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.
|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 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.