Experiment 15
Oxalate Complexes of Chromium(III) and Copper(II)
Theory
Oxalic acid, (1,2-ethanedioic acid), in the form of the dianion,
functions as a bidentate ligand with many transition metal ions.
For example, the Fe(III) complex is the most widely used compound
in chemical actinometry (see the CHEM2111 manual). The Calcium
complex is the deposit that is responsible for cataracts
and kidney stones.
Chromium is an abundant element in the earth's crust. The metal
is used for plating and in chrome steels. The chromates
[chromium(VI)] have many industrial uses as pigments, catalysts
and fungicides. Chromium(III) is a common and stable oxidation state
that displays significant kinetic inertness. The most common geometry
is octahedral with other shapes being quite rare.
Copper is not as abundant (68ppm compared to 122ppm for Cr). It has been
known since prehistoric times and is used extensively in wiring.
The usual stereochemistry is either 4 coordinate or distorted 6
coordinate with 4 short bonds and 2 long bonds in accordance
with the Jahn-Teller theorem.
The solid state structure of the complex with 2 oxalates depends on the cation used:
for Na+ and K+ both are 6 coordinate with links from the carbonyl O
to a neighbouring copper.
2].gif)
sodium salt
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potassium salt
Preparation of
K3[Cr(C2O4)3].3H2O
Note: The discovery of this compound is credited to Dr Wilton George
Turner who in the London winter of 1830-1831 isolated it by accident.
Turner was born in Clarendon, Jamaica in 1810 and died in the Turks Islands
in 1855. He obtained a PhD from the University of Geissen in 1838
(with Justus von Liebig).
It was described thus:
This beautiful salt crystallizes in thin elongated prisms, which appear black
by reflection, blue by transmitted light, and green when reduced to powder.
Its solution is green and red at the same time, except by candlelight,
when it is of a pure red.
Suspend 5.5 g of oxalic acid dihydrate in 10 cm 3 of cold water
in a 600 cm 3 beaker. Add, in small portions with stirring, 1.8 g
of potassium dichromate. CARE ! Cr(VI) is a suspected
carcinogen The orange mixture soon warms up spontaneously,
almost to boiling point, and a vigorous evolution of gas occurs.
When the reaction has ceased, decant one half of the hot
green-black liquid into a 100 cm3 beaker. (Save the
other half for the next preparation.). Stir in 1.1 g of potassium oxalate monohydrate
and when all the solid has dissolved add 2 cm3 of
ethanol. Heat the reaction mixture on a water bath until solid
begins to deposit from solution. Then add, with stirring, 5
cm3 of ethanol and cool in an ice bath. When cold,
filter off the green-black solid and wash with three 5
cm3 portions of cold 2:1 ethanol/water and then with
10 cm3 ethanol (No flames). The
product is dried in air and the yield calculated as a percentage
based on chromium. The overall reaction is:
K2Cr2O7 +
7H2C2O4 +
2K2C2O4 →
2K3[Cr(C2O4)3].3H2O +
6CO2 + H2O
Preparation of
trans-K[Cr(C2O4)2
(H2O)2].3H2O
The solution saved from the first part of the experiment contains
an equilibrium mixture of the cis- and trans-
isomers of the diaquabis(oxalato)chromate(III) ion. The potassium
salt of the trans- isomer is purple and the
cis- isomer is blue-gray. The lower solubility of the
trans- isomer results in its preferential
crystallization.
Slowly evaporate the solution saved, to about one half its bulk
(do not boil) and allow further evaporation to
occur spontaneously at room temperature. By this stage, crystals
should have been deposited, which are filtered off and washed
with ethanol. The product is air-dried and the yield calculated
as a percentage based on chromium.
The yield is small but it is more important to isolate the
pure trans- isomer than to maximize the yield.
Do not evaporate the solution to dryness.
Show your product to a demonstrator. If at the end of the lab day
no product has formed, cover container and store carefully in
your cupboard for the next lab session.
Preparation of
K2[Cu(C2O4)2].2H2O
Prepare two solutions:-
i) Dissolve 3.1 g
of potassium oxalate monohydrate in 15 cm3 of water
ii) Dissolve 2.0 g of
CuSO4.5H2O in 10 cm3 of water.
Heat both solutions to about 60 °C and then slowly add the
copper(II) solution to the oxalate solution with stirring.
(If you reverse the order of mixing the crystals formed are usually found
to be smaller). Cool the mixture in ice water and filter off the blue crystals via a
weighed No.3 sintered glass crucible. Wash with 2 x 5
cm3 portions of ice-cold water and 10 cm3
of ethanol (NO FLAMES !!).
Air dry the product and determine the yield.
Show the product to your demonstrator!
(KEEP THE K2[Cu(C2O4)2].2H2O
FOR NEXT WEEK'S LAB)
Write an equation for the preparation and calculate the
percentage yield based on the quantity of copper sulfate crystals
used.
Questions
- Draw the structures of the cis- and trans-
diaquabisoxalatochromate(III) ions.
- The trioxalatochromate ion is also a mixture of isomers.
Explain and draw the isomers.
- Draw in perspective, the structure of the copper complex.
- Dissolve a small quantity of
K2Cr2O7 in dilute NaOH. Note and
explain your observations.
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