22 Apr 2016

Ligand Field Correlations for Square Pyramidal Oxovanadium(III)

In-Class Activity

Submitted by Matt Whited, Carleton College
Categories
Description: 

Students work in groups to derive the ligand-field diagram for a square-pyramidal vanadium(III) oxo complex using octahedral V(III) as a starting point. The activity helps students to correlate changes in orbital energies as a function of changing ligands and geometry as well as rationalizing why certain geometries can be particularly good (or bad) for particular complexes. The activity also helps students see why oxo complexes of early metals are frequently best described as triple bonds.

Learning Goals: 
  • Students shoud be able to apply symmetry and group theory to understand frontier molecular orbital splittings and degeneracies
  • Students will be able to use correlation to predict the ordering of frontier molecular orbitals for an unknown complex from a known starting point
  • Students will be able to distinguish among σ and π effects and predict their importance in determining frontier MO structure for metal complexes
  • Students will be able to rationalize the preference of metal complexes for particular geometries based on number and type of bonding ligand as well as d-electron count
Implementation Notes: 

I use this activity in a class where we first discuss the effect of changing geometry or ligands on the ligand field splittings for metal complexes using octahedral as a starting point.  Thus, the students have already seen tetrahedral, square-planar, and trigonal bipyramidal ligand field diagrams.

Then I break up the students into groups of 3-4 to work on the activity.  After about 30 minutes of work as I move among the groups to answer questions or correct misconceptions, we circle up to go over the answers, which I write out on the document camera (this part usually spills over into the next class).

Time Required: 
~45 minutes
Evaluation
Evaluation Methods: 

I evaluate the students informally by moving among the groups working on the activity and allowing them to present answers to the different questions, then giving other groups opportunities to affirm or challenge the answers.

I also evaluate the success of this LO by gauging student success on ligand-field questions on a subsequent exam.

Evaluation Results: 

In this context (we have already seen a lot of symmetry and group theory), the students are pretty good at seeing how the symmetry labels for d orbitals change going from Oh to C4v, so I like the way the activity guides them to think about that.

Students are more mixed in their ability to see how orbitals move around as ligands move or change.  I expect this, but I am typically surprised that they still have trouble seeing the connection to crystal field theory and recognizing that the orbitals in this case are all antibonding, so greater overlap means more destabilization.

Finally, the part about triple bonding to the oxo really throws most of them for a loop. Even when they label the orbitals as π*, they tend to forget that there must be a corresponding π-bonding MO.

Since I introduced this activity, it seems that student performance on LF questions on exams has improved slightly, but I only have a few years of data points so it's hard to say for sure.

Creative Commons License: 
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