Is there ever a time where EDTA and a metal ion are not in 1:1 ratios?

Question

My book says that Ca2+ and Mg2+ are in a 1:1 molar ratio with EDTA. However, what about compounds like Co and others? When will I know when it isn't a 1:1 ratio?

Answer 1

Sure -- it depends on the orbital structure of the metal atom.

EDTA is a chelating agent, and you can have as many as three around certain metals, such as Mn, Cu, Fe, and Co.

See the referenced articles for more.

Answer 2

Ah... No. EDTA is a bi-dentate ligand, but it only binds to a single atom.

The EDTA complexation is just like any other combination reaction. You will have a limiting reagent either the metal ion or the EDTA. So to call for a 1:1 mole ratio isn’t technically correct, since your excess reactant skews the whole 1:1 ratio. In the complex it is 1:1.