Submitted by Sheri Lense / University of Wisconsin Oshkosh on Tue, 08/15/2017 - 18:02


I am hoping to include an experiment on the kinetics of the trans-tetraamminedichlorocobalt(III) ion for my Advanced Inorganic Chemistry class this semester.  We have a new NMR spectrometer that I would like the students to have several opportunities to use throughout the semester, so I have tried following the aquation reaction by 1H NMR spectroscopy, as described in the following reference: Orvis, J.A.; Dimentry, B.; Winge, J.; Corbin Mullis, T.  J. Chem. Ed. 2003, 80(7), 803-805.  In this procedure, the starting material is simply dissolved in D2O and  NMR spectra are recorded at varying time intervals.

In the paper by Orvis et al., only one aquation product, cis-[Co(NH3)4(H2O)Cl]Cl2, which gives three resonances in a 2:1:1 ratio, is observed.  However, the 1H NMR spectra I have collected show a much more complex reaction, which is more consistent with a different paper on the acid hydrolysis of the trans-tetraamminedichlorocobalt(III) ion.  In my spectra, collected over the course of two hours, I observe the following:

  • The trans-tetraamminedichlorocobalt(III) ion gives one resonance, at 4.1 ppm, which diminshes over time until it is barely visible.
  • A peak at 4.4 ppm grows in and then dimishes.  According to other sources from the literature, this may be trans-[Co(NH3)4(H2O)Cl]Cl2, which then reacts reversibly to produce  cis-[Co(NH3)4(H2O)Cl]Cl2.
  • Peaks at 4.3, 3.7, and 3.2 also grow in slightly.  These may be assigned to cis-[Co(NH3)4(H2O)Cl]Cl2.  However, as the experiment progress their ratio differs greatly from the 2:1:1 ratio described by Orvis et al., perhaps due to hydrogen-deuterium exchange by the NH3 groups? 
  • I also have a peak grow in at 4.6 ppm that I have not been able to assign this peak based on the literature.  Perhaps cis-[Co(NH3)4(H2O)2]3+, with the second resonance hiding under another peak?

I am wondering if anyone else has tried out this experiment and if so, whether your results matched those reported by Orvis et al. or whether they were more complex.  If you did obtain more complex results, were the students doing the lab able to draw meaningful conclusions from their data?  I would greatly appreciate any experience or insight on this experiment.

Thank you in advance.


Brian Johnson / St. John's University/College of St. Benedict

Sheri:  We have done this expt for a number of years.  Especially recently, we have not been getting good data.  I had been attributing that to the way we had been gathering data.  For time reasons, students were only gathering data for three temperatures.  When they made the Arrhenius plot it was often not very linear and the activation paramaters, particularly DS, dd not agree with the the lit.  Some times it even  had the wrong sign.   I had attributed this to the use of too few data points.  When I had groups share data to get more temps to make the graph, there was little or no improvement.  I had attributed this to  differences in technique in preparing and inserting the NMR samples.I think I like your explanation better.  Originally we did this expt on a different NMR, and it was easier to set the probe temp in advance of inserting the samples, so we may not have been accutately controlling the temp.  Obviously the rate changes significantly over a small temp range.  Brian

Fri, 10/27/2017 - 18:00 Permalink