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CHEM 104B
Spring 2013
Instructor: Prof. John Hartwig
GSIs: Rebecca Green, Dianne Xiao
Problem Set 3: Spectroscopy
1) [MnO4]- and [MnO4]2- show very intense absorbances at 18,320 cm-1 and 22,840 cm-1,
respectively. Explain the origin of these bands and their relative energies.
2) For the following complexes, draw a possible electronic transition and predict the value of its
molar absorptivity, ε. State whether the transition is spin forbidden/allowed and Laporte
forbidden/allowed.
a) [Ti(H2O)6]3+
b) VCl4
c) [Mn(H2O)6]2+
d) [Cu(NH3)6]2+
3) In the last problem set, you analyzed
[Fe(isoxazole)6][ClO4]2, which displays a spin
transition at 213 K. In addition to the magnetic
properties, the UV-Vis absorption bands are
temperature-dependent (see figure on the right). In
particular, the absorption band at 11,800 cm-1
(labeled σ1 in the figure) is much more intense in the
spectra at 220 K and 293 K than in the spectra at
102 K.
Draw a d-orbital splitting diagram and assign the
electronic transition corresponding to this absorption
band. Give its term symbol, and estimate ∆ .
O
4) Determine the ground state term symbols for
transition metal complexes with the following
electronic configurations:
a) d9 (Oh)
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b) d5 low spin (Oh)
c) d5 high spin (Oh)
d) d5 low spin (Td)
e) d5 high spin (Td)
f) d7 low spin (square planar)
5) You are given an enzyme that is known to contain iron at the active site. Based upon a poorly
resolved X-ray structure of this enzyme, it has been determined that the metal exists in an
octahedral geometry with 6 equivalent ligands around the metal. The UV/Vis of this complex
shows a series of low energy and very faint absorptions (ε << 1). Addition of ethylenediamine
(en) to the enzyme results in the liberation of the iron center as (en)3Fen+. The UV/VIS of this
complex shows four d-d absorptions (ε = 20), each of which has a double-hump appearance.
Assuming that no oxidation or reduction occurred upon addition of ethylenediamine, answer the
following questions:
a) What is the oxidation state of the Fe center?
b) Why did the ε of the d-d transitions on the Fe center change so dramatically upon addition of
ethylenediamine?
c) Why do the absorptions in the (en)3Fen+ complex all have a double-hump appearance?
6) Consider the complex: OsCl6.
a) Predict the number of spin allowed d-d transitions that will occur in this complex.
b) There exists one observable charge transfer band in the UV/Vis spectrum of this complex;
which is it and why?
c) Sketch the UV/Vis spectrum (ε vs. λ) for this compound and label the absorption bands.
7) From the reaction of NiBr2 and Ph2EtP, it is possible to isolate green crystals of
[Ni(Ph2EtP)2Br2], which have a magnetic moment of 3.2 BM, and red crystals of [Ni(Ph2EtP)2Br2],
which have a magnetic moment of zero. When either of these is dissolved in dichloromethane at
40 °C, the resulting solution has a magnetic moment of 2.69 BM. Suggest structures for the
green and red crystals and offer an explanation for the solution magnetic moment.
8) The following absorption bands are found in the spectrum of [Cr(ox)6]3-: 262 nm (overlap with
charge transfer), 418 nm, 571 nm. Determine the values of Δo and B.
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