9.31 Amongst the following, the most stable complex is
(i) [Fe(H2O)6]3+ (ii) [Fe(NH3)6]3+
(iii) [Fe(C2O4)3]3– (iv) [FeCl6]3–
In each of the given complexes, Fe is in a +3 oxidation state. As C2O42− is a bidentate chelating ligand, it forms rings and hence
[Fe(C2O4)3]3− is the most stable complex.
The complex which contains chelating ligands are more stable than nonchelated compounds of comparable composition. Hence, option (iii) is the correct answer.