Which of the following is the most stable species?

[Ni(NH3)6]2+ [Fe(CN)6]3- [Fe(C2O4)3]3- [Ni(H2O)6]2+

[Fe(C2O4)3]3-

[Fe(C2O4)3]3- is the most stable among the given complexes as it contains three bidentate ligands (C2O42-: oxalato) forming rings giving chelate effect. Other complexes contain monodentate ligands such as NH3, CN–, H2O. Monodentate ligands does not provide chelate effect.