CHEMISTRY 2000 Topic #1: Bonding – What Holds Atoms Together? Spring 2008 Dr. Susan Lait

2 Molecular Orbitals of Heteronuclear Diatomics The molecular orbitals of heteronuclear diatomics (CO, CN -, etc.) can be predicted using the same principles that we used to construct the molecular orbitals of homonuclear diatomics: Ignore the core electrons Total number of MOs = Total number of AOs Only AOs of similar energy combine to make LCAO-MOs Only AOs of compatible symmetry combine to make LCAO-MOs:  -type AOs (s and p z orbitals) make  bonds  -type AOs (p x and p y orbitals) make  bonds

3 Molecular Orbitals for CO Consider the valence atomic orbitals of carbon and oxygen: Which AOs will combine to make MOs?

4 Molecular Orbitals for CO Using symmetry and energy as our guide, we predict that we will make LCAO-MOs that look something like: We rank these MOs from lowest to highest energy to the best of our ability; however, at this point, we are estimating for a few of the orbitals. We can use quantum mechanical software such as HyperChem to show us the exact geometry of the MOs formed as well as their exact energies.

5 Using HyperChem, we generate a valence MO diagram for CO: Contrary to what Petrucci states, the MO diagram for CO has the orbitals in the same order as N 2 does. This has been verified experimentally. Why does this make sense? Molecular Orbitals for CO ( ) For the heterodiatomics, it can be hard to classify some of the valence orbitals as bonding, antibonding or nonbonding, so we just number them all as  or  (no  * or  *). Can you identify each valence orbital in CO as bonding, nonbonding or antibonding (with respect to the C-O bond)? side view:

6 Molecular Orbitals for CO Write the orbital occupancy for CO. Draw a Lewis structure for CO, and compare the bond order obtained by the Lewis method to the bond order obtained by MO. (Nonbonding electrons aren’t factored into bond order.) Comment on the MO diagram on the previous page in relation to the formal charges for CO.

7 Molecular Orbitals for HF Consider the valence atomic orbitals of hydrogen and fluorine: Which AOs will combine to make MOs? Which AOs will not mix (and therefore still look like an AO)?

8 Molecular Orbitals for HF Using symmetry and energy as our guide, we predict that we will make LCAO-MOs that look something like: There can be no  bonding in HF. Why not? There will still be orbitals with  symmetry in HF. Which ones?

9 Molecular Orbitals for HF Using HyperChem, we generate a MO diagram for HF:

10 Molecular Orbitals for HF Write the orbital occupancy for HF. Draw a Lewis structure for HF, and compare the bond order obtained by the Lewis method to the bond order obtained by MO. How is the MO diagram on the previous page consistent with other features of your Lewis structure?

11 Molecular Orbitals for AlN Consider the valence atomic orbitals of aluminium and nitrogen: Which AOs will combine to make MOs?

12 Molecular Orbitals for AlN What do we expect the MO diagram to look like for AlN? (Don’t forget that the core electrons get numbers – even if we’re not including them on this diagram!)