Iron ligand substitution

[aj47]

AFAIK the addition of dilute ammonia to metal(II) hexaaqua ions will always result in a hydrolysis reaction resulting in the hydroxide precipitiating, while ammonia in excess will result in an ammine complex.

 

However when I was looking up ligand substitution reactions for iron(II) ions, I found that with excess ammonia the hydroxide forms and not the ammine complex. Does anyone know why this is? I would have expected all metal (II) ions to react in the same way.

[Tartaglia]

This is actually a remarkably difficult question to answer.

 

It is actually a thermodynamic question with a lot of potential contributions from a lot of different, physical, chemical and electronic influences. I will however give a number of pointers.

 

Symmetry of the complex matters

The oxidation state matters

The ionic radius

Whether it is a 3d, 4d or 5d metal

Whether the complex is high spin or low spin -ie crystal field stabilisation energy

Whether the ligand is strong field or weak field

Whether ligands can pi accept or pi donate

Solubility considerations, enthalpy, entropy

Whether bridging dimetal or polymetal complexes are possible eg with hydroxide ligands

and probably a lot of other things which haven't occured to me yet

[aj47]

wow ok I thought it would be something really fundamental that I had overlooked but thanks anyway.

 

Actually I asked someone else this question and they vaugley answered it mentioning entropy and relating it to a larger [math] \Delta S [/math] value in the Gibbs free energy equation, for the formation of hexaaqua complexes. However I did not understand this as surely the difference in entropy values would negliable for the ammine and aqua complex, as the corordination numbers are the same and size of the ligands almost identical?

[Tartaglia]

I suspect entropy is not the major factor, but the polarising power of metal ions increases with charge and decreases with ionic radius and the more polarising the metal ion the more the solvent molecules will affected beyond the immediate metal coordination sphere so there could be entropy differences

[aj47]

ok thanks great help. I'm guessing these factors also account for the reason why aluminium can't form ammine complexes aswell.

[Tartaglia]

Ammine complexes of ammonia are not unknown but tend to be adducts rather than cationic complexes. Some of the reasons outlined in my first post do not apply to aluminium as it doesn't have a partially filled d shell.

Aluminium is incredibly oxophilic as the thermodynamic sink of Al is Al2O3. Al3+ is extremely polarising due to its small size and large charge. It will find ripping OH- or O2- out of water a lot easier than ripping NH2- or even N3- out of ammonia as the Pka of H2O is a lot lower than for NH3. Thus it prefers aqua ligands