The Chelate Effect

This is an extension of ligand substitution and links in to entropy and Gibbs Free Energy.

If a multidentate ligand (such as EDTA^4-) replaces a unidentate one (such as water) there is a thermodynamic advantage, even if there is little difference in bond strength.

Before substitution:

M(H₂O)₆²⁺ + EDTA^4-    a total of TWO species in solution.

After substitution:

M(EDTA)^2- + 6 H₂O     a total of SEVEN species in solution.

Even if we assume that there is no change in the bond energy total (which would mean zero enthalpy change) there is a large increase in entropy and so the Gibbs Free Energy would be strongly negative.

See other sheets on Gibbs Free Energy for help with this.

The same kind of thing can work with oxalate (more properly known as ethanedioate) C₂O₄^2- and ethyline diamine (or "en") (more properly known as 1,2 diaminoethane) H₂NCH₂CH₂NH_2 .