This activity is meant to teach students an MO theory interpretation of hypervalency that goes beyond the simple (and somewhat unsatisfying) explanation that atoms that are in the third row and below use d-orbitals for bonding in addition to s- and p-orbitals. Specifically, students will be learning how to construct MO diagrams for multicenter bonding schemes (i.e., 3c4e).
I have seen other articles in J. Chem. Ed. about how to treat hypervalent molecules, or even how to construct multicenter bonding schemes, but not one that uses what I believe to be a more systematic approach that I am tentatively calling the "missing orbital method".
This activity builds upon the ideas presented in Adam Johnson's VIPER activity and J. Chem. Ed. paper on building LGOs, which my class completes two classes before undertaking this activity.
A student should be able to apply their knowledge of group theory to construct ligand group orbitals of polyatomic molecules
A student should be able to construct a molecular orbital diagram using ligand group orbitals.
A student should be able to identify orbitals that do not participate in bonding in hypervalent molecules.
A student should be able to interpret the meaning of bonding and nonbonding orbitals.
Some students find molecular models helpful in visualizing orbital overlap
All my students found it very helpful to have a table of the orbital hybridizations for the different molecular geometries, which can be found in Adam Johnson's J. Chem. Ed. article.
It is important that students are grounded in constructing LGOs before attempting this activity, because it requires them to quickly build them and combine them with orbitals on the central atom to form MOs. My class gets a week of lecture and activities on LGOs, MO theory, and hypervalent molecules before this activity.
Students are graded on this activity based on the questions and how well they constructed the LGOs and translated them into a molecular orbital diagram, with a specific focus on which LGO was chosen to be nonbonding.
The year before creating this activity, students in this course in had a question on their exam where they were asked to construct the MO diagram of XeF4, being warned that the molecule was hypervalent. They had seen lectures about hypervalency, but never got an opportunity to practice this systematized approach.
This year, my students received essentially the same question on their exam. Comparing results from the two years, every one of my current students scored higher on that question than anyone in the class last year.
One particularly sticky point for many students is seeing how px and py overlap with atoms on a trigonal plane. Since the molecule in this activity is trigonal bipyramidal, about half of the class struggled with drawing the appropriate LGOs. Others were so stuck on the rules they learned about LGOs previously that they forgot to eliminate one of the bonding LGOs before constructing the MO diagram. The last two questions in the activity were also problematic, but that may have been because students were burned out by the end or ran out of time.