Chemistry students make rainbow of colors in lab experiment

milticolored liquids in various beakers

By Josephine Eshon
Science Communicator & Graduate Student (Schomaker Group)

Who knew chemists could make all the colors in the rainbow by changing a single parameter?

Now students can rediscover their childhood fascination through a new undergraduate inorganic chemistry laboratory experiment that emerged as a way to access all of the colors in the rainbow.

Researchers, Evan Warzecha, Timothy C. Berto, Chad C. Wilkinson, and John F. Berry, at the University of Wisconsin-Madison, developed this new method with the addition of multiple ligands to a dichloromethane solution of rhodium acetate (Rh2OAc4), a common catalyst in many organic and inorganic reactions. Rhodium acetate has two open coordination sites along the rhodium-rhodium vector. These sites allow various ligands to bind to rhodium. Ligands, including sodium isonicotinate, PPh3, p-toluenesulfonylmethyl isocyanide, benzaldehyde, ethanol, and acetonitrile, were explored. The color of the resulting rhodium acetate ligand complex is dependent on the identity of the ligand. The different colors result from the different electronic structures of the metal complexes.

student working in lab

When asked where the idea for the experiment originate, Berry said from speaking with Bassam Shakhashiri, a UW-Madison chemist known for exciting demonstrations.

“Every year, Bassam features a different element in his Christmas show, and for the show on rhodium, he asked if I knew about any colorful experiments that feature that element,” Berry said, “I knew about the different colors that you get when you dissolve rhodium acetate in different solvents. So I asked an undergraduate (Evan Warzecha) who was working with me at the time to see if he could generate all of the color hues of the rainbow using rhodium acetate. Evan figured out how to do it, and the rest is history!”

This experiment illustrates ligand field theory, molecule orbital theory, synthetic chemistry, absorption spectroscopy, molecular orbital theory, metal-metal bonding, and solvatochromism. All of these concepts are topics covered in inorganic chemistry classes at UW-Madison. At the end of the experiment, students conduct UV-vis spectra for each solution, to explain the color of the resulting rhodium acetate ligand solution. This experiment has been conducted for five semesters and takes roughly one hour to complete.

According to Wilkinson, student success is measured by “assessing the student data collected, i.e. do the spectra collected look as intended and are students correctly interpreting the features of the spectra and their understanding of ligand field theory though a deeper level of data processing.” Furthermore, Wilkinson states, “grading for this experiment is not difficult… The feedback provided by the experiment helps them to address misconceptions students may have.”