A10.4 Polydentate Ligands and Chelation
Polydentate ligands form more than one coordinate bond to transition metal atoms or ions. This process is known as chelation.
Figure 10.1 Polydentate ligands
Ethan-1,2-diamine is a bidentate ligand – having two nitrogen atoms, it can form 2 coordinate bonds to a metal atom. It can be abbreviated to (en) in formulas.
So if (en) bonds to 6 coordination sites, it forms complex ions like
$\textsf{[Ni(en)}_\textsf3\textsf{]}{^\textsf{2+}}$
. This structure is known as a chelate.
Figure 10.2 Chelate Structure
EDTA is a hexadentate ligand and one molecule can bind to all 6 coordination sites of a transition metal.
It forms the
$\textsf{ EDTA}^\textsf{4-}$
ion.
Figure 10.3 EDTA
Figure 10.4 an EDTA / M chelate
If M is
$\textsf{Fe}{^\textsf{2+}}$
, the chelate is
$\textsf{[Fe(EDTA)]}^\textsf{2-}$
A chelate with a polydentate ligand is more stable than a complex ion with multiple monodentate ligands.
$\textsf{[Fe(H}_\textsf2\textsf{O)}_\textsf6\textsf{]}^\textsf{2+}\textsf{+ EDTA}^\textsf{4-}$
$\textsf{[Fe(EDTA)]}^\textsf{2-}\textsf{+ 6H}_\textsf{2}\textsf{O}$
In the equation above EDTA is replacing six water ligands. Therefore the number of molecules is increasing as the reaction takes place. This increases the entropy of the system, making the formation of the EDTA complex a favourable process.

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### Haber Weiss and Fenton reaction mechanisms

(an example of the toxic effects of transition metal ions) The Haber–Weiss reaction can occur in cells and generates hydroxyl radicals •OH from hydrogen peroxide (
$\textsf{H}_\textsf{2}\textsf{O}_\textsf{2}$
) and superoxide free radicals (•
$\textsf{O}^\textsf{-}_\textsf{2}$
).
The reaction is normally very slow but is catalysed by transition metal ions which are therefore a possible source for “Oxidative stress” which is thought to be involved in the development of many illnesses like Cancer, Parkinson’s disease and Alzheimer’s disease. Oxidative stress affects a biological system's ability to detoxify the reactive intermediates or to repair the resulting damage of this stress. The first step of the mechanism involves reduction of
$\textsf{Fe}^\textsf{3+}$
to
$\textsf{Fe}^\textsf{2+}$
ions to
$\textsf{Fe}^\textsf{3+}\textsf{+}$
$\textsf{O}_\textsf2^\textsf{-}$
$\textsf{Fe}^\textsf{2+}\textsf{+ O}_\textsf{2}$
The second step is known as the Fenton reaction
$\textsf{Fe}^\textsf{2+}\textsf{ + H}_\textsf{2}\textsf{O}_\textsf{2}$
$\textsf{Fe}^\textsf{3+}\textsf{+ OH}^\textsf{-}\textsf{ +}$
$\textsf{OH}$
And the overall reaction is therefore:
$\textsf{O}_2^\textsf{2-}\textsf{+ H}_\textsf{2}\textsf{O}_\textsf{2}$
→ •
$\textsf{OH + OH}^\textsf{-}\textsf{+ O}_\textsf{2}$
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