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 hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals. The donor atoms in EDTA4- are the two N atoms, and the four, negatively charged O atoms.
When this ion bonds to a metal atom, the two N atoms, and four of the O atoms, are used.
In this complex, a single EDTA4- 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.
EDTA4- is used to "trap" trace amounts of transition metals that could potentially catalyze the decomposition of the product.