The chelate effect
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Water, ammonia and chloride ions are all called monodentate ligands because there is a single dative covalent bond between the ligand and the central metal atom. (Monodentate literally means 'one toothed' because the ligand only 'bites' the metal atom in one place). In other ligands, however, more than one bond to the metal atom is involved, and so these ligands are called polydentate ligands.
1,2-diaminoethane, often abbreviated to 'en', is a neutral, bidentate ligand. It has the formula H2NCH2CH2NH2.
Where will the lone pairs of electrons be that enable 1,2-diaminoethane to form two dative covalent bonds to metal atoms or ions?
There is a lone pair of electrons on each nitrogen atom in 1,2-diaminoethane.
When 1,2-diaminoethane is added to a solution of copper(II) sulfate a ligand exchange reaction occurs. One 1,2-diaminoethane ligand replaces two water ligands:
[Cu(H2O)6]2+ + en [Cu(en)(H2O)4]2+ + 2H2O
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The structure of the complex is called a chelate after the Greek word for crab's claw.
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Why might a crab's claw be a good way to describe the structure of the complex?
A crab's claw is a good description because you can imagine the copper ion being gripped by the claw of a 1,2-diaminoethane molecule.
It's interesting to compare the lg Kstab values for complexes with mono- and bidentate ligands.
| ion | lg Kstab |
| [Cu(NH3)2(H2O)4]2+ | 7.86 |
| [Cu(en)(H2O)4]2+ | 10.6 |
The increase in stability resulting from complexes formed with polydentate ligands is called the chelate effect. The reasons for the chelate effect are explained in the following animation.
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