11 Apr 2013

MO Theory for Organometallic Compounds: Pentalene

In-Class Activity

Submitted by Zachary Tonzetich, University of Texas at San Antonio

This is an in-class exercise for upper level inorganic students designed to highlight aspects of symmetry, group theory, MO theory, and Hückel theory. The exercise is an expansion of a Problem Set question I give to my Advanced Inorganic Chemistry class. In this activity, students will develop the MO diagram for the π system of the pentalene dianion using the Hückel approach. They will then consider the effect of folding the ring system using a Walsh diagram. Finally, they will use their results to construct the qualitative MO diagram for the sandwich complex, [Ti(η8-C8H6)2] and discussion whether it violates the 18-electron rule. Pertinent references to literature studies are provided to demonstrate to the students how this qualitative approach is in general agreement with more sophisticated studies.

PDF icon Pentalene Example.pdf1.41 MB
Learning Goals: 

This activity is designed to cement various learning objectives in symmetry, group theory, electronic structure, and organometallic chemistry.

Students should be able to follow the flow of the activity without getting lost or confused.

Students should be able to reproduce aspects of the activity on their own without instructor guidance (Hückel solution in D2h, qualitative Walsh diagram, qualitative MO diagram).

Students should also be able to apply the treatment to other non-traditional "sandwhich type complexes".

Equipment needs: 

None, although polynomial solver programs from the internet are useful (see web resources).

Implementation Notes: 

I use these types of examples prior to exams to bring various concepts together and demonstrate real applications of the material taught in class.

Time Required: 
75 minute lecture period
Evaluation Methods: 

Assessment is made through similar problems on exams and/or problem sets.

Evaluation Results: 

My students are generally able to follow along well and comprehend the various reasoning employed in this example. Transfer of these concepts (Hückel approach, derivation of SALCs, etc.) to exams and problem set questions is usually successful although many students have a difficult time estimating the magnitude of orbital overlap when constructing MO diagrams. This is also reflected in some confusion concerning Walsh diagrams.

Creative Commons License: 
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