My Notes
Categories
This is a kinesthetic activity in which students must utilize knowledge of the σ-donating, π-donating and π-accepting ability of ligands in order to rank the ligands in the spectrochemical series. Students are each assigned a ligand on a card. Suggested ligands are I-, Br-, Cl-, F-, ONO-, NO2- OH-, H2O, pyridine, NH3, ethylenediamine, bipyridine, phenanthroline, PPh3, CN- and CO. Each student must evaluate the π-accepting, π-donating and σ-donating ability of their ligand based on molecular orbital theory and electronegativity. Students can confer with each other and are encouraged to draw molecular orbital diagrams to assist in their decision-making.
In the second part of this activity, students must use their answers from the first part to line up in order from weakest to strongest field ligand as a class. They can talk with each other but cannot use any other resources. Once students have decided on their order, I show them where each ligand falls in the spectrochemical series on an overhead projector and we compare the order that the class has come up with to the known spectrochemical series. Questions that students have asked in the past include why water is a stronger field ligand than hydroxide and why the ligand field strength of the halide ions decreases as the size of the halide ligand increases.
After establishing the correct ranking of ligands in the spectrochemical series, I ask students with π-acceptor ligands to step forward, π-donor ligands to step back and σ-donor only to stay in place and then ask for one volunteer from each group to draw an ionic or molecular orbital diagram that shows how their ligand acts as a π-acceptor or π-donor. One misconception that has come up at this point is that any ligand that falls on the stronger field side of the spectrochemical series (such as ethylenediamine) is a π-acceptor. This is a good opportunity to review that students can refer to an orbital diagram of a ligand to confirm whether it can act as a π-donor or π-acceptor and how the σ-donating, π-donating and π-accepting ability of a ligand affects the ligand-field splitting parameter in the context of a molecular orbital diagram. In an optional fourth part to this activity, students are asked to show on a molecular orbital diagram for an octahedral molecule how their ligand will affect the energy difference between the d orbitals (∆) of a coordination complex.
Attachment | Size |
---|---|
Ligand lineup activity.docx | 14.21 KB |
The learning goals for this activity are for students to:
- A student should be able to apply his/her knowledge of concepts such as orbital theory and electronegativity to evaluate the σ-donating, π-donating and π-accepting ability of ligands.
- A student should be able to utilize knowledge of the σ-donating, π-donating and π-accepting ability of ligands in order to rank the ligands in the spectrochemical series.
- A student should be able to explain how the σ-donating, π-donating and π-accepting ability of a ligand affects the ligand-field splitting parameter in the context of a molecular orbital theory.
- A student should be able to communicate scientific concepts to his or her peers.
No special equipment is required for this activity.
I did this activity with my advanced inorganic chemistry class as a review after we covered the spectrochemical series in the course reading and in lecture. Another suggestion, adapted from "Athletic Periodic Trends Review" by Lori Watson, is to sound a bell if the students' ordering is correct and a buzzer if their ordering is incorrect. If incorrect, the students have to try again.
Evaluation
This activity itself was not graded, but students were assessed using problems on a homework set due later that week that covered ligand-field splitting and the spectrochemical series, exam questions that covered ligand-field splitting and the spectrochemical series and a laboratory activity and report in which students determined how using ligands of varying ligand field strength affected the color and UV-visible spectroscopy of a series of coordination complexes.
In general, students were able to determine the approximate position of most common ligands in the spectrochemical series and most were able to explain how the σ-donating, π-donating and π-accepting ability of a ligand affects the ligand-field splitting parameter in the context of a molecular orbital diagram. A common misconception is that any ligand that falls on the stronger field side of the spectrochemical series (such as ethylenediamine) is a π-acceptor. The next time I run this activity, I plan to reinforce that for small ligands students can refer to an orbital diagram of a ligand to confirm whether it can act as a π-donor or π-acceptor.