In this activity, the provided d orbital splitting patterns need to be matched with ligand geometries. Students are provided with the d orbital splitting diagrams for 6 ligand geometries (octahedral, trigonal bipyramidal, square pyramidal, tetrahedral, square planar, and linear). A web browser is used to view an animation (developed by Flick Coleman) which allows for the visualization of the relationship between the positions of the metal d orbitals and the ligands. Given this information, students should then be able to qualitatively rank the orbitals from highest to lowest energy. Once the orbitals are ranked in terms of energy, the pattern can then be matched to the provded splitting diagrams.
After completing this in-class activity, students should be able to:
- visualize the positions of the ligands relative to the metal's d orbitals;
- determine the energies of the orbitals based upon electron repulsion relative to the barycenter;
- qualitatively rank the d orbitals in terms of their energies for a variety of ligand fields;
- explain why the observed splitting pattern in produced for each ligand geometry.
This activity has been used both as an in-class exercise and a homework assignment, and I have found that it works much better as an in-class activity.
In the class previous to the one where the activity is completed, I work through the splitting diagram for an octahedral complex. The web-based animation is used to show the relative positions of the orbitals and the ligands. While projecting the image on a whiteboard, I illustrate the geometric arrangement of ligands using a cube and I place the metal at the center of the cube and the ligands at the center of each of the 6 faces of the cube. (This allows me to give them an idea about distance between the ligands and the orbitals.) In preparation for the next class, I ask them to think about how the ligand-orbital interactions would be different in a tetrahedral arrangement of ligands. I also remind the students to bring their laptops to the following class.
When arriving in class, I break the students up into groups of two and I ask them to work on the assignment together. They discuss the animations and can consult with other groups or me if they get stuck. The exercise is usually completed in one 50-minute lecture.
There is no formal evaluation done for this in-class activity, but student comprehension is gauged by the quality of the discussion.
When this activity was assigned as a take-home problem set, most students had very little trouble determining octahedral (covered previously), square planar (3 out of 4), and square pyramidal (3 out of 4) splitting patterns. They had a bit more trouble determining the other splitting patterns, with an average of only 1 out of 4 getting each of the others correct.
I have used this actvity in class for several years (with about 40 students) and have found that the students benefit from the ability to discuss the activity with other students. Their discussions focus on the repulsion that would be felt between electrons in various d orbitals and the ligands in each ligand field. The vast majority of students (greater than 85%) easily determined which orbitals will lie above and below the barycenter. More than two-thirds of the students have been able to distinguish the relative positions of the orbitals based upon ligand-d electron repulsion and pair all of the diagrams with the ligand field correctly after discussing it with their fellow classmates.