CHEMISTRY 2000
Topic #1: Bonding – What Holds Atoms Together?
Spring 2008
Dr. Susan Lait
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:
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
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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 pz orbitals) make  bonds
 -type AOs (px and py orbitals) make  bonds
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Molecular Orbitals for CO

Consider the valence atomic orbitals of carbon and oxygen:

Which AOs will combine to make MOs?
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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
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as well as their exact energies.
Molecular Orbitals for CO

Using HyperChem, we generate a valence MO diagram 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 *).
(side view:
)
Can you identify each
valence orbital in CO as
bonding, nonbonding or
antibonding (with
respect to the C-O
bond)?
Contrary to what Petrucci states, the MO diagram for CO has
the orbitals in the same order as N2 does. This has been
verified experimentally. Why does this make sense?
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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.
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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)?
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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?
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Molecular Orbitals for HF

Using HyperChem, we generate a MO diagram for HF:
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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?
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Molecular Orbitals for AlN

Consider the valence atomic orbitals of aluminium and nitrogen:

Which AOs will combine to make MOs?
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Molecular Orbitals for AlN
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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!)
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