EDTA – A sexidentate ligand
Some ligands can bond to a metal atom using more than two pairs of electrons. An example is ethylenediamminetetraacetate ion (EDTA4-), the Lewis structure of which is shown below. EDTA4- forms very stable complexes with most of the transition metals.
This sexidentate ligand forms very stable complexes (usually octahedral structures) with multivalent transition metal ions. The donor atoms in EDTA4- are the two N atoms, and the four, negatively charged O atoms.
When the tetravalent EDTA ion bonds to a divalent or greater metal atom, the two N atoms and four of the oxygen atoms are used. Just as with the oxalate ion the electron density of the delocalized electron cloud of the deprotonated carboxylic groups is localized fixing the hybridization of one the oxygens to sp3. In this complex, a single EDTA4- ion forms 6 bonds to the Fe3+ cation. The CN of 6 is an octahedral structure.
EDTA4- is used to "trap" trace amounts of transition metals that could potentially catalyze the decomposition of the product.
The sodium salt of EDTA4- (i.e., Na4EDTA) can be found in many commercial products including: