Question

Which of the following complexes is most stable?
A.${\left[ {Fe{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$
B.${\left[ {Fe{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$
C.${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$
D.${\left[ {FeC{l_6}} \right]^{3 - }}$

Answer 1

Hint: The oxidation of $Fe$ in the complexes given is +3. Find which ligand acts as the chelating ligand. Chelation is the type of bonding of ions and molecules to metal ions. This bonding involves the presence of two or more separate coordinate bonds between the multiple bonded ligand and central atom.

Complete Step by step answer: There are two types of stability in coordination compounds – Thermodynamic stability and kinetic stability. Thermodynamic stability is the change in energy on moving from reactants to products and the kinetic stability can be defined as the stability which refers to the reactivity, generally ligand substitution.
We know that, in the complexes the stability increases by the chelation.
Therefore, chelation can be defined as the type of bonding of ions to metal ions. It involves the formation or presence of two or more separate coordinate bonds between the polydentate ligand and central atom.
The chelate effect is the enhanced affinity of chelating ligands for the metal ion compared with the affinity of collection of similar non – chelating ligands for the same metal.
We have been given with the four complex which are –
${\left[ {Fe{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$
${\left[ {Fe{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$
${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$
${\left[ {FeC{l_6}} \right]^{3 - }}$
From the above complexes we can see that the oxidation state of iron is +3.
Now, from the above complexes, we can conclude that there is only one chelating ligand which is ${C_2}O_4^{2 - }$. ${C_2}O_4^{2 - }$ is the bidentate chelating ligand. Therefore, when it forms the complex with $Fe$ it forms a ring as it acts as the chelating ligand. Hence, the most stable complex is ${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }}$.

Therefore, the correct option is (C).

Note: The factors which affect the stability of complex are –
-Charge on the central metal ion
-Nature of the metal ion
-Basic nature of the ligand
-Presence of the chelate rings
-Effect of multi dentate cyclic ligands