24.2+-+Ligands+with+More+Than+One+Donor+Atom

__**24.2 - Ligands with More Than One Donor Atom**__ Most ligands bond to only a single coordination site on a metal ion, and are known as **monodentates** (Latin, one-toothed). However, some ligands can bond to a metal ion at multiple coordination sites simultaneously, usually as a result of their donor atoms being located far apart from each other. Ligands that bond at two coordination sites are called **bidentates**, and, in general, ligands that bond at more than one coordination site are called **polydentates**. Polydentates are also known as **chelating agents**.

Due to their ability to coordinate at multiple sites on a metal ion, chelating agents form more stable compounds than monodentates do. From an equilibrium standpoint, bonding of a chelating agent to a metal ion has a much higher formation constant (K of formation) than that of a similar monodentate, indicating that the reaction proceeds much farther right. This implies an increase in stability, and is known as the **chelate effect**.
 * Chelating Agents**

The process of a chelating agent bonding with a metal ion is more exothermic and has a greater positive entropy change than that of monodentates forming the same bonds. As a result, it has a negative change in free energy (i.e. the chelating agent process is more spontaneous). The entropy change is easily explained. When a metal ion is in solution, individual H2O molecules bind to each coordination site (H2O acts as a monodentate). However, when a chelating agent is added the metal ions favor the bonds with the chelating agent over the bonds with H2O. Chelating agent molecules can bind to multiple coordination sites, replacing several H2O molecules with a single molecule. The replaced H2O molecules are freed from the ion, increasing the total number of particles, and thus increasing the entropy, of the system.
 * The Chelate Effect and Thermodynamics**

There are many useful applications of chelating agents. Chelating agents are often added to solutions to prevent metal ions from reacting with other species present, effectively hiding the metal ions form other particles as they bind to the coordination sites. Some chelating agents are included in medicines and are used to remove harmful metals from living systems. Chelating agents are also found naturally in biological systems, and are present in chlorophyll (key to photosynthesis) and hemoglobin (essential for oxygen transport in the blood).
 * Uses of Chelating Agents**


 * Links to More Coordination Chemistry**
 * Classifications of Ligands**
 * Visual Representations of Ligands**
 * The Lewis Acid-Base Concept:Coordination Compounds**


 * References**
 * 1) Brown, Theodore L., and H. Eugene LeMay. Chemistry: The Central Science. Upper Saddle River, NJ: Pearson Prentice Hall, 2008. Print.
 * 2) "Classifications of Ligands." Web. 20 Mar. 2011. .
 * 3) "Coordination Chemistry." Western Oregon University. Web. 20 Mar. 2011. .
 * 4) "Transition Metals." Denk Group - University of Guelph. Web. 20 Mar. 2011. .