Among the following, which is considered the most stable complex?

The complex ion [Fe2(C2O4)3] 3- is regarded as the most stable complex among the given options due to several reasons related to its structure and bonding characteristics.

Firstly, [Fe2(C2O4)3] involves iron in a higher oxidation state, potentially forming a strong metal-ligand interaction. The oxalate ion (C2O4) is a bidentate ligand that forms chelate rings with the metal center. This chelation significantly enhances the stability of the complex because of the chelate effect, where the formation of multiple bonds between the metal and the ligand makes the complex less prone to dissociation.

Additionally, the presence of two iron centers allows for effective distribution of charge and coordination, enhancing the overall stability of the complex. Iron(III), common in oxalate complexes, has a tendency to stabilize itself with suitable ligands like oxalate, leading to increased stability.

In contrast, while [Fe(CN)6] 3- is also a very stable complex due to the strong field strength of cyanide ligands, it typically forms under specific conditions and is not as stable as the chelate formation observed in the oxalate complex. The [Cu