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 C2​O42−​ is a bidentate chelating ligand, it forms rings and hence 

[Fe(C2​O4​)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.