EDTA

Some ligands can bond to a metal atom using more than two pairs of electrons. An example is ethylenediamminetetraacetate ion (EDTA^4-), the Lewis structure of which is shown below. EDTA^4- forms very stable complexes with most of the transition metals.

EDTA^4-
This hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals. The donor atoms in EDTA^4- are the two N atoms, and the four, negatively charged O atoms.

= Fe

= C

= O

= H

= N

EDTA^4-
When this ion bonds to a metal atom, the two N atoms, and four of the O atoms, are used.

[Fe(EDTA)]^2-
In this complex, a single EDTA^4- ion forms 6 bonds to the Fe atom (i.e., 2 Fe-N bonds and 4 Fe-O bonds). The coordination number of 6 results in an octahedral structure.

Applications

The sodium salt of EDTA^4- (i.e., Na₄EDTA) can be found in many commercial products including:

soap
beer
mayonnaise

EDTA^4- is used to "trap" trace amounts of transition metals that could potentially catalyze the decomposition of the product.