Students construct computer models of two transition metal complexes, solve their electronic structures, and inspect the resulting d-type molecular orbitals to identify which are non-bonding, sigma* antibonding, or pi* antibonding. After constructing a molecular orbital diagram, they determine which of the two complexes is likely to absorb light at a longer wavelength.
Students should be able to
- carry out electronic structure calculations of transition metal cation complexes
- identify d-type orbitals within a transition metal complex
- explain the role of pi-antibonding orbitals in reducing the crystal field splitting
The exercise is designed to be run on a laptop with Spartan Student version 5.0.1
This learning object is the third in a series of Spartan-based modules in an integrated general chemistry course*. In the preceding two modules, students learn to interpret electrostatic potential mappings of molecules ("Intermolecular forces") and to interpret molecular orbitals ("The MO picture of bonding"). Thus, it is assumed for this activity that students come in already familiar with how to build molecules in Spartan, and how to interpret simple molecular orbitals and orbital diagrams.
*This course is directed at students who took a significant amount of high school chemistry (e.g. AP Chemistry). It is not assumed students have prior expertise in electronic structure calculations, however.
Students are asked to summarize their analyses carried out during their investigations. Central to that analysis is identification of the five metal-centered d-orbitals (d-MOs) in the transition metal complex, based on the shape and degeneracies of those MOs. In addition, students are asked to write responses to the following self-assessments:
- Define the terms transition metal cation complex, crystal field splitting, metal-centered d-orbitals, sigma antibonding geometry, pi antibonding geometry, doubly degenerate, and triply degenerate.
- Describe shape and energy differences between n-d-MO, s*-d-MO, and p*-d-MOs, with an example.
- Describe qualitatively how sigma and pi antibonding affect the crystal field splitting (Δo) and the wavelength of absorbed light.
On exams, when shown pictures of transition metal complex MOs, most students were able to pick out those that correspond to the "metal d orbitals", even when those MO contained considerable ligand character. About 1/2 of students were able to accurately distinguish sigma-antibonding vs pi-antibonding character in these MOs.
In conversations, most students also appeared to clearly understand broader ideas associated with MO theory, e.g., the idea that an MO shows how electron density is distributed over various parts of a molecule, and that these orbitals can contain up to two electrons.
During the activity itself, many students initially struggled with mechanics of interpreting the data as presented by Spartan, i.e., what the energy ladder means, where the energies (in eV) are given. For some, more substantive questions arose regarding the difference between the colors associated with different part of an MO (i.e., the phase of the wave function) vs the coloration in an electrostatic potential mapping; as long as enough time is set aside for the activity, these questions provide opportunities for interesting and in-depth discussion.