Experiment 6
Deduction of a Spectrochemical Series
from Octahedral Chromium(III) Complexes
Spectrochemical Series - 1
EXPERIMENT 6: Deduction of a Spectrochemical Series
from Octahedral Chromium(III) Complexes
Aims
to prepare (pseudo) octahedral complexes of chromium(III)
to measure their solution UV-visible spectra
to construct a spectrochemical series for the ligands utilised
Introduction
The simplest experimental illustration of the spectrochemical series should be the
spectroscopic investigation of octahedral or tetrahedral complexes of d 1 or d9 ions, where a
single spectral transition, of energy 10 Dq, is predicted. There are, however problems for both
cases. The octahedral complexes of Ti(III) (d1)are highly sensitive to air oxidation and a study
of their spectra necessitates the use of inert atmosphere techniques for preparation and
handling. The complexes of Cu(II) (d9) generally deviate from either octahedral or tetrahedral
symmetry. A good compromise between experimental simplicity and easy spectroscopic
analysis exists in the case of Cr(III) complexes. Crystal field theory predicts that the 4F ground
state of the free chromium ion is split into three levels as shown below:
4P
4T
1g
4T
1g
4F
4T
2g
4A
2g
The energy separation between the two lower levels, 4A2g and 4T2g is 10 Dq. Thus in the
Cr(III) case, although we expect to find a more complicated spectrum than in the case of d1 or
d9 complexes, the energy of the longest wavelength peak gives the crystal field parameter 10
Dq.
Experimental
a) Preparation of [Cr(en)3]Cl3
- Wash 0.5-1.0 g of zinc granules with 6M HCl (to remove surface ZnO),
- Place the zinc granules in a flask with CrCl3.6H2O (2.66 g) and add ethylenediamine (10 cm 3)
and methanol (10 cm3).
- Reflux for 1 hour.
- Cool the solution to room temperature.
- Collect the yellow product (contaminated with a black solid) by filtration.
- Wash with 10% ethylenediamine in methanol until the washings are colourless.
- Wash with a small portion of ether and suck air through the product until dry.
- Recrystallise by dissolving in the minimum quantity of a 1:1 methanol/water mixture at 60C,
filtering and cooling on an ice-salt bath to crystallise the product.
Spectrochemical Series - 2
The temperature is critical in this process, since above 60C the complex decomposes to a
deep red solution.
- Record the yield (g, %).
If your product does not precipitate, take 1 cm 3 of the yellow solution and retain (for running
a UV-visible spectrum if no solid is obtained). Treat the remaining solution with MeOH and
cool again – yellow solid should now appear.
b) Preparation of K3[Cr(ox)3]
- Dissolve oxalic acid dihydrate (4.5 g) in distilled water (10 cm 3) with gentle warming.
- While the solution is still warm add potassium dichromate (1.5 g) in small portions.
- After the vigorous reaction has subsided, heat gently to boiling and add potassium oxalate
(1.8 g).
- When the potassium oxalate has dissolved allow the solution to cool to room temperature.
- Add ethanol (3 cm3) and cool on an ice bath. Blue-black crystals should separate.
- Filter, wash first with ethanol/water (1:1 V/V, 50 cm 3) and then with pure ethanol (25 cm 3)
and air dry.
- Record the yield (g, %).
c) Preparation of [Cr(acac)3]
- Dissolve CrCl3.6H2O (2 g) in water (40 cm 3) and add urea (10 g) and acetylacetone (8 cm 3).
- Heat the mixture on a water bath for 1 - 2 hours. Deep maroon crystals should form.
- Cool the flask to room temperature.
- Collect the product by filtration, wash with water and air dry.
- Record the yield (g, %).
- Determine the melting point of your product.
Spectra
- Using small graduated flasks, accurately prepare solutions of the four compounds below at
the concentrations indicated. The concentrations given are only a guide and you may require
to dilute some of them in order to get the absorbance on scale.
1.
2.
3.
4.
[Cr(OH2)6] (NO3)3.3H2O
[Cr(en)3]Cl3.2H2O
K3[Cr(ox)3]
[Cr(acac)3]
0.04 M in water
0.04 M in water
0.01 M in water
0.01 M in CHCl3.
- Run the UV-visible spectrum of each solution.
Questions
1.
2.
3.
4.
Why is zinc used in the preparation of [Cr(en)3]Cl3?
In the preparation of K3[Cr(ox)3], how is Cr(VI) in the dichromate starting material
reduced to chromium(III) in the product? Write balanced redox equations for the
process.
What is the spectrochemical series?
From the positions of the lowest energy (highest wavelength) absorption maxima in
the UV-visible spectra obtain values of 10 Dq (in cm-1) for each species.
Spectrochemical Series - 3
The energy of the absorption in cm-1 is given by the following expression
E (cm-1) = 107 /  (nm)
and the molar absorptivity is given by Beer’s Law:
A = cl
4.
A = absorbance
 = molar absorptivity (dm3 mol-1 cm-1)
c = concentration (mol dm -3)
l = pathlength (cm)
Derive a spectrochemical series for the ligands involved.
Spectrochemical Series - 4