Complex Formation--A Study of Competition for Cation
Introduction
Complexes are formed between cations and neutral molecules, or ions of opposite charge, i.e.
anions. The neutral molecules, or anions, surrounding the central cation are called ligand, and
each contains at least one atom bearing a pair of electrons, which can be donated to the central
cation, forming a dative, (coordinate), bond. The ligand is said to co-ordinate to the central ion.
If only one atom, per ligand molecule or ion, has a pair of electrons which can be donated to the
central ion, the ligand is said to be unidentate if each ligand can form two dative bonds, it is called
bidentate, and so on. Some ligands can form as six dative bonds, with the central ion. A ligand
which has two, or more atoms each capable of forming a dative bond with the same central ion, is
called a chelate ligand, and the complex formed is called a chelate complex.The names and
structures of the ligands studied in these experiments are as follows:
oxalate
salicyclic acid
1,2-diaminoethane
(ethylenediamine)
Ethylenediamine tetracetate ion (EDTA)
thiocyanate
water
Some ligands can co-ordinate more strongly to the central ion than others, and the point of these
experiments is to determine the relative strength of co-ordination of these ligands.
Apparatus:
250 cm3 conical flask, test-tubes, 10.0 cm3 pipette, burette, 50 cm3
Chemicals:
0.1M aqueous sodium, or ammonium oxalate (10 cm3), 0.1 M aqueous 1,2-diaminoethane (10 cm3),
0.1 M aqueous potassium thiocyanate (10 cm3), 1% w/v salicylic acid in acetone (5 cm3),
0.1 M aqueous EDTA (60 cm3), 0.05 M ammonium iron(III) sulphate, 0.10 M iron(III) chloride (45
cm3)
Procedure
A. Complexes with Cu2+ as the central ion
1.
Place 2 cm3 of aqueous copper(II) sulphate in a test tube and add a solution of oxalate ions
(e.g. aqueous ammonium oxalate, until no further change occurs.
2.
Add aqueous 1,2-diaminoethane to the above until no further change occurs.
3.
Add aqueous EDTA to this solution.
Question for discussion
Try to work out the order of increasing strength of co-ordination.
What is coordinated to the copper(II) ion in the solution you started with? Add this to your list in its
correct position in the above order.
4. Confirm the order by starting with a strong co-ordinating ligand, and adding what you think
is a weaker one.
How do your experimental results in step (4) confirm the order?
Questions for discussion
1. There are features that the strong co-ordinating ligands have in common. State these features.
Explain the strength of these strong ligands.
2. How do these features lead to the formation of very stable complexes?
B. Complexes with Fe3+ as the central ion
Devise experiments to find the relative strength as co-ordinating ligands of (a) water, (b) the
thiocyanate, (c) the oxalate ion and others you wish to use. Give your method (not detailed
experimental steps) in the following space:- [You should have your experiment planned before
the practical.]
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C. Competition between cations
Take about 2 cm3 of aqueous copper(II) sulphate and add 1,2-diaminoethane until the colour
intensity is a maximum. Then add dilute sulphuric acid. Record your observations
D. Determination of the formula of a complex ion using competitive complexing
To about 2 cm3 of an aqueous solution of iron(III) ions add a few drops of a solution of salicylic
acid in acetone. Ten add aqueous EDTA until no further change occurs. Pipette 10.0 cm3 of 0.10
M Fe3+ solution into a 250 cm3 conical flask, add about 100 cm3 of distilled water, followed by
about 1 cm3 of salicylic acid in acetone. Titrate against 0.10M EDTA until a clear yellow solution
is obtained. Record your titration data and colour change of the end point.
Questions for discussion (section C and D)
1. Explain the experimental result and give equation if appropriate.
2. What is the formula of the Fe3+/EDTA complex?
3. Explain how the salicylic acid can be used to indicate the end-point of this titration? Give
equation if appropriate.
END