_____University of Puget Sound
Department of Chemistry
Chem 230
EXP. 10– SPECTROSCOPY OF TRANSITION METAL COMPLEXES
LABORATORY OBJECTIVES AND ASSESSMENTS
1. Understand how the color of transition metal complexes depends on the strength
of the ligand.
a. Use visible spectra to determine qualitative differences in ligand strength.
b. Determine a spectrochemical series of ligands.
2. Understand how absorbance is related to energy attributed to “Crystal Field
Splitting” in metal-ligand complexes.
a. Measure absorption spectra of metal-ligand complexes to determine
wavelengths associated with ligand strength.
b. Calculate the energy associated with d-d electronic transitions (Δo).
c. Determine a semiquantitative spectrochemical series of ligands.
3. Understand ligand binding in terms of Lewis structures.
a. Draw the Lewis structure of a metal ion ligand
b. Draw the 3D structure of an octahedral transition metal complex.
c. Identify Lewis base/ligand groups.
d. Recognize mono/multi-dentate ligands.
INTRODUCTION
Read Gilbert, Chapter. 17-8, pages 868-878. This experiment was adapted for this
lab from Inorganic Chemistry, Gary Wulfsberg, University Science Books, 2000, pp 938939.
Many transition metal complexes are colored. The color arises due to the fact that
these complexes absorb in the visible region of the electromagnetic spectrum. The
energy of the absorbed light can be attributed to “Crystal Field Splitting”. In this lab,
we will focus on transition metal complexes in which the metal adopts an octahedral
geometry (recall VSEPR). Due to the presence of the ligands in this geometry, the d
orbitals on the metal are split into two groups; 3 orbitals of lower energy (called t2g)
and 2 orbitals of higher energy (called eg). The color observed for the metal complexes
can be attributed to the excitation of an electron from one of the t2g orbitals to an eg
orbital. The energy gap between the two sets of orbitals (Δo) can be calculated from the
wavelength of absorption. In this lab, you will examine a series of complexes for a
given transition metal ion and correlate the ligand with the splitting energy (Δo).
eg
dz2
dx2-y2
Δ
t2g
3d orbitals in free ion
Average potential energy of 3d
orbitals raised in octahedral ligand
field
dxy
dyz
dxz
3d orbitals splitting in octahedral
ligand field
Figure 1. Splitting of d-orbital energies by an octahedral field of ligands.
Exp. 10 – Spectroscopy of Transition Metal Complexes
1
EXPERIMENTAL PROCEDURE
WORK WITH PARTNER
FOR EACH NUMBERED STEP CAREFULLY RECORD ALL OBSERVATIONS IN YOUR
NOTEBOOK
PART A. PREPARATION OF COMPLEXES AND THEIR ORDERING BY VISUAL
INSPECTION
1.
2.
3.
4.
5.
6.
You and your partner will be assigned one of the following metal ions: Ni2+,
Co2+, or Cu2+.
Clean seven small test tubes and give them a final rinse with deionized water.
Place the test tubes in a rack. To each test tube add 2 mL of 0.1 M solution of
your metal ion. In turn, add 2 mL of one of the following solutions to each of the
test tubes: 1) deionized water; 2) 1.0 M sodium glycinate; 3) saturated K2C2O4; 4)
1.0 M NH3, buffered; 5) 1.0 M pyridine; 6) 1.0 M EDTA; 7) 1.0 M buffered 1,2diaminoethane (commonly named ethylenediamine or en, for short). Carefully
stir each test tube, by holding in one hand and tapping the bottom of the test
tube with the tip of index finger of other hand, to mix its contents.
Clearly label each test tube and record the color of each test tube.
Arrange the seven test tubes, by color, in order of the visible spectrum
(beginning with the lowest energy color). In Table 1 record the resulting order
of ligands.
Find the other groups in your lab that are forming the same metal complexes
and discuss the following; Does the ordering of the ligands with your metal ion
agree with the observations made by the other groups? If not, try to obtain
some consensus before going on.
Now as a whole lab section, determine if the ligand ordering for Ni2+ complexes
is reasonably consistent with the ordering of the ligands for complexes of the
Co2+ ion and of the Cu2+ ion. Try to make slight rearrangements in the ligand
ordering such that they are more nearly metal-independent. Record this metalindependent listing of ligands in order. This is a qualitative spectrochemical series
of ligands.
PART B. COLLECTION AND ANALYSIS OF THE SPECTRA
7.
8.
9.
There is uncertainty associated with the qualitative ordering of ligands due to
the limited ability of our eyes to discriminate similar colors. Hence, the visual
ordering will be checked by collecting the spectra of these complexes with a
spectrophotometer.
Each group assigned to the same metal ion will collect the UV-visible data for
each of the seven ligands (solutions in test tubes). Please sure to record the
wavelength of each peak. Note: Since many of the cobalt complexes undergo air
oxidation, the Co2+ group will have to remake their seven solutions and
immediately collect the spectral data. Each member of each metal ion group will
have 7 spectra, please make sure they are clearly labeled and that wavelengths
of peaks are recorded.
DISPOSE ALL THE METAL SOLUTIONS IN THE APPROPRIATE
LABELED WASTE CONTAINERS.
Convert the wavelength measurements to energy units of reciprocal centimeters
using the equation:
Exp. 10 – Spectroscopy of Transition Metal Complexes
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Wavenumber in cm-1 = 107/wavelength in nanometers
(1)
10. In the Cu2+ complexes, only one (possibly asymmetrical) peak due to a d-d
electronic transition will be observed. The energy of this transition in reciprocal
centimeters is the octahedral ligand field splitting, Δo. More than one d-d
transition will be observed with the other metal complexes; however, the lowest
energy peak in these complexes equals Δo. Record Δo for each the seven
complexes.
11. The energy of Δo can be approximately factored into two components, an f factor
due to the ligand and a g factor due to the metal ion:
Δo = fligand x gion
(2)
By convention, an f factor of 1.000 is assigned to the water molecule as a ligand.
From the Δo value for the hydrated metal ion, compute the g factor for your
metal ion, and record it.
12. Using the experimental g factor you just calculated, calculate the f factors for the
other six ligands.
13. The f factors finally allow us to make the spectrochemical series
semiquantitative. In Table 1 list the seven ligands in increasing order of their f
values, indicating small and large differences between corresponding ligands
with “<” and “<<”, respectively.
14. A ligand binds to a metal ion by donating a pair of electrons to the metal.
Ligands that have more than one atom with a pair of electrons for donation can
bind in a poly- or multi-dentate geometry.
WHAT TO DO
TO BE TURNED IN IN LECTURE ON MONDAY APRIL 30
THE FORMAL REPORT FOR Exp. 9- Evaluation Methods for Detecting NH4+:
This is a formal lab report including a brief introductory paragraph stating the
purpose of this lab in a few sentences as well as Results, and Discussion. Treat the
report as an analysis of the two ammonium detection methods. You should include all
pertinent information needed in determining which method to use for what level of
ammonium. You should include a discussion on the relative pros and cons of each
method such as sensitivity and interferences.
See the Lab Info Moodle page or the handout for Exp. 9-Second Week for
Details.
For Exp. 10
Before Lab: Read this experiment and read Gilbert, Chapter 17-8, pages 868-878.
Set up your lab notebook appropriately for this experiment including a title bar.
You should also include the following information:
•
A reagent table with the following reagents:
Nickel(II) nitrate, Ni(NO3)2•6H2O
Copper(II) sulfate, CuSO4 •5H2O
Cobalt(II) chloride, CoCl2 •6H2O
Sodium glycinate, Na+NH2CH2CO2Potassium oxalate, K2C2O4
Exp. 10 – Spectroscopy of Transition Metal Complexes
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Pyridine, C5H5N
1,2-diaminoethane, H2NCH2CH2NH2
sodium ethylenediaminetetraacetate, (Na+)4[(-O2CCH2)2NCH2CH2N(CH2CO2-)2]
ammonia, NH3
BE SURE TO NOTE THE DISPOSAL PROCEDURES IN YOUR TABLE.
•
Prepare the following table in your notebook before coming to lab. This table
must be one full page, landscape-format, in your notebook.
•
A Printout of the Report Sheet, see below. (Print back-to-back.)
Table 1: Summary of data for the Transition Metal complexes of ___________
Ligand
H2O
NH2CH2CO2-
C2O42-
NH3
C5H5N
EDTA
H2NCH2CH2NH2
Color of TM complex
Ligands Ordered by E
(Visual) Low to High
Wavelength(s) λ, nm
Energy, cm-1
Δo
f factor
Ligands Ordered by E
(f-factor) Low to High
During the Lab: Carry out the procedure as written. Enter all relevant
observations and data directly into your laboratory notebook. Complete your
calculations and provide at least one set of sample calculations. Clearly and
completely answer each question.
Complete the class data table and prepare for the lab discussion.
To be Turned In at the end of class: Staple in a neat packet the following; Report
Sheet, and the copy pages containing 1) the completed summary table, 2) a sample
set of calculations, and 3) observations associated with this lab
Exp. 10 – Spectroscopy of Transition Metal Complexes
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REPORT SHEET -EXP. 11– Spectroscopy of Transition Metal Complexes
Name_____________________________________
Lab Section___________
Q.1.
Draw the line (Lewis Dot) structure representations for each ligand—H2O,
NH2CH2CO2-, C2O42-, NH3, C5H5N, [(-O2CCH2)2NCH2CH2N(CH2CO2-)2],
H2NCH2CH2NH2. Include lone pairs on atoms that have them. Label each
ligand as monodentate, bidentate, tridentate, hexadentate, etc...
Q.2.
a) What kind of acid-base interaction describes the type of bonding exhibited
by the metal complexes?
b) Which is acting as the acid (the ligand or the metal ion)? Which is acting as
the base?
Q.3.
Each of the metal complexes formed were octahedral. Sketch each of the
complexes using the following templates:
Exp. 10 – Spectroscopy of Transition Metal Complexes
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Q.4.
Comment on the periodicity of the semiquantitative spectrochemical series.
Can you note any grouping of nitrogen- and/or oxygen ligands in the
spectrochemical series? Which ligand atoms generally give the greatest Δo?
Q.5.
Compare the order of your two spectrochemical series. Do the ligands come in
the same order in the qualitative spectrochemical series you obtained by
inspection in Part A as the f factors in Part B? Note the any differences.
Exp. 10 – Spectroscopy of Transition Metal Complexes
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