# Molecular Orbitals of Square-Planar Tetrahydrides

## In-Class Activity

Submitted by Matt Whited, Carleton CollegeThis 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.

Attachment | Size |
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MOs of square planar tetrahydrides Group Activity.docx | 28.58 KB |

Deriving Ligand Group Orbitals (Handout).pdf | 93.7 KB |

* 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.

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 B_{1g}-symmetric LGO can be generated either by inspection of the B_{1g}-symmetric d(x^{2}-y^{2}) 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.

## Comments

## This must be included!

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