Experiment 7
Isomerism and Coordination Geometries
Explored by Spectroscopic and Magnetic
Measurements
Isomerism and Geometry - 1
Experiment 7: Isomerism and Coordination Geometries
Explored by Spectroscopic and Magnetic Measurements
Aims
to prepare cis and trans-isomers of a cobalt(III) complex
to compare the structures and spectroscopic properties of the above
to identify different coordination geometries by use of spectroscopic and magnetic
measurements
Introduction
Geometric isomerism is an important feature of transition metal coordination chemistry and
can strongly influence properties such as reactivity, solubility and stability of complex ions.
One example is the compound Pt(NH3)2Cl2 which in the cis-form is a potent anti-cancer
treatment whilst in the trans-form is inactive. In this experiment cis and trans isomers of a
cobalt(III) amine complex are investigated. Cobalt(III) centres generally have a low spin 3d6
electron configuration which renders them relatively inert to substitution and isomerisation and
thus their isomers are long lived and easily studied.
The combination of magnetic measurements and electronic spectroscopy is a powerful tool
for the elucidation of the structures of transition metal complexes: both coordination geometry
and oxidation state can generally be deduced. Nickel(II) complexes have 3d8 electron
configurations and can adopt octahedral, tetrahedral and square planar geometries. the dorbital splitting and electron arrangements are very different in each (below) allowing the
different geometries to be distinguished by a combination of UV-visible spectroscopy and
magnetic measurements.
Energy
Tetrahedral
Octahedral
Isomerism and Geometry - 2
Square Planar
Experimental
a) Preparation of trans-[Co(en)2Cl2]Cl
- Dissolve CoCl2.6H2O (16 g) in water (50 cm3) in a 250 cm3 Buchner flask.
- Prepare 60 cm3 of a 10 % solution of 1,2-diaminoethane in water.
NOTE: 1,2-diaminoethane (IUPAC name) used to be known as ethylenediamine, hence
the abbreviation “en”. The old name continues to be used by many chemists.
- Slowly add the 10 % solution of 1,2-diaminoethane to the cobalt chloride solution with
stirring
- Draw air through the solution for 10 to 12 hours.
To do this, place a cork bung with a glass tube through it (obtain from centre bench) into the
top of the Buchner flask making sure the bottom of the glass tube is below the surface of
the solution. Attach the Buchner flask to a water pump and switch fully on. Air will now be
sucked through the solution, oxidising cobalt(II) to cobalt(III).
- Next lab session, add 35 cm3 of conc. hydrochloric acid.
- Transfer the contents of the flask to an evaporating basin and allow the solution to
evaporate on the steam bath until a crust forms on the surface.
- Allow to cool and stand overnight.
- Filter off the green crystals of the hydrochloride of the trans-isomer, wash with ethanol and
diethyl ether.
- Dry overnight in the oven at 110C. A dull green powder is produced.
- Record the yield (g, %) and run infra-red (nujol mull) and solid state (reflectance) UV-visible
spectra.
Retain about a quarter of your sample for spectroscopic investigation (above) and proceed to
convert the remainder to the cis form.
b) Preparation of cis-[Co(en)2Cl2]Cl
- Dissolve approx three quarters of your trans-isomer in the minimum quantity of 50 %
aqueous ethanol.
- Neutralise the solution by adding dilute NaOH solution.
- Evaporate to dryness on a steam bath.
- Redissolve in 50% aqueous ethanol and evaporate to dryness once more.
- Wash out any unreacted trans-isomer with a little water.
- Filter, wash with diethylether and air dry.
- Record the yield (g, %) and run infra-red (nujol mull) and solid-state (reflectance) UV-visible
spectra.
c) Spectra of Nickel Complexes
The materials required for the experiment are supplied. They are:
1.
Solid NiCl2.6H2O
2.
Solid Ni(DMG)2*
3.
Solid [Et4N]2[NiCl4].
4.
Solution of NiCl2.6H2O in H2O
5.
Solution of Ni(DMG)2 in CHCl3
6.
Solution of [Et4N]2[NiCl4] in CH3NO2 (dilute)
7.
Solution of [Et4N]2[NiCl4] in CH3NO2 (conc)
*
DMG = dimethylglyoximate; HO-N=C(Me)-C(Me)=N-O-
Isomerism and Geometry - 3
- Measure the powder magnetic moments of compounds 1-3 using the method described in
appendix 1. Note that [Et4N]2[NiCl4] is highly deliquescent. Make sure you stopper the flask
very rapidly after taking your sample.
- After consulting the demonstrator, measure the reflectance spectra of the three solid
compounds.
- Measure the transmission spectra of the four solutions using the appropriate UV visible
spectrometers.
- Using the given molarities, calculate the molar absorptivities of each solution, and tabulate
these together with the energies of the absorptions (in cm -1).
Questions
1.
2.
3.
4.
5.
6.
Describe by means of equations the reactions involved in the preparations of the two
cobalt complexes..
Sketch the structures of the two cations.
In the light of their symmetry properties comment on the spectroscopic data you have
obtained. (Hint: consider the nitrogen environments - are they all the same? What
effect will that have on the IR spectrum?)
Compare the reflectance and transmission spectra for each of the nickel compounds
using solution 7 in the case of [Et4N]2[NiCl4] Are they similar?
Assuming similar geometries in the solution and solid, use the magnetic moments,
molar absorptivities and energy positions (in cm -1) of the bands to predict the likely
geometry of each compound explaining how you use each measurement to support
your prediction. Some information on UV-visible spectroscopy is given in appendix 3.
Consider the solution spectra obtained for 6 and 7. In what way are they different?
Suggest a reason for the change. (Hint: how does the molar absorptivity change?
What does that suggest about the geometries of the two species?)
Isomerism and Geometry - 4