[js493cam]

Hi,

In the following equilibria:

Cd(2+) + 4NH3 <-> [Cd(NH3)4]2+ (1)
Cd(2+) + 2en <-> [Cd(en)2]2+     (2)           (en = NH2-(CH2)2-NH2)

The enthalpy change is approximately identical ( -55.2 and -55.7 kJ/mol) because ammonia and en have almost identical field parameters, while the entropy change for (1) and (2) is hugely different (-41.9 and +10.4 J/K/mol respectively). This means that reaction (2) goes much more to the right hand side than (1), by about three orders of magnitude. This is the chelate effect. I think I understand why (1) has a negative entropy change - i.e. disorder will decrease when you bind four ammonias to the cadmium. I understand that the entropy change of (2) must be much less than (1), as you release fewer molecules of en when you push the reaction to the left hand side, resulting in less disorder. What I'm not happy with is why the entropy change of (2) is actually POSITIVE. Can anyone enlighten me as to why reaction (2) is favoured entropically?

Thanks

j

[Rudi]

Hi,

in the case of ammonia, four NH3 replace four H2O and so the number of independent species remains the same.

Ethylenediamine is, however, a bidentate ligand and, thus, two ligands replace four H2O, increasing the number of independent species in the system, which causes a negative entropy (for the reaction from left to right).

[js493cam]

Ah, that kinda makes sense. It probably helps if you write out the waters in the equation rather than just assuming them. Thanks!

[renge ishyo]

Also, the solvation of the ligand is different when it is bound to the metal. For example, when ethylene diamine (en) is not bound it is a straight chain molecule in solution. This means that more water molecules have to surround it compared to the number of molecules that have to surround it after it closes on itself to form a stable ring. The excess water molecules that are released when this molecule is in a more compact ring form versus the straight form also contributes to the increase in entropy in favor of the chelating complex.