## Quadruple Bond Acrobatics

Submitted by Lori Watson, Earlham CollegeStudents are typically asked one multiple chose or short answer question where they identify which d orbitals are involved in metal-metal quadruple bonding and/or idetify/draw the interaction. They will also use these concepts in a more applied way in both problem set and exam in depth questions where they must explain particular structural or spectroscopic evidence using, for example, the ligand geometry forced by the eclipsed conformation of the dx2-y2 remaining d orbital.

Students generally perform very well on the basic identification/d-orbital interaction question that mostly tests recal of the facts. There is a range of performance on more complex application problems, though students usually correctly identify the role of the quadruple bond orbitals and geometry as a factor. Common challenges involve misidentification of axes, and an inability to think through how changes to variables like metal identity or oxidation state, or ligand sterics, may further contribute to observed bonding or structural data.

Four pairs of students represent quadruple bonding in metal complexes by "forming bonds" with a variety of physical methods involving actions like facing each other while holding hands (sigma bond), touch hands and feet of their partner "above and below" the plane (two pi bonds), touching hands and feet while facing each other (delta bond). This results in a "Twister"-like pile of students resembling the quadruple bonding interaction

Procedure:

1. Ask for 8 volunteers who are comfortable touching each other (holding hands, touching foot to foot)

2. Start with the shortest pair of students, and proceed through all four pairs having them do the following:

- Sigma bond: have two students face each other at a comfortable distance, holding both hands. The held hands represent electron density along the internuclear axis. This is dz2
- Pi bonds: have two pairs of students form the dxz and dyz bonds by having two students stand behind each of the first pair. They will represent pi electron density above and below the internuclear axis by touching hands together on either side (dxz) or a hand and foot above and below the axis respectively (dyz), where the y axis points toward the ceiling. Unless your students can levitate, one foot must remain on the floor at all times--so the dxz orbital interaction is challenging, and one "lobe" (represented by the foot stick out toward the back) will not be properly represented.
- Delta bond: have the tallest students face each other, one behind each of the previous three students on their side. Have them spread out their feet and hands at approximate right angles to each other, and then touch both hands palm to palm together above the z axis, and both feet together below th z axis. To do this, the previous pairs of students will have to move even closer together, and the dxy orbitals will need to "bend" toward each other. Students will observe that it's difficult to make good contact palm to palm. Quadruple bonds are weaker!

3. Let the class dissolve into giggles, and then debrief. How did each group of students have to move? Which orbital was "left out"? How would be expect incoming ligands to bind? Why? Could you have quintuple bonds? (Hint: yes) What would happen if the incoming ligands were too large to be eclipsed? (Hint: will tend to form staggered, triple bonded metal-metal complexes instead).

4. Give the class time to sketch out all four orbitals involved in a metal-metal quadruple bond in their notes.

A student should be able to identify and draw the d orbitals involved in quadruple bonding, including their interactions. They should be able to explain why quadruple bonds are shorter than corresponding triple bonds and where and which d orbital will be involved in bonding to ligands.

8 willing students who consent to physical contact with each other (holding hands, touching foot to foot). It works best to begin with the shortest pair of students and proceed toward the tallest pair of students.

This works best when begining with the shortest pair of students and proceeding toward the tallest pair of students.

Please see attached pictures for a step-by-step guide to movement.