Chemistry 417-01
Exam 1
Inorganic Chemistry
September 24, 2001
-------KEY-------
Name
GROUND RULES:
a. Closed book; closed notes; no consulting
b. allowed: calculator; model set; point group flowchart
c. 180 minute (3 h) time limit
d. Due Friday Sept. 28, 5:00 p.m.
e. please fill in and sign:
time started
time finished
I have abided by the ground rules in taking this exam:
1.
Determine the shape and point group of the following molecules and ions. Tell whether the molecule
or ion is polar or non-polar.
(a)
1,3,5-trifluorobenzene
(b)
H3 O+
(c)
IF4 -
(d)
SnCl2
Exam 1
page 2
When octahedral tungsten hexacarbonyl (W(CO)6) is refluxed in propionitrile (EtC≡N), the product is
W(CO)3(N≡CEt)3. (Kubas, G. J. Inorg. Chem. 1983, 22, 692-694.) The product is either merW(CO)3(EtCN)3 or fac-W(CO)3(EtCN)3 which belong to different molecular point groups (C2v and
C3v respectively when we ignore the atoms of the ethyl groups in assigning symmetry point groups).
In principle, one can determine from analysis of the infrared spectrum which of the drawings below
corresponds to the true structure.
Note: The z-axis is specified in the first structure. In the second structure, let the z-axis be the principal
axis of rotation. Think of how you set up ammonia, a simpler C3v molecule.
Et
CO
C
CO
z
N
CO
EtC≡N
W
N≡CEt
≡
2.
W
CO
EtC≡N
N
≡
OC
OC
C
Et
≡
N
C
Et
mer-W(CO)3(EtCN)3
C2v
fac-W(CO)3(EtCN)3
C3v
The observed product shows 3 carbonyl stretching frequencies in the infrared spectrum, at 2021, 1898 and
1840 cm-1. Use group theory to determine the irreducible representations of the stretches expected for
each of the two species in their respective point group. Does your analysis allow you to suggest one
structure over the other as the true structure? Or, is the information obtained from group theory
ambiguous? (Character tables are on the last page of the exam)
C2v
E
C2
σv(xz)
σ’v(yz)
Γ
3
1
1
1
C3v
E
2 C3
3 σv
Γ
3
0
1
Exam 1
3.
page 3
Consider the following possible electron arrangements for a p3 configuration:
A
B
C
D
CIRCLE the correct answers. Some may have multiple answers.
(a) which is the ground state?
A
B
C
D
(b) which represent excited states?
A
B
C
D
(c) which represent impossible states?
A
B
C
D
(d) in which state is exchange energy maximized?
A
B
C
D
4. Questions about atomic structure.
(a) Please write the ground state electron configuration for the following atoms and ions: Se, Se2-, V, V4+ .
The use of the inert core formalism is fine.
(b) Using concepts from the class, please describe briefly the main reason(s) that Ca atoms are larger than
Zn atoms.
Exam 1
5.
page 4
A molecule you are interested in (say trans-1,2-dichloroethene) has C2h symmetry. In preparation for
constructing a molecular orbital diagram, or perhaps some other purpose one might need to know the
symmetries of orbitals on atoms. With that in mind, please determine the irreducible representations in
C2h to which the s, px, py, and pz orbitals each belong.
Here is the C2h character table. (The two last columns are blank because that is where you would
normally find the information I am asking for. You will determine by inspection the irreducible
representations of the orbitals.)
C2h
E
C2
i
σh
Ag
1
1
1
1
Bg
1
-1
1
-1
Au
1
1
-1
-1
Bu
1
-1
-1
1
Determine Γs, Γpx, Γpy, Γpz by applying the symmetry operations to each orbital and noting the
change in position and/or phase (if any). Recall that the orbital should be centered at the origin of the
Cartesian coordinates. Each irreducible representation you obtain should match one of the irreducible
representations in the table above.
Exam 1
6.
page 5
Draw resonance structures for the isoelectronic compounds NSO- and SNO -. Assign formal charges.
For each ion, choose the best resonance structure. Of the two ions, which do you predict to be the
more stable, and very briefly why?
Exam 1
page 6
Selected Character Tables:
C2v
E
C2
σv(xz)
σ’v(yz)
Α1
1
1
1
1
z
x2, y2, z2
Α2
1
1
-1
-1
Rz
xy
Β1
1
-1
1
-1
x, Ry
xz
Β2
1
-1
-1
1
y, Rx
yz
C3v
E
2 C3
3 σv
Α1
1
1
1
z
Α2
1
1
-1
Rz
E
2
-1
0
(x, y),
(Rx, Ry)
x2 + y2, z2
(x2 - y2, xy),
(xz, yz)