[torturedillini]

Hello. I am writing a lab report based on the results of a cobalt experiment I recently completed. Basically, I was given an unknown cobalt complex. I have limited my unknown to either [Co(NH3)₅(NO₂)]Cl₂ or  [Co(NH3)₅(ONO)]Cl₂. These are linkage isomers. I did four characterization tests. I determined the molar mass, the charge of the complex ion, the number of ammonia ligands and the determination of the non-ammonia ligands. For the last one, I used a diode array spectrophotometer to measure my known and unknown solutions. My known is [Co(NH₃)₅Cl]Cl₂. I measured the visible absorption spectrums of each solution. I also collected some data: the peaks of the spectrum, but I don't know what to do with this information. Help please.

[macman104]

Did you use an IR or just visible?  When I did this experiment a few weeks ago, we used an IR to track the changes in the N-O stretching frequencies.

[torturedillini]

Just the ultraviolet and visible parts. I don't know what the peaks are though or what to do with this information.

[AWK]

http://en.wikipedia.org/wiki/Linkage_isomers
http://www.public.asu.edu/~jpbirk/CHM-115_BLB/Chpt24/sld041.htm
http://www.dartmouth.edu/~chemlab/chem6/cobalt3b/full_text/intro.html
http://wwwchem.uwimona.edu.jm:1104/spectra/Colinkage.html

[golddigger50]

So you have a 6 coordinate complex (cobalt has 6 ligands).  5 ligands are ammine groups NH3.  the other one is either NO2 or ONO.  These two ligands have different energies in the "Spectrochemical Series" (look it up).  Wave lengths are related to energy, short wavelength has high frequency and high energy, opposite for longwavelenth.  So say you see two peaks, 300nm and 700nm.  the 300 is higher in energy.

So Look up a Jorgensen table and calculate your Dq.  10Dq = Delta o (Octahedral splitting energy).  You will calculate this value for each of your unknowns and see which is higher in energy and what the octahedral splitting field energy is.

Next you will use a Tanabe-Sugano diagram to find the Delta o of the unknown.  You may be able to find a calculator program online that allows you to punch in longwave and short wave values.  Your compound should have two peaks, since this cobalt is a d6 (+3 oxidation) complex and low spin.  According to Jorgensen this means that it has 6 electrons in the T2g orbitals and no unpaired electrons and is diamagnetic (will have 2 peaks in UV-Vis, and repel magnets).

so once you find the Octahedral splitting field energy using the Tanbe-sugano diagrams, you compare your results to the Jorgensen theoretical value, that you calculated and see which compound has the closest value.


I hope this helps someone, I know it is way late.