Studying Transition Metal Complexes
Objectives:
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to investigate stoichiometry using color measurement
to study bonding in transition metal complexes
to determine the formula of a transition metal complex
Principles:
Determining the formula of a complex ion by color measurement
The cationic forms of metallic elements such as iron, cobalt, copper, and chromium form
larger ionic compounds with ligands (anions and neutral molecules). These are called
complex ions. For example, Co+3 ion interacts with 6 ammonia molecules forming the
hexaamminecobalt(III) ion:
The equation describing this would be:
Co+3 + 6NH3  Co(NH3)6+3.
Metallic elements tend to form 2, 4, or 6 bonds with ligands. The number of bonds
formed is called the coordination number. Iron in oxyhemoglobin, cobalt in Vitamin B12,
and magnesium in chlorophyll all have coordination number 6. Copper often has
coordination number 4 while silver and gold can have coordination number 2.
The ligands also have their own terminology. Some ligands (e.g. H2O, NH3, amines,
halides, nitrite) form only one bond with a metal. We call these monodentate (one-tooth).
So the formula of a monodentate ligand with a metal having coordination number 6
would be: MX6 (e.g. Co(H2O)6+2 or [Fe(H2O)5Cl]+2 ) Each ligand could bind to one of the
6 possible positions on the metal ion.
Some ligands are bidentate (two-toothed). Ethylenediamine is such a molecule. Its
abbreviation is en. The formula of en with a metal atom having coordination number 6 is
M(en)3. (e.g. Co(en)3+3). Three ethylenediamines bonded to 1 cobalt ion.
We can have tridentate ligands (diethylenetriamine) and polydentate ligands (EDTA).
The purpose of this experiment is to determine the stoichiometry of a complex ion
formation reaction. In the example above, the stoichiometry is that 6 ammonia molecules
would react with one Co+3 ion.
Color of complex ions:
The color of a complex ion is often distinctly different than the ion that is not complexed
(or an ion complexed merely with the solvent). The color occurs when an electron in the
metal ion absorbs the energy of visible light causing it to "jump" from one energy level to
a higher one. This subtracts that color from the visible spectrum as the light is absorbed.
A single photon of light is absorbed by an electron causing the electron to "jump" to a
higher energy level. The energy difference is  which equals the photon’s energy.

Different ligands cause the spacing between the steps to increase or decrease, changing
the wavelength of light needed to cause a jump (and therefore the color of that light).

E

Ligand 2 requires a higher energy light to cause the electron to jump because the energy
difference is larger. This would be reflected in a lower max.
Substances to investigate (suggested list):
copper(II) sulfate
EDTA
diethylenetriamine
triethylenetetramine
ethylenediamine
sodium salicylate
sodium acetylsalicylate
imidazole
Instruments and chemicals needed:
safety glasses
a laboratory notebook to record observations
a ball point pen
we will supply you:
0.1M copper(II) sulfate
0.1M solution of the appropriate ligand
UV-visible spectrophotometer
What to do: We will supply solutions of metal ion and ligand that are 0.1661M. These
solutions would have 100x1019 ions or 100 E ions where E stands for exa, 1019. Your
team will be assigned one ligand to study. You are to determine the formula of the metal
ligand complex.
1. For a given ligand, determine the wavelength of maximum absorbance by mixing
a small amount of metal ion (5 drops) with a large amount of ligand (5 mL). Scan
the region between 320 and 810nm noting the wavelength giving the maximum
absorbance. Record the wavelength giving the maximum absorbance. This is the
max you will use for the experiment.
2. You will use the balance to prepare solutions.
3. Place the graduated cylinder on the balance and press the tare bar Add Cu+2
solution and reweigh. The amount of Cu+2 solution should vary between 1 drop
and less than 1 gram.
4. Then add ligand until the total mass is 1.00 gram
5. Add water to the 5 mL mark on the graduated cylinder
6. Pour into a vial, pour back into the cylinder, and back into the vial. This mixes the
solution.
You should have 10 solutions with various concentrations of copper and ligand:
0.05g Cu+2 , 0.10 g Cu+2 etc. which correspond to 0.05(100E) = 5E ions of copper in the
first case and 10E ions of Cu in the second case.
Measure absorbance at each concentration for each solution you prepared at max.
Graph absorbance versus number of metal ions and determine the number of metal ions
that give the maximum absorbance.
This is what the ideal graph looks like:
If you don't get a nice triangle, make additional solutions to add to your data.
What does the value of the number of copper ions at the maximum absorbance tell
you?
It gives you a good clue on the number of ligands attached to the metal ion. Suppose the
maximum number of copper ions is half-way on the graph. That would be 50E ions of
copper. We are doing the experiment so that the total number of ions in the solution is
always 100E. So that 50 ions of copper and the other ions are the ligand (also 50E). 50%
of the ions are copper meaning the other 50% are ligand or 1 copper for 1 ligand. The
formula would be CuL.
What if the maximum is at 20 copper ions? Then there would be 80 ligand ions or a ratio
of 1 copper to 4 ligands. The formula would be CuL4
One complicating factor is that with some ligands an precipitate forms at low
concentrations of ligand. You can do three things if this happens:
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filter the solution
let the solution settle and draw off the top clear layer
graph the cloudy solution
If you chose the latter, the graph will look like this:
Finally we want you discuss your results with others in the class to determine if there is a
relation between ligand and wavelength of maximum absorbance.
Note: You can dilute the stock solutions directly by a factor of 10 and then add volumes
to a constant total volume as an alternative way to do the experiment.
Using data from the ligand experiment to determine which ligands would lower the
energy level difference compared to the metal ion in water and which raise the energy
level difference by calculating the energy level difference for each max.
For example:
Lab Questions:
1. Calculate the number of copper ions in each vial from the mass of copper and the
concentration of the solution (100E ions/g).
2. Graph the absorbance versus number of copper ions for the solutions you
prepared and determine the maximum in the absorbance readings.
3. Determine the coordination number for copper in your complex and the type of
ligand in the complex also (e.g. CN=6, tridentate, bidentate…).
4. Compare the max values of complexes prepared by other students. What is their
color? What is the energy of transition? What is the order of the ability to separate
the energy levels (greatest to weakest)?