Exam #3 Topics and Sample Questions
Ionic Compound Bonding and Structure
Symmetry: Identifying symmetry elements and determining point groups
Coordination Chemistry: Structures and isomerism
Coordination Chemistry: Bonding and spectroscopy
Some Questions:
Ionic Compounds
1. The formula for lattice energy, Uo, is:
Compare each term for the two salts: MgBr2 and KBr
a) Circle for which each is larger:
Z+Z-
MgBr2
or
KBr
or
same
M
MgBr2
or
KBr
or
same
(1 – 1/n)
MgBr2
or
KBr
or
same
KBr
or
same
b) Which overall do you expect to have a stronger lattice energy?
MgBr2
or
c) Which term in the equation do you think is most important in any difference between them?
Z+Z- term
d. What is the purpose of the Madelung constant? Account for all of the electrostatic interactions in the
extended ionic structure.
2. The solid spheres represent O; the hollow spheres represent Ti.
What is the formula of the compound? What is the oxidation state of the Ti?
O has 4 face ions + 2 internal ions = 4(1/2) + 2 = 4 O ions
Ti has 8 corner ions + 1 internal ion = 8(1/8) + 1 = 2 Ti Ions
Formula = Ti2O4 = TiO2
Because O is 2-, Ti is 4+.
3. A Bragg’s Law question:
Symmetry
8. List the ALL symmetry elements and
point group for each of these molecules.
Rotation axes
Mirror planes
Inversion center
One C4 axis
4 sigma-v
no
One C4 axis
Four C2 axes
1 sigma-h
4 sigma-v
yes
BrF5
Point group: C4v
XeF4
Point group: D4h
9. All five molecules below are planar (flat). Which two molecules have
the same point groups as this transition metal compound? Circle them
and note what the point group is here: _____
Point Groups: C2v
C2v
Cs
Which structure above has the Cs symmetry point group? Draw an arrow to it.
Which molecule(s) have an inversion center? Write an “i” below any that do.
D2h, i
Transition Metal Chemistry
10a. Consider the EDTA (ethylenediaminetetraacetic acid) anion, and the schematic of a
coordination complex it forms.
What is the coordination number of the complex? 6, because the metal has six bonds
What is the denticity of the EDTA ligand? Give a word or a number. The word should not
include the phrase “poly.”
Hexadentate because the one ligand bonds through 6 atoms
If the CH2CH2 bridge between the two N atoms was to break in the metal-bonded EDTA, the
coordination number would be 6 and the denticity would be two tridentate ligands (because
there are now two ligands, each bonding through three atoms).
10b. Co(trien)(NH3)22+
three views:
Coordination Number: 6
Denticities of each ligand: one tetradentate ligand and two monodentate ligands
11.
All ions exist in aqueous solution as octahedral compounds, where the six ligands are water
molecules. Consider the Cu2+ ion in water.
Considering this complex, answer the following questions:
What species is the Lewis acid?
Cu2+ ion
What species is the Lewis base?
Water molecules
What is the donor?
Water molecules
What is the acceptor?
Cu2+ ion
What is the charge on Cu?
2+ because waters are neutral
What is the d-electron configuration?
d9
12. Consider the compound, Ni(NH3)4Cl(SCN). The SCN ligand can potentially bond via the S-end or the
N-end.
a. Draw a constitutional isomer (connectivity based) of this compound.
What specific kind of isomer have you drawn?
Turn NCS around so the S bonds to the metal. These are then linkage
isomers.
b. Draw a stereoisomer (spatial based) of this compound.
Swap the Cl and the lower NH3 ligand to form the trans isomer.
13. Another popular ligand in transition metal chemistry is terpyridine, abbreviated “trpy.” Its organic
line structure is given below.
a. What is the maximum denticity of
this ligand?
tridentate
b. One compound containing trpy is shown
below: Ni(trpy)Cl3-.
How many other isomers of this compound are possible? Explain. No. The trpy ligand needs to stay
planar, so stays in only the meridinal configuration.
c. Would a similar compound that used three pyridine ligands instead of tery be more stable or less
stable?
Pyridine:
Less stable because multidentate ligands are more stable due to the chelate effect.
What is the chelate effect? Look it up.
d. Here is an isomer of trpy. Is it expected to behave similarly or differently than trpy?
Explain in what ways it would be similar and in what ways it would differ.
No, it would not. The lower N is not oriented so that it can bond with the metal center.
14a. Below is a molecular orbital diagram for a transition metal compound. Label it and explain
everything you can about it. Fill in electrons if the compound were Cr(NH3)63+.
Cr is Cr3+, and is a d3 ion.
14b. How many unpaired electrons will FeCl63- have? Fill in the d-orbital portion of the MO diagram to
show how you get the answer.
Fe is Fe3+, d5
15a. The metal orbital portion of MO diagrams for two complexes of Ni2+ are shown below.
a. Which has stronger metal-ligand bonding?
Left one; you know this because of the larger d-orbital splitting
b. Which will absorb light at longer wavelengths?
Right one, because the smaller delta-o absorbs lower energy, longer wavelength light
15b. If Zn2+(aq) could be oxidized to Zn3+(aq) (it can’t, really, but if it could…) would it likely
result in a colored aqueous solution?
Recall that (aq) means the metal ion is bonded to six water ligands.
Use the molecular orbital diagram to support your answer.
Zn2+ is colorless because the d orbitals are filled and no d-d transition is possible.
Zn3+ would be d9, and would have a vacancy in the higher energy eg orbitals, so a d-d transition would be
possible, and it would absorb visible light, just like Cu2+.
15c. We discussed in class that the Mn2+ ion in aqueous solution is colorless, unlike almost all the other
transition metal ions. If Mn2+(aq) is reacted with an oxidizing agent, the solution rapidly becomes deeply
colored. Use the molecular orbital diagrams below to explain why.
Yes. Mn2+ is colorless because it is d5 high spin and any d-d transition is spin-forbidden. When oxidized,
Mn3+ will be d4 and will have spin-allowed transitions.
15d. Explain why Re2+(aq) might be a colored solution, even though it, like Mn2+, is a d5 metal ion.
16. Determine the oxidation state and d-electron configuration on metals in coordination compounds.
Fe is Fe(CN)63Mn in Mn2(CO)9
17. Predict relative stability using the chelate effect, and explain it.
Which is more stable: Ni(NH3)62+ or Ni(en)32+?