Determining coordination sphere from UV, CFSE

[lat3ralus65]

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?

 

[Nino MMP]

The first thing you should do is to look up the literature for UV-Vis absorbance spectra of known complexes with the same ligands and coordination numbers. This will give you an idea of what the expected absorbance spectra for a tetrahedral and octahedral complex with the same ligands would be. Once you have that information, then you can compare the absorbance spectrum of your unknown complex to the expected spectra and see which one is more closely aligned. This should give you a good indication of whether the complex is tetrahedral or octahedral.

 

[Blueshift5]

There are a few ways to determine the coordination sphere of a complex using UV-vis spectroscopy and crystal field splitting energy (CFSE). One approach is to compare the observed absorption peaks with those of known tetrahedral and octahedral complexes. For example, tetrahedral complexes typically have a lower energy peak (longer wavelength) than octahedral complexes due to their smaller CFSE. In your case, the complex with an absorption at 525 nm is likely tetrahedral because it has a lower energy peak compared to the complex with an absorption at 545 nm.

Another approach is to use the Tanabe-Sugano diagram, which plots the CFSE as a function of ligand field strength. The position of the absorption peak on the diagram can indicate the coordination sphere of the complex. For example, a peak at 525 nm would correspond to a tetrahedral complex, while a peak at 545 nm would correspond to an octahedral complex.

In order to determine whether the complex is high-spin or low-spin, you can look at the shape of the absorption peak. High-spin complexes typically have a broader, more intense peak compared to low-spin complexes, which have a narrower, less intense peak. You can also calculate the CFSE using the absorption peak and compare it to the expected values for high-spin and low-spin complexes.

Overall, determining the coordination sphere and spin state of a complex from UV-vis and CFSE data can be challenging and may require additional information or techniques. It is important to carefully analyze the data and consider multiple factors before making a conclusion.