[CO(en)₃]³⁺ is more stable than [CO(CH₃NH₂)]³⁺ to make more stable complex because of chelate effect.
[CO(en)₃]³⁺ is a metal chelate due to presece of bidentate ligand ethylene diamine ligand. Metal chelates are more stable than complexes containing only monodentate ligands because of the chelate effect, we expect ethylenediamine to form a stronger complex with Co than ammonia.
The Chelate Effect is that complexes resulting from coordination with the chelating ligand is much more thermodynamically stable than complexes with non-chelating ligands.
The chelate effect defined as the augmented affinity of chelating ligands for any metalic ion in comparison of the affinity of a similar nonchelating ligands for the same metallic ion.
