IB Inorganic Chemistry
St Anne’s and Oriel Colleges
MT 2016
Spectroscopy and Magnetism
Suggested reading:
• Infrared and Raman Spectra of Inorganic and Coordination Compounds, K Nakamoto
• Ligand Field Theory and its Applications, B N Figgis & M A Hitchman
• Magnetochemistry, A F Orchard (Primer)
• Magnetic Materials, N A Spaldin
I suggest also consulting the notes / problems for the advanced option course on Molecular Spectroscopy: in practice much of the vibrational/electronic component of that course is examinable in
the core course.
Questions:
(i) Explain the relationship between Orgel and Tanabe–Sugano diagrams. How might the latter
be used to estimate the crystal field splitting parameter ∆oct from an experimentally-measured
UV/Vis absorption spectrum?
(ii) Outline the ways in which you might compare and contrast the spectroscopy and magnetism
of Mn and Eu (i.e. as part of a broader essay question on the chemistry of these two elements).
Make sure to illustrate your answer with relevant examples. To what extent do your observations reflect more general similarities and differences between the transition metals and the
lanthanides?
(iii) Explain why, in principle, no octahedral transition-metal complex should derive any colour
from d → d transitions. Give an example of a stretching mode of the arbitrary [ML6 ]n+
complex that would allow the system to interact with visible light. By what mechanism(s)
is/are spin-forbidden d → d transitions ultimately allowed?
(iv) Discuss the role of spin–orbit coupling in the magnetism of the 3d transition metals.
(v) The measured magnetic susceptibility of K3 [Cr(CN)6 ] at 298 K is 2.42 × 10−7 m3 kg−1 . Given
that the value of NA µ0 µ2B /3kB is 1.57 × 10−6 m3 K mol−1 , calculate the effective magnetic
moment of the complex and comment on the value that you obtain.
(vi) The adduct [SeCl6 ]2− can be obtained from the reaction of SeCl4 with two equivalents of an
alkali metal chloride and its IR and Raman data (in cm−1 ) obtained:
IR:
Raman:
280
299
150
255
165
By considering possible regular geometries compatible with the stoichiometry EX6 , comment
on the likely structure of the hexachloroselenate dianion.
IB Inorganic Chemistry
St Anne’s and Oriel Colleges
MT 2016
(vii) What factors govern site ordering in spinels? In what way(s) might site ordering affect the
magnetic properties of spinels containing paramagnetic transition metal ions? Magnetic measurements appear to show that the ferrite CoFe2 O4 has 3.4 unpaired spin per formula unit.
Rationalise this observation.
(viii) The table below gives the magnetic properties of the following series of iron-containing compounds (not necessarily in the same order ), together with an indication of how these properties
vary with temperature.
[Fe(η 5 -C5 H5 )2 ]
[Et4 N]2 [FeCl4 ]
K2 [FeO4 ]
[Fe(NCS)2 (bipy)2 ]
KFe(SO4 )2 ·12H2 O
K3 [Fe(CN)6 ]
[FeCl2 (diars)2 ][BF4 ]2
Identify the compounds A–G, giving reasons for your choice.
Compound
magnetic moment / µB
at 300 K
at 80 K
A
5.40
5.40
B
2.25
1.90
C
5.89
5.89
D
5.23
0.94
E
3.01
3.01
F
2.76
2.63
G
diamagnetic
√
√
√
√
√
[ 3 = 1.73; 8 = 2.83; 15 = 3.87; 24 = 4.90; 35 = 5.92]
bipy = bipyridyl; diars = 1,2-C6 H4 (AsMe2 )2
IB Inorganic Chemistry
St Anne’s and Oriel Colleges
MT 2016
Past paper questions
(2014 IB Examination—Please answer ALL parts):
2.
Answer ALL parts (a) – (d).
(a) For each of the following complexes state the full term symbol of
the electronic ground state and predict the number of spinallowed d-d transitions in each case.
[Ni(H2O)6]2+
[Cr(H2O)6]3+
[Mn(H2O)6]2+
[CoCl4]2–
[Fe(H2O)6]2+
[7]
(b) Arrange the following species in order of increasing extinction
coefficient in the visible region, giving your reasons.
Fe3+(aq)
MnO4–(aq)
Pr3+(aq)
cis-[Co(en)2F2]+
trans-[Co(en)2F2]+
[5]
–
(c) Explain the values (cm 1) of the lowest energy, intense absorption
–
–
bands (εmax > 1000 mol 1 dm3 cm 1) of the following complexes.
[FeCl4]2–
[CoCl4]2–
[NiCl4]2–
[CuCl4]2–
[ZnCl4]2–
45,500
42,700
35,000
24,400
56,700
[IrCl6]2–
[PtCl6]2–
20,500
35,900
[7]
(d) Arrange the following cobalt species in order of their increasing
effective magnetic moment at 300K. Rationalise
any deviations
Question
continues
from simple ‘spin-only’ behaviour in these species.
DCHC 2789
[CoCl4]2–
Cs3CoF6
5
[CoI4]2–
[Co(NH3)6]3+
Turn over
[6]
!
!
IB Inorganic Chemistry
St Anne’s and Oriel Colleges
MT 2016
SECTION B
(2008 IB Examination—Please answer ALL parts):
Answer THREE of the questions 5 – 11
5. Answer ALL of the parts (a) – (c).
(a) Which electronic configurations are expected to lead to Jahn-Teller
distortions in octahedral and tetrahedral complexes? Include both highspin and low-spin configurations but not excited states. For which of
these geometries does one expect the effect to be greater?
[4]
(b) The absorption spectra of number of transition metal complexes are
shown below. Assuming the spectra A – D arise from d-d transitions
assign them to the hexa-aqua complexes formed by Cu(II), Ti(III),
Ni(II) and Cr(III), giving your reasons.
[8]
10
Extinction Coefficient (ε), dm3 mol–1 cm–1
A
0
15
B
10
5
0
5
C
0
10
D
5
0
10,000!
15,000
20,000
25,000
Wavenumber/ cm–1
(c) The effective magnetic moments, (µeff /µB) measured at 298K for A – E
are given below. Match the following empirical formulae
these,
Question to
continues
giving your reasons:
[8]
DCHC 2780
-5-
K2ReF6
K4[Mn(CN)6].3H2O
PrO2
[FeO4]2–
Turn over
Cr(η6-C6H6)2
Compound
µ eff /µ B
Temperature
Dependent
Moment
A
3.50
No
B
0
No
C
2.83
No
D
2.18
Yes
E
2.53
No
6. Answer ALL of the parts (a) – (c), giving in each case a full explanation of
your reasoning.
(a) Sketch the solution phase NMR spectra that you would expect for the
following (for the nucleus indicated):
[2 x 3]
19
o