[anna123]

Just wondering if anyone could tell me why chelates are more stable than complexes containing unidendate lingands. Also, why are metal (III) chelates more stable than metal (II) chelates?

Thanks alot for any help!

[anna123]

Oh are they more stable because their formation involves an increase in entropy? I'm still not sure about the other bit though!

[danhirons]

Because:

M(H20)6 2+ + 3en ---> M(en)3 2+ + 6H2O

i.e. 4 to 7 species, therefore increase in entropy is highly favourable.



Regarding the other point, I think it's just basic electrostatics. If you're pairing M(III) with say an edta 4- hexadentate ligand, then pure electrostatic attractions will be stronger when the metal is in the higher oxidation state.

[charco]

(Original post by anna123)
Just wondering if anyone could tell me why chelates are more stable than complexes containing unidendate lingands. Also, why are metal (III) chelates more stable than metal (II) chelates?

Thanks alot for any help!

As the other posters say - entropy...

.. but there is also the logical contribution from statistical likelihood of collision.

A multidentate complex may lose bonding from one ligand position, but the other bonds maintain the detached atom with a potentially bonding lone pair in the same spacial region increasing its likelihood of re-attachment.

[anna123]

Ok thank you both very much for your help!

[JMaydom]

Entropy is the major driving force but enthaply plays a role.... If the ligands are charged, then there is a disfavourable interaction in bringing the multiple negatively charged ligands in close proximity to each other. In an equivalent charged chelate, these charges are already close together, hence enthalpy is more negative