2018 Inorganic Chemistry (I) Final exam 2019/01/07
1. Which of the following molecules or ions contain (a) a C3 axis but no !h
plane, and (b) a C3 axis and a !h plane:NH3; SO3; PBr3; AlCl3; [SO4] 2-;
[NO3] -?(6)
2. In 1993, the [Pt(CO)4]2+ ion was reported for the first time [G. Hwang et al.
(1993) Inorg. Chem., vol. 32, p. 4667]. One strong absorption at 2235 cm-1
in the IR spectrum was assigned to νCO, and this was absent in the Raman
spectrum. In the Raman spectrum, two absorption (νCO) at 2257 and 2281
cm-1 (absent in the IR spectrum) were observed. Show that these data are
consistent with [Pt(CO)4]2+ having D4h symmetry.(10)
3. Haemoglobin is the iron-containing metalloprotein responsible for
transporting O2 in the bloodstream of mammals. When O2 binds to the Fe
centre, it does so in an ‘end-on’ manner and gives rise to a band in the IR
spectrum at 1107cm-1. No absorption is present in the IR spectrum of
gaseous O2. Rationalize these observations. (It is not necessary to discuss
the bonding in O2 or oxyhaemoglobin.) (10)
4. (a) PF5 has D3h symmetry. What is its structure? (b) Suggest an
appropriate bonding scheme for PF5 within VB theory, giving appropriate
resonance structures.(10)
5. (a) BCl3 has D3h symmetry. Draw the structure of BCl3 and give values for
the bond angles. NCl3 has C3v symmetry. Is it possible to state the bond
angles from this information? (b) Derive the symmetry labels for the atomic
orbitals on B in BCl3 and on N in NCl3. (10)
6. Cyclobutadiene, C4H4, is unstable but can be stabilized in complexes such
as (C4H4)Fe(CO)3. In such complexes, C4H4 is planar and has equal C-C
bond lengths:
(a) After the formation of C-H and C-C σ-bonds in C4H4, what orbitals are
available for -bonding? (b) Assuming D4h symmetry for C4H4, derive the
symmetries of the four π-MOs. Derive equations for the normalized
wavefunctions that describe these MOs, and sketch representations of the
four orbitals. (10)
7. The industrial manufacture of NH3 from N2 and H2 is carried out on a huge
scale using heterogeneous catalysis, i.e. the reaction between gaseous N2
and H2 is carried out over a solid catalyst. (a) Construct an MO diagram for
N2 and use the diagram to explain why N2 is a chemically insert species.
(b) To what point group does NH3 belong ? (c) Using the appropriate
character table on the last page of the exam , construct a set of ligand
group orbitals for a triangular H3-fragment. Give symmetry labels to the
LGOs. (d) Construct an MO diagram for NH3 showing the interactions
between the N atomic orbitals and the LGOs of the H3-fragment. Use the
MO diagram to determine the N-H bond order and to confirm that NH3 is
diamagnetic.(10)
8. Draw the Lewis structure and give the point group of the following molecules
(with highest possible symmetry.) (4)
(a) Cyclohexane (b) tribromobenzene
9. Diborane belongs to D2h Point group and has a type of bonding described
as three center, two electron bonding. Please explain this interaction by
following steps:
(a) Consider the bridging hydrogens, show the representation of their
group orbitals (Г(1s)) and then reduce the representations to the
irreducible representation. (2)
(b) Consider the boron atoms, show the representation of their group
orbitals (Г(2s), Г(2pz), Г(2px)) and then reduce the representations to
the irreducible representation. (4)
(c) Qualitatively construct the MO diagram of diborane with 1s orbitals of
two bridging hydrogen & 2s, 2px and pz orbitals of two borons. (4)
(d) Fill the electrons into the MO diagram. (remember, two electrons on
each boron form two B-H bonds with nonbriding hydrogen) (2)
(e) Briefly explain the interaction of three center two electron bonding
using MO diagram. (2)
(f) In this MO diagram, qualitatively draw the shape of electron density for
the lowest bonding MO (ag) and highest antibonding MO (b3u). (2)
10. A schematic MO diagram for XeF4 constructed by using only the 5p orbitals
of “Xe” and the 2p orbitals of the four “F” is given on the last page of the
exam.
(a) Write down the number of p valence electrons from Xe and the four F
atomic centers on the exam sheet and place the p electrons directly into the
MO diagram that is provided. (2)
(b) What is the net number of bonding electrons between Xe and the four
fluorines in XeF4 according to the MO diagram using only the p-valence
orbitals? (2)
(c) Determine the reducible representation for the fluorine out of plane group
pi orbitals derived from the fluorine p orbitals in D4h symmetry. (4)
(d) Determine the Xe 5d orbitals that have the correct symmetry to interact
with the fluorine group pi orbital sets. (4)
(e) What are the irreducible representations for the occupied molecular
orbitals that are completely localized on F when only p valence orbitals are
considered? (2)
Eu
A2u
Eu Eu
Eg Eg
B2u
Eu
B1g
Eu
A1g
A2u
A2u
Eu
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