18 Apr 2014

Molecular Orbitals of Square-Planar Tetrahydrides

In-Class Activity

Submitted by Matt Whited, Carleton College

This in-class activity walks students through the preparation of a molecular-orbital diagram for methane in a square-planar environment.  The students generate ligand-group orbitals (LGOs) for the set of 4 H(1s) orbitals and then interact these with carbon, ultimately finding that such a geometry is strongly disfavored because it does not maximize H/C bonding and leaves a lone pair on C.

The activity then moves on to a published square-planar nickel tetrahydride (granted that the published version is stabilized by bonding to two other Ni centers, but it is interesting nonetheless).  Students find that inclusion of d orbitals restores 4-fold bonding for the square-planar molecule.

Learning Goals: 

* Students should be able to identify the symmetry point group for a molecule and use this information, together with a character table, to determine the symmetries of orbitals on the central atom as well as LGOs for the set of equivalent surrounding atoms.

* Students will be able to construct a simple molecular-orbital diagram using basic principles of MO theory, including symmetry constraints.

* Students will gain an appreciation for the importance of molecular geometry in determining a molecular-orbital picture and dictating stability.

* Students will be able to predict differences in bonding utilizing d orbitals versus s and p only.

Implementation Notes: 

I have used this LO twice with my post-pchem inorganic class.  I split the students into groups of 3-4, then we work through generating the LGOs (questions 1&2) together.  In their groups, they find symmetry matches for these LGOs on C and Ni and use those findings to construct molecular-orbital diagrams.

I teach students both projection operator (algebraic) and generator function (visual) approaches to generating LGOs (and all SALCs, for that matter), and I find that square-planar methane provides a good opportunity to show them how the B1g-symmetric LGO can be generated either by inspection of the B1g-symmetric d(x2-y2) orbital or by using the projection operator. Of course, this is also a nice opportunity to show students that a generator function is just a tool for helping us visualize a SALC, so it doesn't matter that carbon doesn't have any low-lying d orbitals.

I have attached a handout that I provide for my students on generating LGOs.

Time Required: 
20-30 minutes
Evaluation Methods: 

Students were informally assessed, primarily in a formative manner as I strolled around the classroom asking and answering questions for individual groups. The groups were also informally assessed through their answers to the questions in the handout.

Evaluation Results: 

I do this activity fairly early in my treatment of MO theory, so some students still have trouble relating what they find in character tables to what they need to include in an MO diagram.

Some groups also got hung up on energetic ordering of atomic orbitals, even though these should be similar to what we saw earlier for methane.

However, the students were generally successful in finding that there can only be 3 bonds in square-planar methane, leading to a much less favorable structure.

Creative Commons License: 
Creative Commons Licence


This must be included! (planar methane from Periodic Table of Videos) https://www.youtube.com/watch?v=b_L9DcGiXyc

And this is the paper that the video references:

Cooper, Oliver J., Wooles, Ashley J., McMaster, J., Lewis, W., Blake,
Alexander J. and Liddle, Stephen T. (2010), A Monomeric Dilithio
Methandiide with a Distorted trans-Planar Four-Coordinate Carbon.
Angew. Chem. Int. Ed., 49: 5570–5573. DOI: 10.1002/anie.201002483

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