In general, for a metal ion with a coordination number of 6, ethylenediamine complexes are more stable than ammonia complexes. This is because ethylenediamine is a bidentate ligand, meaning it can donate two electron pairs to the metal ion, while ammonia is a monodentate ligand and can only donate one electron pair. The chelate effect, which refers to the increased stability of a complex with a multidentate ligand compared to a complex with multiple monodentate ligands, contributes to the enhanced stability of ethylenediamine complexes.
When considering the reaction of these complexes with sodium hydroxide solution, the basicity of the ligands also plays a role. Ammonia is a weaker base than ethylenediamine, meaning it is less likely to protonate in aqueous solution. As a result, ammonia complexes are more likely to undergo ligand exchange reactions with hydroxide ions from sodium hydroxide solution, leading to the formation of metal hydroxide complexes.
In contrast, ethylenediamine is a stronger base and its complexes are less likely to undergo ligand exchange reactions with hydroxide ions. Therefore, ethylenediamine complexes are generally more stable in the presence of sodium hydroxide solution compared to ammonia complexes.
