Here are a few selected questions that have appeared on previous Chem 220 exams. I make no
claims as to they’re being comprehensive in their scope, but they will give you an idea of the
sorts of things I might ask you to do.
One representation of the structure of Tabun (or Agent GA) is shown.
Tabun belongs to the same class of nerve gases as Sarin and VX,
which act as acetyl-cholinestearase inhibitors.
O CH3
CH3CH2
P N
C CH3
a) What two orbitals overlap to form the P-C bond to the cyano
group?
N
b) What two orbitals overlap to form the P-C bond to the ethyl group?
c) Which P-C bond do you expect to be the shortest, and why?
d) The observed geometry at the N(CH3)2 group is planar, with bond angles near 120º. What
does this suggest about the hybridization of atomic orbitals at N? Draw a resonance form of
Tabun that accounts for this observation, including formal charges if required.
Below is a qualitative molecular orbital diagram for NH3, which has C3v symmetry. Symmetry
adapted linear combinations of the hydrogen 1s orbitals in this molecule are shown.
a) Which orbital is the LUMO of NH3? Describe this orbital
as either bonding, non-bonding, or anti-bonding.
3a1
2e
a1
b) Consider the following chemical reaction:
NH3 + [H]– ⎯→ [NH2]– + H2
Rationalize this reactivity in terms of the MOs of NH3
2a1
c) The 2a1 orbital is lower energy that it would be if the NH3
were a planar (D3h) molecule. Explain why this specific
orbital is stabilized upon a change in symmetry from D3h to
C3v.
e
1e
1a1
The molecule at right is bis(pyrazolyl)pyridine (NNN). Used as a
ligand, it binds to a metal with three N atoms in a planar arrangement,
so that (NNN)2M will have an (approximately) octahedral geometry.
The structure of the [(NNN)2Fe]2+ ion is highly unusual, in that it
has different bond lengths at different temperatures. Below
260K, the average Fe-N bond length is 217 pm, while above
260K, the Fe-N bond length is 195 pm. The compound changes
its number of unpaired electrons at 260K.
N
N
N
N
N
a) For the low-temperature form of [(NNN)2Fe]2+, sketch a
qualitative crystal field splitting diagram for the d-orbitals,
and calculate the crystal field stabilization energy in Dq units.
If pairing energy terms are required in your answer, include
them. Assume a perfectly octahedral geometry.
b) Explain why the high-temperature and low-temperature forms have such different bond
lengths.
Consider the interhalogen ion [ICl2]–, which has a bond angle of 180º. A qualitative valence
molecular orbital diagram for the complex is shown.
e) Use a Lewis structure to rationalize the observed geometry
of [ICl2]–. Include formal charges.
f) Use valence bond theory to rationalize the observed
geometry of [ICl2]–, including a promotion/hybridization
diagram for the I centre. What is the hybridization of atomic
orbitals at the I atom? (What was the formal charge on I?)
g) Use valence bond theory to explain that the I-Cl bond length
in [ICl2]– (255 pm) is longer than that in ICl (232 pm).
h) Which orbital is the HOMO of [ICl2]–, and what is its
bonding nature? (e.g. σ-bonding, π*-antibonding, etc.)
i) The valence bond description of [ICl2]– involves only σbonding, with no π-bonds. The MO description agrees, but
does so using a different interpretation. Describe how the
MO description arrives at the conclusion of zero π-bonds in [ICl2]–.
3σu
3σg
2πu
1πg
1πu
2σu
2σg
1σu
1σg
Consider the octahedral cobalt(III) complexes [Co(H2O)6]3+ and [Co(NH3)6]3+. One is yelloworange and one is blue. Both have zero unpaired electrons.
c) In terms of valence bond theory, what two orbitals overlap to form each Co-N bond in
[Co(NH3)6]3+ ?
d) For [Co(H2O)6]3+, sketch a qualitative crystal field splitting diagram, including symmetry
labels (e.g. a1g) of the d-orbitals, and calculate the crystal field stabilization energy in Dq
units. (The answers are the same as for [Co(NH3)6]3+.)
e) Assign the correct colour to each compound, and explain your qualitative reasoning for your
assignments.
f) In terms of molecular orbital theory, what is the bonding character (e.g. non-bonding,
π∗-antibonding, etc.) of the LUMO of [Co(H2O)6]3+ ?
g) In terms of molecular orbital theory, explain why [Co(H2O)6]2+ (with three unpaired
electrons) undergoes ligand substitution reactions at a rate a billion times (!) faster than
[Co(H2O)6]3+. (There is also a crystal field theory explanation for this observation, which
will be awarded partial credit.)
For each of the following compounds,
a) Draw a valid Lewis structure (of which there may be more than one)
b) name the geometry predicted by VSEPR
c) determine the formal charge at the central atom in your structure
d) provide the hybridization of valence atomic orbitals at each central atom required to
account for the predicted structure
ClO2–
XeOF4
SiCl4
Cl2SO
PO43–
CO2
ICl2+
Given your understanding of molecular orbital diagrams, provide an example of a compound
where the addition of an electron results in the following change, and provide an explanation for
the change.
a) stronger bonds
b) weaker bonds
c) a change in geometry of the molecule
Using whatever theory you like, explain the observation that when heated to the gas phase,
mercury (Hg) exists as monoatomic atoms, but gold (Au) forms diatomic Au2 molecules.
Explain the fact that when F– dissociates from nitryl fluoride (FNO2) to leave the nitryl cation
([NO2]+), the N-O bond length decreases. (N-O length 123 pm in FNO2, 115 pm in [NO2]+).
Given this result, how much information can you infer about the nature of the LUMO in [NO2]+?