Animated Solution for Chemistry - Coordination Compounds: The total number of coordination sites in ethylenediaminetetraacetate [EDTA${}^{4-}$] is ... .

When we talk about coordination chemistry, few ligands are as famous and widely used as EDTA (Ethylenediaminetetraacetic acid). In its fully deprotonated form, it exists as the ${\text{EDTA}}^{4-}$ anion.

To understand its incredible chelating power, we must first look at its structure. The name itself gives away its anatomy: an ethylenediamine core (two nitrogen atoms connected by a two-carbon chain) with four acetate groups attached to the nitrogens.

The denticity of a ligand refers to the number of donor atoms it uses to bind to a single central metal ion. A donor atom must possess at least one lone pair of electrons that it can share to form a coordinate covalent bond.

Let's scan the ${\text{EDTA}}^{4-}$ molecule for these electron-rich sites:

1. The Nitrogen Atoms: The central ethylenediamine backbone contains two nitrogen atoms. Each nitrogen atom has a lone pair of electrons, making them excellent Lewis bases. This gives us our first $2$ coordination sites. 2. The Oxygen Atoms: Attached to the nitrogens are four acetate groups ($-{\text{CH}}_2{\text{COO}}^{-}$). Each acetate group features a negatively charged oxygen atom. These anionic oxygens are highly nucleophilic and readily donate their electron pairs to a metal cation. With four such groups, we get an additional $4$ coordination sites.

Hexadentate Chelation

Adding them up, we have $2$ nitrogen donors and $4$ oxygen donors, yielding a total of $6$ coordination sites.

Because it can attach to a central metal ion from six different directions simultaneously, ${\text{EDTA}}^{4-}$ is classified as a hexadentate ligand. When it wraps around a metal ion, it forms a highly stable, cage-like structure containing multiple five-membered chelate rings. This phenomenon, known as the chelate effect, makes EDTA complexes exceptionally stable.

The hexadentate nature of EDTA makes it an invaluable tool in both industry and medicine. It is used in complexometric titrations to determine water hardness (by binding ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ ions), in heavy metal poisoning treatments (chelation therapy), and even as a preservative in food and cosmetics to sequester trace metals that would otherwise catalyze oxidation.