Complexes and ligands
A coordination complex consists of a central metal ion bonded to molecules or ions with lone pairs (ligands). Ligands with one donor site are monodentate (NH₃, H₂O); those with several are multidentate, and the ring complexes they form are called chelates, from the Greek for a crab's claw (Morgan & Drew, 1920).
The chelate effect
[Ni(en)₃]²⁺ has a far larger stability constant than [Ni(NH₃)₆]²⁺. One multidentate ligand displaces several water molecules, increasing the number of free particles and thus entropy. Five- and six-membered rings are the most stable.
Common chelators and uses
- EDTA: hexadentate; used for Ca²⁺/Mg²⁺ titration (water hardness), lead poisoning treatment and food preservation.
- Medicine: dimercaprol (BAL) for arsenic and mercury, deferoxamine for iron overload.
- Environment: amine-rich polymers such as chitosan bind heavy metals and remove them from wastewater (see adsorption and deacetylation).
- Biology: heme in hemoglobin is a porphyrin chelating iron; chlorophyll chelates magnesium.
References
- Morgan, G. T., & Drew, H. D. K. (1920). Researches on residual affinity and co-ordination. Part II. Journal of the Chemical Society, Transactions, 117, 1456-1465.
- Schwarzenbach, G. (1952). Der Chelateffekt. Helvetica Chimica Acta, 35(7), 2344-2359.
- Housecroft, C. E., & Sharpe, A. G. (2018). Inorganic Chemistry (5th ed.). Pearson.
- Flora, S. J. S., & Pachauri, V. (2010). Chelation in metal intoxication. International Journal of Environmental Research and Public Health, 7(7), 2745-2788.