Why is the stability of `[Ni(en)_(3)]^(2+)` more than that of `[Ni(NH_(3))_(6)]^(2+)`?

Explanation

The stability of the complex of metal ion `Ni^(2+)` with a bidentate lignad, en is significantly greater than that for the complex of the same metal ion with two monodentate ligands, `NH_(3)`. The reason is chelate effect. In case of `[Ni(en)_(3)]^(2+)` ion, formation of five-membered rings enhances its stability. In fact the equilibrium constant for the formation of `[Ni(en)_(3)]^(2+)` is almost `10^(10)` times that for the formation of `[Ni(NH_(3))_(6)]^(2+)` ion.
`Ni^(2+)(aq)+6NH_(3)(aq)hArr[Ni(NH_(3))_(6)]^(2+),K_(f)=4.07xx10^(8)`
`Ni^(2+)(aq)+"2en (aq)"hArr [Ni(en)_(3)]^(2+)'K_(f)=1.90xx10^(18)`