Practical Final Chem V01BL Spring 2015
Instructor: Dr. Nick Blake
Read this before the practical final, you will be working with either a partner or two
partners, be clear about what you are doing, you will receive no help during the final
INTRODUCTION
Cu2+ is thought to have a coordination number of 6 meaning that its complexes in
solution are octahedral. Cu2+ has a [Ar]3d9 configuration (it has 9 valence electrons
all in the d orbitals). In an octahedral complex, the 5 d-orbitals split into two sets of
degenerate orbitals. The 3 lowest energy orbitals are the dxy, dyz and dxz. In daylight
an electron can be excited from one of the lower d-orbitals to the higher d-orbital as
shown in Fig. 1 below.
Figure 1: The splitting of the d-orbitals in Cu2+ in an octahedral coordination compound splits into two
groups as shown. The ion, being d9 leaves all d-orbitals filled except for one that is partially filled. An
electron from the lower energy d-orbitals can be excited to the higher orbital by absorbing a photon of
the correct frequency f.
The energy splitting E (also called oct) depends on the ligands bound to the Cu2+.
As you can see below, the splitting for Cu(H2O)62+(aq) where H2O is the ligand ,is
smaller than for Cu(NH3)4(H2O)22+ where NH3 is the main ligand. The splitting E is
normally somewhere in the visible or in the near infra-red, and this is why Cu2+ is
generally colored in solution
Figure 2: How the splitting of the d-orbitals in Cu2+ depends on the ligands
Figures 3 and 4 below show spectra for the two complexes shown in above in Fig. 2
Figure 3 UV/Vis absorption spectrum of [Cu(H2O)6]2+
Figure 4: UV/Vis absorption spectrum of [Cu(NH3)4(H2O)2]2+
Notice in Figure 3 that the long wavelength peak is centered around 800 nm for
Cu(H2O)62+(aq), while for Cu(NH3)4(H2O)22+ it is centered around 600 nm. The
absorption spectrum can be used to estimate the size of this splitting using the
Planck equation
DE º D oct
1.988 ´10 -16 J × nm
=
=
l
l (nm)
h×c
where h = Planck’s Constant, c = speed of light, and  is the wavelength of the
absorption peak in meters (not in nanometers). The equation on the right can be
used if (nm) is in nanometers
The bigger  is the smaller E is and vice versa.
Fig. 5 shows the spectra obtained in Exp. 13 on coordination compounds where the
absorption spectra for different coordination compounds of Cu2+ were measured
Figure 5: The Absorption spectra of Cu2+ with various ligands, sali, (salicylate anion), diethylenetriamine
(dien), ethylenediamine (en), ethylenediaminetetraacetic acid (EDTA) and ammonia (NH 3)
Most of the coordination compounds show one absorption band which is what we
would expect, but two do not - Cu(Sali)22- which has band at 800 nm and one at 400
nm, and Cu(en)y2+ which has absorption bands at 600 nm, 700 nm and 800 nm.
In the case of Cu(Sali)22- the absorption at 400 nm is nothing to do with the dorbitals - it is due to electron transitions in the salicylate ligand, so as with the
others it only shows one peak due to the d-electrons on the Cu2+ ion.
This leaves the Cu(en)y2+, where y is left undetermined but could be y = 1 or 2 or 3.
If we compare the spectrum of Cu(en)y2+, with the other spectra we see that the 600
nm absorption peak is in the same position as that seen for Cu(NH3)4(H2O)22+ while
the peak at 800 nm is close to where the peak is seen for Cu(H2O)62+(aq)
Could it be that instead of making one coordination compound we are seeing more
than one compound?
When we made the Cu(en)y2+ it was make by mixing
Cu(H2O)62+(aq) with an aqueous solution of the en (1,2diaminoethane or ethylenediamine), see Fig. 6.
Figure 6: Ethylenediamine (en)
En is called a chelating agent, it is bidentate meaning each N can act as a Lewis base
and bond with the Cu2+. In principle it means that up to 3 en could bind to the Cu2+
(see figs. 7 and 8 below)
Figure 7: A schematic 3-d image of [Cu(en)3]2+ the
red lines with the circles at either end represent
the ethylenediamine, the circles are the N atoms
that donate electron pairs to the Cu2+ ion
Figure 7: The two other possible coordination compounds the en can make with Cu2+, left
[Cu(H2O)4(en)1]2+ and right [Cu(H2O)2(en)2]2+
This means we could envisage 3 sequential reactions that could occur
If this was true, then we could have up to 4 complexes present in our solution when
we mix the two reactants (the Cu(H2O)62+(aq) and the dien), these are
[Cu(H O) ] , [Cu(H O) en] , [Cu(H O) (en) ]
2+
2
2+
6
2
4
2+
2
2
2
, and [ Cu(en)3 ] (aq)
2+
In principle each of these would have it’s own absorption band. Since N containing
ligands give rise to a stronger metal-ligand bond than with H2O, we would expect
[Cu(en)3 ]2+ (aq) to have the larger oct, [Cu(H 2O)2 (en)2 ]2+ the next largest, followed
by
[Cu(H O) en ]
2+
2
4
with [ Cu(H 2O)6 ] being the smallest.
2+
OBJECTIVE
The objective of this experiment is to determine the formula for each of the 3 peaks
seen in the absorption spectrum for the coordination compound(s) made by the
Cu(H2O)62+(aq) and the dien. Use Job’s method, as outlined in Part C of Experiment
13 on Coordination compounds and in the PowerPoint for that experiment
Part A
First measure the spectrum for the reaction of Cu2+ with the 1,2 diaminoethane
(ethylenediamine), using the procedure from part B of Exp. 13 and identify the
absorption peaks to be investigated, we will call these A1, A2 and A3, at wavelengths
1,  2 and  3 respectively. Print the graph and label it Figure 1, on this graph
show the peaks A1, A2 and A3 and their wavelengths 1,  2 and  3
Part B
Second measure the spectrum of Cu2+ alone, print the graph and label it Figure 2
Part C
Third, make 10 solutions with a different [dien]/[Cu2+] ratio (as outlined in Part C of
Exp. 13), and measure the absorbances at the 3 peak positions found in part A
above. Put all of the spectra on one graph and print it, label it Figure 3
Part D
Construct a table of the following format
Table 1: Solutions used for Part C, along with the measured absorbances for the 3 absorption peaks
N
Mass
0.1661M
Cu2+
(g)
Mass of
0.1661
M
en (g)
[en]/[Cu2+]
Absorbance
A1
 = ________
Absorbance
A2
 = ________
Absorbance
A3
 = ________
1
2
3
4
5
6
7
8
9
10
make 3 scatter plots in Excel where the y-axis is either A1, A2 or A3, and the x-axis is
[en]/[Cu2+], label these Figure 4, Figure 5 and Figure 6, and on each graph give the
wavelength at which the absorption was measured, as well as the composition of the
Cu(en)y2+ coordination complex at that wavelength.
Results Section
This will include
 Table 1
 Figure 1
 Figure 2
 Figure 3
 Figure 4
 Figure 5
 Figure 6
Note that I need to initial every figure you create
Analysis Section
 Show a sample calculation for obtaining [en]/[Cu2+]

I want you to identify, y, the number of en ligands that are complexing with
the Cu2+ in A1, A2 and A3. Are they all due to the same coordination
compound or not?

Explain in the analysis section how you determine y.

Calculate oct for the three absorption bands in kJ/mol

Present these results in a table like the following
Wavelength (nm) y in Cu(en)y2+ oct (kJ/Mol)
1
2
3
Conclusion Section
I want you to write a conclusion, where you

answer the objectives of the experiment (what where the wavelengths of the
absorption bands, what was the composition of the bands (If you need to
refer to one of the graphs refer to it as Fig. 1, Fig. 2 etc.)

Do they make sense? (Does the position of the peak in wavelength jibe with
the composition of the coordination compound)

Did the experiment fulfill the objectives or not? If not, why not and what
additional experiment would you do.

Where there any anomalies if so what?
The Report
The report can be either hand written or typed. If it is hand written it has to be
clear, neat and easy to read, marks will be deducted for sloppiness.
The report has to be in your own words. It should have the following sections

Objective

Procedures (Cite Exp 13 in the Ventura College Chem V01BL Manual)

Results (Make sure I have initialed your figures)

Analysis

Conclusion
It should be handed in by 7:00 pm Tuesday May 13 in my mailbox in Math and
Sciences, or emailed to nblake@vcccd.edu, with photographed copies of the figures