Chelate complexes

EDTA metal ion chelate complex

In complex chemistry, the term chelate complexes - or chelates for short - stands for compounds in which a multidentate ligand (has more than one free electron pair) occupies at least two coordination sites (binding sites) on the central atom. In this case, the ligand is called a chelator (Greek Χηλή, chele for “crab claw ”). The central atom is usually a doubly positively charged metal ion ( ^e.g. Fe ²⁺ , Cu ²⁺ ). The ligands and central atom are linked by coordinative bonds . This means that the binding electron pair is provided by the ligand alone.

Chelate complexes are more stable than identical complexes with monodentate ligands that are not linked to one another. This “chelation effect” has two causes. On the one hand, the decrease in entropy during complex formation is lower, which has a thermodynamic stabilization effect. In the example above, a complex is formed from two particles. In the case of non-chelating ligands, seven particles would combine to form a complex. On the other hand, a chelating ligand can only remove itself from the central atom after all bonds have been broken, which means that the chelator is much more difficult to dissociate from the metal ion. This also increases the likelihood of immediate recombination after cleavage.

Examples of this are found in nature such as heme , chlorophyll or vitamin B12 . In medicine, it is important in the context of chelation therapy . B. EDTA (ethylenediaminetetraacetate). It is also used in analytical chemistry for titrations in chelatometry .

Chelating ligands

Important chelating ligands are:

Ligand	Abbr.	Tenacity
Acetylacetone	acac	2
Ethylenediamine	en	2
2- (2-aminoethylamino) ethanol	AEEA	3
Diethylenetriamine	serve	3
Iminodiacetate	ida	3
Triethylenetetramine	trien, TETA	4th
Triaminotriethylamine	tren	4th
Nitrilotriacetate	nta	4th
Bis (salicylidene) ethylenediamine	salen	4th
Ethylenediaminotriacetate	ted	5
Ethylenediaminetetraacetate	EDTA	6th
Diethylenetriamine pentaacetate	DTPA	8th
Triethylenetetramine hexaacetate	TTHA	10
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate	DOTA	8th
Oxalate	ox	2
Tartrate	tart	2
Citrate	cit	3
Dimethylglyoxime	dmg	2
8-hydroxyquinoline	oxine	2
2,2'-bipyridine	bpy	2
1,10-phenanthroline	phen	2
Dimercaptosuccinic acid	DMSA	2
1,2-bis (diphenylphosphino) ethane	dppe	2

Individual evidence

1. ↑ Wolfgang Forth, Dietrich Henschler , Walter Rummel: General and special pharmacology and toxicology . First edition: Bibliographisches Institut 1975.