12 Jan 2015

Cobalt-Ammine complexes and theories of bonding in metals

Lab Experiment

Submitted by Erica Gunn, Simmons College
Categories
Description: 

This is a two-week lab in which students synthesize and then characterize three Werner cobalt complexes using IR, UV/VIS and computer calculations using Spartan. Syntheses are based on procedures from:

Angelici, R. J. Synthesis and Technique in Inorganic Chemistry. University Science Books, 1996, pp 13-17.

Borer, L.L.; Erdman, H.W.; Norris, C.; Williams, J.; Worrell, J. Synthesis of trans-Tetraamminedichlorocobalt (III) chloride, Inorganic Syntheses, Vol 31, 1997, pp 270-271.

Slowinski, E.; Wolsey, W.; Rossi, R. Chemical Principles in the Laboratory 11th ed. Cengage Learning, 2016. 

Students were randomly assigned to synthesize one of the three Co complexes in week 1, and then worked in groups to characterize their complexes in week 2.* They were also required to compare the results for their complex with students in other groups. This latter process was partially completed in lab, and then student data was collected in a shared folder on Google Drive to allow all students access to data that they did not personally collect while writing their lab reports. 

 

* In my iteration of this lab, students also measured the kinetics of aquation of the three complexes, with and without solid state catalysts. That portion of the lab still requires optimization, and was removed for simplicity. See instructor notes for a more thorough discussion.

Learning Goals: 

Students will be introduced to models of bonding in coordination complexes using Werner's cobalt ammine complexes.

Each student will synthesize one cobalt ammine complex, and analyze the product using UV/VIS and IR spectroscopy. 

Students will also carry out computations using Spartan to predict the spectra, view the molecular orbitals, and visualize IR vibrational modes for the molecule, and will compare this data to their experimental results. 

Equipment needs: 

A list of chemicals and equipment required is given in the prep notes (assuming a class of 12 students).

Implementation Notes: 

See Instructor notes document.

Time Required: 
2 4-hour lab periods
Evaluation
Evaluation Methods: 

Lab notebooks were collected and graded for all students. In addition to a condensed introduction and thorough lab procedure/observations section, students wrote a short conclusion and discussion of experimental error and answered the postlab questions. Experimental data was compared to literature and classmates' data (where possible), as well as to computational results. 

Evaluation Results: 

See instructor notes document. 

Almost all students were able to synthesize their complexes with little or no difficulty. One student heated too much during the evaporation stage and did not obtain product. Several students obtained low yields for synthesis A, likely due to formation of the cis- instead of the trans product. Reheating the solutions to a higher temperature produced the green trans isomer instead. 

Students were able to obtain UV/VIS and IR data for their complexes, and were interested to compare these experimental results with the computational spectra (Spartan was especially helpful to visualize the vibrational modes to understand how they are connected to symmetry). Our ability to interpret the IR spectra was somewhat limited due to the wavelength range for our instrument and poor preparation of the KBr pellets. We were able to identify some differences in the spectra, but the real identifying peaks fall below 400 cm-1 and were not measurable with our instrument. 

For my iteration of this lab, students also measured the rates of aquation of the three complexes synthesized. This experiment highlighted the dramatic differences in reactivity for the complexes. 

In general, students found this lab straightforward and easy to follow, and these examples served as an anchor for several class discussions (symmetry of vibrational modes, trans effect in kinetics of ligand substitution, isomerism in coordination complexes, etc.). 

Creative Commons License: 
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