LIGAND EXCHANGE IN COPPER (II) COMPLEXES

A2 Experiment 14.4(3)

Ligand Exchange in Copper (II) Complexes

introducton

Copper ions form stable complexes with a range of ligands. The co-ordination number is usually 6 (octahedral), but can be 4 (tetrahedral). A stability constant, K, can be defined for the displacement of water ligands from hydrated copper (II) ions by other ligands. This is illustrated by the equilibrium below for chloride ions:-

[Cu(H2O)6] 2+ (aq) + 4 Cl (aq) ( [CuCl4] 2- (aq) + 6 H2O (l)The values of the stability constants for different ligands vary over a very wide range, so it is more common to use log K values instead. The larger the value for log K the further equilibrium lies to the right, i.e., the stronger the ligand is compared to water and the more stable the new complex is. The table below shows 6 ligands with their log K values:-

LIGANDlog K

H2Owatermonodentate-

Cl -chloridemonodentate5.6

NH3ammoniamonodentate13.1

2-hydroxybenzoate (salicylate)bidentate16.9

1,2-dihydroxybenzene (catchetol)bidentate25.0

(edta) 4-ethylenediaminetetraacetatehexadentate18.8

In this practical you will observe the colours of the copper complexes made with these ligands and how one ligand can displace another.

apparatus

Test tube rack(s) and plenty of test tubes (get 6 to start with)

Teat pipettes

White tile to help you see the colours better

safety

Concentrated hydrochloric acid and concentrated ammonia solutions are corrosive and produce harmful fumes. Handle these in the fume cupboard.

Catchetol is made up in sodium hydroxide solution, which is caustic. Wear full eye protection and gloves. Avoid spillages onto the wooden bench or skin. Wash away any spillages with plenty of water.

procedure

APut one teat pipette full of 0.5M copper (II) sulphate into a test tube and keep for reference.

What is the colour of the CuSO4 solution?

What complex ion is present?

BPut one teat pipette full of the CuSO4 solution into a clean test tube. Add concentrated HCl drop by drop until the colour stops changing (fume cupboard)

What is the colour of the solution now?

What complex ion is present?

CPour half of your solution from B into a test tube half full of distilled water (and keep the rest for part D)

What is the colour of the solution now?

What complex ion is present?

Explain the change in terms of the equilibrium shown on Page 1.

DTo the solution left from part B, add concentrated ammonia solution dropwise until there is no further change in colour (fume cupboard). Keep this solution for part F.

Describe everything you observed whilst adding the conc. NH3 solution.

Which complex ion is present now?

If a stronger ligand can displace a weaker one, which order would you place the three used so far?

Is this consistent with the log K data?

EPut one teat pipette full of the CuSO4 solution into a clean test tube. Add a solution of the ligand edta until the colour stops changing.

What is the colour of the [Cu(edta)] 2- complex?

Use the log K data to predict what colour change you would expect if you added some edta solution to the solution left from part D

FTo the solution left from part D, add edta solution until there is no further change in colour.

What is the colour of the solution now?

What complex ion is present?

Write a balanced equation for the reaction which has occurred.

GCollect 4 clean test tubes and put one teat pipette full of the copper (II) sulphate solution into each. Add dilute (bench) ammonia solution to the first tube until there is no further change in colour. To the second add edta. Add sodium 2-hydroxybenzoate (sodium salicylate) to the third and add 1,2-dihydroxy benzene (catchetol) to the fourth. (Care: the catchetol is dissolved in NaOH)

What colours are the four complexes?

Ammonia

Edta

2-hydroxybenzoate (salicylate)

1,2-dihydroxybenzene (catchetol)

HUse the log K data to predict the sequence of colour changes that would take place if you started with the original CuSO4 solution and then added ammonia solution followed by sodium 2-hydroxybenzoate, then edta and finally 1,2-dihydroxybenzene.

To check your prediction, put one teat pipette full of the original CuSO4 solution into a clean BOILING tube. Add each ligand in turn dropwise until there is no further colour change at each stage (you may need reasonably large quantities in some cases).

Was your prediction correct?

further questions

1) Ammonia, water and chloride ions are all monodentate ligands. Draw the three-dimensional structure of each of the complex ions formed from these ligands.

2) 1,2-dihydroxybenzene, like phenol, is a weak acid and in alkaline solution is ionised (loses two H+ ions from the hydroxyl groups). It acts as a bidentate ligand. Draw the structure of the complex formed with this ligand and show the charge clearly.

COO -

OH

OH

OH

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