I have synthesized some Cobalt(III) form the oxidation of cobalt (II) in this case when i take 1 mol ligand & one mole cobalt (II) compound containing tetradentate ligand the ligand coordinate to the metal center at axiel position and form a structure (expected as square pyramidal) if i take 2 mole ligand & 1 mole cobalt(II) compound it form octahedral complex. when i take the NMR it shows the correct number of proton count. can we find any difference in UV- Visible spectroscopy?then how to interpret ? if it possible kindly send relevant article.
UV-Vis spectroscopy is not an absolute analysis method (i.e. for the identification of a species). You can only differentiate five and six-coordinate Co-complexes if you have reliable references (spectra) with strictly analogous (known) ligand complexes and all of these show marked difference in 5 and 6-coordinated states.
You should check the composition of the complex. Regardless of the stoichiometry, is not usually the Co (III) complexes are five coordinated. They are typically octahedral.
There are thousands of octahedral Co(III) complexes and except three or four special cases they are all diamagnetic. That means that you can use NMR (proton, carbon, phosphorus ...) to check the composition of your product. I am sure that your complex contains octahedrally coordinated cobalt(III), irrespective of the stoichiometry of the starting compounds.
I understand that the ligand you are using is a monodentate one, but what are the other ligands in the Co(III) complex? As Emira and Wolfgang stated the pentcoordinate Co(III) complexes are not likely to form. Octahedral Co(III) are the most frequent complexes due to the high CFSE of the low-spin d6 configuration. You have mentioned that you observed penta and hexa coordinate complexes. How did you characterize the complexes? What is the solvent you are using. There is a possibility of coordination of the solvent to convert the penta to hexacoordinate.
Water is around and it is a pretty good ligand. Proton NMR does not help you to see the coordinated water.
Should you have a Co(II)aquo4 (3d7) or an octahedral Co(II)aquo6 (II) or any other geometry you are wasting your time trying 1H-NMR. The analogous Co(III)Aquo4 or Co(III)Aquo5 geometries may give you some info, provided they are NOT high spin (aquo is weak field). Any other ligand Glycine, imidazole ... aminoacids combined with Co(III) sure will give you some indication of 1H present in the 1st coordination sphere. However, not so fast, the frigging Co(III) has some small diamagnetic contributions that alter the NMR spectra. Kinda find it interesting. Should you have line broadening, typical, got to try low temperatures, its amazing. Figure it yourself why. The real challenge is a mixed oxidation state (CoII-O-CoIII). Enjoy the pentacoordinated 3d6, the only place you can find pentacoordinated Co(II) are the enzymes. I would buy a Paramagnetic NMR Spectra book. Consider also that Co(III) is a Lewis acid, and will destabilize the protons on the 1st coord sphere (H2O), thus changing the chemical shift. Yes you can do it, I've been there.
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