In Coordination Chemistry, transition metals bind with ions or certain compounds in aqueous solutions. These ions are known as ligands, and together the whole compound is called a coordination complex. A few compounds have the ability to bind to the central metal atom more than once and are described as polydentate, which literally translates to mean "many teethed." These compounds are described thus because they bind on to a central atom at multiple points. EDTA is such a polydentate ligand. More specifically, EDTA is a hexadentate ligand, since hexa- means six and the ligand attaches six times.
EDTA, or ethylenediaminetetraacetic acid, binds a total of 6 places on an ion in solution. Molecules that bind at multiple points in a coordination complex are said to be chelated.
Figure 1: The fully-protonated form of EDTA4-, H4EDTA. The non-protonated form results with the removal of the hydrogen atoms on each of the four carboxylic acid groups. In a non-protonated molecule, the EDTA has extra, unpaired electrons on the four oxygen atoms that have single bonds with the carbons and on the two nitrogen atoms.
Molecules that chelate tend to be more thermodynamically stable, and chelating compounds tend to displace monodentates in coordination complexes in solution. This is due, in part, to the fact that entropy-favoring reactions are spontaneous, and "want" to happen more. For instance, see the following sample equation in which EDTA displaces the water ligands:
Fe(H2O)63+ + EDTA4- ↔ [Fe(EDTA)]-+ 6H2O
Here, an aqua-complex of iron is changed into an iron molecule with a single EDTA molecule surrounding it. This is because on the left side of the equation, there are two molecules - the iron complex and the EDTA - while on the right side of the equation there are seven molecules - the new iron complex and the six water molecules displaced by the EDTA ligand. Entropy-favored reactions are spontaneously occurring, and an increase in the number of molecules is an increase in entropy. Thus, the above reaction is spontaneous, with a K value exceeding 1025. The general trend, as would logically follow, would imply that polydentate ligands that have more "teeth" or binding points to the central atoms are more thermodynamically favored / stable, and can thus be used to displace weaker, monodentate ligands. EDTA can bind a total of six times, such as in the equation above with iron, so it is a VERY good molecule to displace other ligands.
Figure 2: An EDTA molecule binds a total of six times to a generic central metal, labeled M. Notice that the atoms with the unpaired electrons (the two nitrogens and four oxygens) make these coordinate covalent bonds. Credits go to Wikipedia for picture.
See the video below explaining chelation:
As it is so good at displacing molecules in coordination complexes, EDTA can be used to prevent undesired metals in trace amounts - called impurities - from reacting and having detrimental effects on products. This is known as sequestering. For instance, in regards to cosmetics, EDTA serves to increase the cosmetic product's resistance towards molecules in the air. Simmilarly, in personal care and skin care products, EDTA binds to free metal ions and serves as a purifying agent and perservative. It basically reduces the "hardness" (or presence of metal cations) in tap water so that other ingredients in shampoos and soaps can work to cleanse more efficiently. Along the same lines, EDTA is used in laundry detergents to soften water that comes into contact with it so the other active ingredients can cleanse better. In textiles, EDTA prevents the discoloring of dyed fabrics by removing harmful free metal ions and it also gets rid of residue left on industrial equipment that must be used at high temperatures (i.e., broilers). In general, EDTA reduces the reactivity of a metal, preventing any unwanted effects that may result from its presence. EDTA is used in a salt form, most likely in disodium or calcium disodium EDTA.
In addition to its usefulness in industries, EDTA can also be utilized in medicine. Doctors can prescribe EDTA treatments for patients suffering from lead poisoning. Such a treatment is known as chelation therapy, in which EDTA renders the toxic ions present in the body harmless. The EDTA is administered intravenously and makes its way through the blood stream. Given its hexadentate nature, EDTA has a molecular structure much like a claw. Because of this very structure, the EDTA pulls toxic heavy metals detected in the bloodstream towards itself and attaches itself to these metal ions. This attachment forms a compound that can be excreted from the body through urine, not allowing them to bind to enzymes and cytochromes. A chelation therapy may take many sittings and may last anywhere from one to three hours per sitting. Not only can chelation therapy aid in excreting harmful lead ions from the body, but it can also aid in safely getting rid of mercury, chromium, cobalt, nickel, zinc, arsenic and thallium ions from the bloodstream. In cases of excess consumption of digoxin, a medication used to treat atrial fibrillation, atrial flutter and even heart failure, EDTA has been used to clear the bloodstream of the unused ions.
EDTA is very useful as a polydentate ligand. Unfortunately, it is now so overused that it is considered to be an environmental pollutant. Left alone, it degrades eventually to ethylenediaminetriacetic acid, losing one acidic group and becoming toxic after it forms diketopiperizide. EDTA levels are currently being monitored with mass-spectrometry analysis procedures, though it is considered to have very low level acute toxicity.
Highlight text for the correct answer.
1. How many times does EDTA bind in a coordination complex?
Answer: EDTA is a hexadentate ligand, which means that it binds six times. It binds twice at the nitrogens and four at the oxygens.
2. List in order of most to least thermodynamically stable as a ligand: ONO, en (ethylenediamine), EDTA.
Answer: EDTA (which binds six times), en (which binds two times), ONO (which binds one time)
3. What are the most common forms of EDTA?
Answer: EDTA is used most commonly as salts and in a dry form.
4. Why is EDTA so overused? Why is this a problem?
Answer: EDTA is a great chelating agent, allowing multiple bindings in a coordination complex. This gives it the ability to displace other undesirable ligands due to entropy and thermodynamics, and is thus used in laboratories, factories, and in medicine. The problem with its overuse is that it degrades into a toxin. By having excess of this, more toxin is created and left in the environment.
5. Bonus Question: What does Professor Larsen commonly refer to EDTA as?
Answer: "The Kraken" because it is the ultimate ligand (it binds six times, which is a very large amount for ligand binding).
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