system with both ocathedral and tetrahedral geometry in an accurate fashion.  It is imperative to know the bond energies in order to calculate the absorbance.  There must be a conjugated system involved. I have not studied this enough to give any hard numbers here but this is what I mean. Let's say that each degree of conjugation provides an absorbance of 100nm and is additive. To absorb at the 600nm wavelength you would need six degrees of conjugation. That is obviously an absurd genralization/simplification but it lies at the root of what my approach would be.

[Alpha-Omega]

Sure you CAN do that...you do not need to use ChemDraw....that is NOT what the QUESTION DEMANDS...

He is given UV-vis data.  he also had his question phrased differently before...Had it exactly as it appears on www.physicsforums.com as of 03/30/2008 LINK:  http://www.physicsforums.com/showthread.php?p=1670401#post1670401.

Probably a stupid question, but for some reason I can't think of how to determine whether a complex is tetrahedral or octahedral given only UV-vis absorptions. We have [Co2(MIMT)2(NO3)2] and [Co2(MIMT)4(NO3)2], and I know (it's given) that the first one (absorption at 525 nm) is tetrahedral, so I can figure out the crystal field stabilization energy from there. The second one had an absorption at 545 nm. I have no idea how to take that number and determine whether the molecule is tetrahedral or octahedral (high-spin or low-spin). What do I do?

So all he needs to do is use the UV-Vis data and CFT.  He has a few diagrams and CFSE calculations to do.  But it is a simple procedure.  This is a very elementray Physical Inorganic question and is given in every Intro to Inorganic Chemistry course.

BTW:  If you do a search on the internet you will find dozens of hits with similar species used in Inorganic Laboratory Experiments where determiation of the structure (geometry of the experimental analyte) requires the student take his experimental UV-Vis data and using CFT diagrams and stabilization calculations....determine the struture of the species/analytes.

The question states:

How can I tell if a complex [Co2(MIMT)4(NO3)2] is tetrahedral or octahedral from knowing only the UV absorbance (545 nm) and the fact that the complex [Co2(MIMT)2(NO3)2] is tetrahedral (UV absorbance 525 nm)?

All you need is the UV-Vis data and you have to use CFT.

I mean I could also use DFT and state-of-the-art computational software 

LINK:  http://www.boulder.nist.gov/div838/groups.html   

Nice Paper:  http://ethesis.unifr.ch/theses/downloads.php?file=RauzyC.pdf

But why complicate things.....HOW ABSURD!!!!!!!