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 C21J 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.
sodium salt 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 solublity 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

  1. Draw the structures of the cis- and trans- diaquabisoxalatochromate(III) ions.
  2. The trioxalatochromate ion is also a mixture of isomers. Explain and draw the isomers.
  3. Draw in perspective, the structure of the copper complex.
  4. Dissolve a small quantity of K2Cr2O7 in dilute NaOH. Note and explain your observations.

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