Lecture 9: Methane, Borane, and Ammonia

Read section 4.5 from your textbook, then go to WebCT and take Quiz 5. This quiz is due by 10 PM on Sunday, February 4.

The key to the bonding in these molecules is to make group orbitals from the atoms (hydrogen atoms with these molecules) that are bonded to the central atom. These group orbitals must have the correct symmetry to interact with the atomic orbitals of the central atom. The group orbital must transform like the central atoms's atomic orbital in the symmetry operations of the point group.

Methane

The carbon in methane has 2s and 2p valence orbitals. The 4 hydrogen atoms each have a 1s orbital. The following discussion will show you how we can use symmetry to combine the orbitals on carbon with the orbitals on hydrogen to make 4 bonding and 4 antibonding molecular orbitals.




Imagine methane in a cube with the x,y and z axes passing through the faces of the cube.

The hydrogen atoms are located on opposite vertices.




Make linear combinations of carbon 2s and hydrogen 1s orbitals.

The positive combination is shown here. (2 MOs possible)

Click on the picture to see the other combination.




Make linear combinations of carbon 2p and hydrogen 1s orbitals.

The 2pz + 1s combination is shown here. (6 MOs possible)

Click on the picture to see the other combinations.

Including all the molecular orbitals from linear combination of atomic orbitals gives us the molecular orbital diagram shown below. We can fill each orbital with 2 electrons. Methane has 8 electrons from the valence shells of hydrogen and carbon. We fill the lower 4 orbitals (bonding MOs) and the high energy orbitals (antibonding) are empty.



Click here to see pictures of the calculated electron density in methane.

Borane

First, analyze the shape and symmetry of the boron and its orbitals.
    (a) Lewis structure

    (b) Shape and symmetry

    (c) What orbitals are available on the central atom? How do they transform in the molecule's point group?



    (d) What group orbitals can be made from the atoms around the central atom?





    (e) What group orbitals have the correct symmetry to combine with the central atom's atomic orbitals? How do these orbitals transform in the molecule's point group symmetry?



    Note that the boron 2pz orbital is out of the plane of the hydrogen group orbitals and it can't combine with them. It will remain as a non-bonding molecular orbital.

      (f) Draw the molecular orbital diagram.



      Add the valence electrons.



    Ammonia

    Try this one yourself.
    1. Draw the Lewis structure of NH3.

    2. Use VSEPR to determine the shape and symmetry. Remember that the lone pair is important! What is the point group?

    3. What orbitals are available on the central atom? This is easy. It is the same as for boron. However, if you look at how these orbitals transform in the the symmetry of the point group, you see that the 2pz orbital has the symmetry of the 2s orbital.

    4. What group orbitals can be made from the atoms around the central atom? Again pretty much like the group orbitals in borane except that the group orbitals are below the x-y plane. How do these orbitals transform in the molecule's point group symmetry? Draw the orbitals.

    5. What group orbitals have the correct symmetry to combine with the central atom's atomic orbitals?

    6. Draw the molecular orbital diagram. Then check yourself by looking at page 114 in your textbook.