Students easily associate intensity of color with the concentration of a solute from their work in previous general chemistry and analytical chemistry courses. My goal in this exercise is to have them learn that the magnitude of the molar extinction coefficient is a measure of the intensity of the absorption and a function of the quantum mechanical allowedness of the transition. Physically observing the relative intensity of three solutions of the same concentration forces them to struggle with the concept of intensity of an absorption in contrast to the concept of the color that results from the energy of that absorption(s). In the faculty only files I have included an exam question I have given to evaluate student comprehension of this concept. This is a question I aspire for 100% perfect scores, however my data show that 3 students in a class of 15 (so around 20%) did not get a perfect score on this problem.
The following is a simple in-class “demonstration” that I use to segue between d to d and charge transfer transitions. After teaching about d to d transitions and Tanabe-Sugano Diagrams, I show my students three solutions that I have put in large test tubes before class. The three solutions I place in the test tubes are:
a. 10 ml of 0.1M Co(H2O)62+
b. 10 ml of 0.1M Cu(H2O)62+
c. 10 ml of a freshly prepared 0.1 M KMnO4 solution
We review what we have learned about using Tanabe Sugano diagrams to predict the maximum number of possible d to d transitions that could be observed for a given metal ion (with a given oxidation state, d electron configuration, and proposed high or low spin state) in the visible spectrum that give rise to color.
We then use observations of the KMnO4 solution to segue to the discussion of a different type of electronic transition: Charge Transfer.
I have also included an exam question that could be used to evaluate student understanding of the concept highlighted in the demonstration.
Demonstration to Segue Between d to d and CT Transitions
Learning Objectives: Going into this exercise:
1. Students should be able to determine the oxidation state and d electron count for a given set of metal complexes.
2. Students should be able to propose whether a metal complex should be high or low spin (or state whether high and/or low spin are irrelevant designations) for a given metal d electron configuration.
3. If they have learned about Tanabe-Sugano diagrams, students should be able to propose the maximum number of d à d transitions that could be observed for a given metal ion with a given d electron configuration and a given LS or HS designation.
3. Students should be able to visually compare three solutions of different metal complexes with the same concentration and recognize that they vary in color as well as intensity.
Following the Demonstration:
Students should be open minded and prepared to learn about why many transition metal complexes have the beautiful and intense colors that they do (over and beyond the color imparted by d to d transitions that fall in the visible).