Chelate Effect
Chelating ligands give much larger values of formation constants
[Ni(H2O)6]2+ + 6NH3 ? [Ni(NH3)6]2+ + 6H2O Kf = 4 ? 108
[Ni(H2O)6]2+ + 3en ? [Ni(en)3]2+ + 6H2O Kf = 2 ? 1018
Sequestering agents are chelating agents that are used to remove unwanted metal ions.
In medicine sequestering agents are used to selectively remove toxic metal ions (e.g. Hg2+ and Pb2+) while leaving biologically important metals.