Question

In the complexes ${\left[ {Fe{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$ , ${\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{3 - }}$ , ${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$ and ${\left[ {FeC{l_6}} \right]^{3 - }}$, the most stable is:
A. ${\left[ {Fe{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$
B. ${\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{3 - }}$
C. ${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$
D. ${\left[ {FeC{l_6}} \right]^{3 - }}$

Answer 1

We have to know that the Chelate Effect states that complexes that result from coordination containing the chelating ligand are thermodynamically more stable than complexes that contain non-chelating ligands.

Complete step by step answer:
We need to know that a metal-containing ring is known as a chelate ring. We can say a polydentate ligand is a chelating agent, and complexes that have polydentate ligands are known as the chelate complexes.
Experimentally, we can observe that metal complexes of polydentate ligands are considerably more stable than the corresponding complexes of chemically like monodentate ligands; this rise in stability is known as the chelate effect.
The stability of a chelate complex is based on the size of the chelate rings. For ligands with a flexible organic backbone such as ethylenediamine, complexes that comprises of five-membered chelate rings, that has almost no strain, are considerably more stable than complexes that has six-membered chelate rings, that in turn is more stable than complexes containing four- or seven-membered rings.
From the above explanations, we can conclude that the stability of complexes increases with the respect to the number of chelates present in them.
We can say that metal chelates are more stable than non chelates. We know oxalate is a bidentate ligand and it forms metal chelate.
Among the given complexes, the complex ${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$ has more number of chelates, and since the most stable complex would be ${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$ .
Another factor that influences the stability of the complex is the nature of the ligand.
We can say a strong ligand or a strong field ligand is a ligand that can result in a higher crystal field splitting.
We can say a weak ligand or a weak field ligand is a ligand that can result in a lower crystal field splitting.

So, the correct answer is Option C.

Note: We need to know that the binding of a strong field ligand causes a higher difference between the higher and lower energy level orbitals.
Examples: $C{N^-}$ (cyanide ligands), $N{O_2}^-$ (nitro ligand) and $CO$ (carbonyl ligands)
The binding of a weak field ligand causes a lower difference between the higher and lower energy level orbitals since the low difference between the two orbital levels causes repulsions between electrons in those energy levels, the higher energy orbitals could be easily filled with electrons when compared to that of electrons in low energy orbitals.
Examples: ${I^-}$ (iodide ligand), $B{r^-}$ (bromide ligand), etc. The other factors that affect the stability of the complexes are,
1.Nature of the metal ion.
2.Charge present on the central metal ion.
3.Multi-dentate cyclic ligands.