Ligands

Ligands are ions or neutral molecules that bond to a central metal atom or ion. Ligands act as Lewis bases (electron pair donors), and the central atom acts as a Lewis acid (electron pair acceptor). Ligands have at least one donor atom with an electron pair used to form covalent bonds with the central atom. Ligands can be anions, cations, or neutral molecules.

Monodentate Ligands

A monodentate ligand has only one donor atom used to bond to the central metal atom or ion. The term "monodentate" can be translated as "one tooth," referring to the ligand binding to the center through only one atom. Some examples of monodentate ligands are: chloride ions (referred to as chloro when it is a ligand), water (referred to as aqua when it is a ligand), hydroxide ions (referred to as hydroxo when it is a ligand), and ammonia (referred to as ammine when it is a ligand).

        

                 Fig. 1. Central atom with six monodentate ligands attached. (Image courtesy of Wikimedia Commons.)   

Bidentate Ligands

Bidentate ligands have two donor atoms which allow them to bind to a central metal atom or ion at two points. Common examples of bidentate ligands are ethylenediamine (en), and the oxalate ion (ox). Shown below is a diagram of ethylenediamine: the nitrogen (blue) atoms on the edges each have two free electrons that can be used to bond to a central metal atom or ion.

Fig. 2. Ethylenediamine an example of a bidentate ligand. (Image courtesy of Wikimedia Commons.)

Polydentate Ligands

Polydentate ligands range in the number of atoms used to bond to a central metal atom or ion. EDTA, a hexadentate ligand, is an example of a polydentate ligand that has six donor atoms with electron pairs that can be used to bond to a central metal atom or ion.

EDTA is a polydentate ligand. (Image courtesy of Wikimedia Commons.)

Chelation

Chelation is a process in which a polydentate ligand bonds to a metal ion, forming a ring. The complex produced by this process is called a chelate, and the polydentate ligand is referred to as a chelating agent.

Metal-EDTA Chelate. (Image courtesy of Wikimedia Commons.)

Ligand Nomenclature

For a more in-depth study of ligand nomenclature, read the module on Nomenclature of Coordination Complexes

1. If a complex has an ion ligand, an "-o" ending is added. For example, "-ide" is changed to "-o," "-ite" is changed to "-ito", and "-ate" to "-ato." Hence, the bromide ion (Br^-) becomes bromo, the nitrite ion (NO₂^-) becomes nitrito, and the sulfate ion (SO₄^2-) becomes sulfato.
2. When a complex has a neutral molecule ligand, the molecule keeps its original name. For example, ethylenediamine is a ligand, and the ligand is still called ethylendiamine.
3. Prefixes mono=1, di=2, tri=3, tetra=4, penta=5, and hexa=6, are used to specify the number of ligands. If a ligand name has a prefix within itself such as ethylendiamine, place a parenthesis around the name and add bis=2, tris=3, tetrakis=4, in front of the molecule. For example, if there was 2 (en) molecules, the name would be bis(ethylenediamine).
4. When naming a complex, ligands are the first to be named. If there is more than one ligand, list them in alphabetical order. Next comes the metal ion or atom. The oxidation state follows the name in roman numerals.                                    

Problems

1. Do ligands act like Lewis acids or Lewis bases? Why?
2. Do ligands form ionic bonds with the central metal atom?
3. What are chelating agents?
4. What is a monodentate ligand?
5. Describe polydentate ligands and provide an example.
6. What are hexadentate ligands?
7. Name this complex [Cu(NH₃)₄]SO₄.
8. Name this complex  [Co(en)₃](NO₃)₂.

Answers

1. Ligands act like Lewis bases because they share their electron pairs (electron donors) with the central metal atom.
2. No, ligands do not form ionic bonds the with the central metal atom. Rather, they form covalent bonds with the central metal atom because they share electron pairs.
3. Chelating agents are ligands that have two or more atoms with donating electron pairs that are able to attach a metal ion at the same time. These chelating ligands are monodentate and tridentate ligands
4. A monodentate ligand is a ligand that uses only one pair of electrons to bond to the central metal atom or ion.
5. Polydentate ligands are ligands which are able to donate more than two electron pairs to the central metal they bond to. EDTA is an example of a polydentate ligand.
6. Hexadentate ligands are ligands which have six lone pairs of electrons which can all bond to the central metal atom.
7. tetraamminecopper(II) sulfate
8. Tris(ethylenediamine)cobalt(II) nitrate

References

1. Petrucci, Harwood, Herring, Madura. General Chemistry Principles & Modern Applications. Prentice Hall. New Jersey, 2007
2. Cox, Tony. (2004). Instant notes in inorganic chemistry. Oxford, UK: Taylor & Francis.
3. Libraries, Association, Robert Williams, and J. Silva. Bringing chemistry to life. Oxford University Press, USA, 1999. Print.
4. Moeller, Therald, Douville, Judith, & Libraries, Association. (1988). Inorganic Chemistry: A Modern Introduction. Amer Library Assn.
5. Bowker, R., Warmus, Mieczysław, Muzzy, Adrienne, LOCALIZADO, AUTOR, Hopkinson, Barbara, Saur, K, Izod, Irene, Hopkinson, Barbara, Saur, K, Books, K, & Company, K. (1994). Inorganic Chemistry Concepts. K G Saur Verlag Gmbh & Co.
6. Porterfield, William. (1984). Inorganic chemistry. Addison Wesley Publishing Company.

Outside Sources

1. Electronic Structure of Coordination Complexes http://www.youtube.com/watch?v=mAPFhZpnV58
2. Transition Metal Complexes http://www.youtube.com/watch?v=UvWBuryKlSk&feature=related
3. Coordination Complexes A http://www.youtube.com/watch?v=v7MbkMi3aMQ&feature=related
4. Coordination Complexes B http://www.youtube.com/watch?v=nYHHpONul80&feature=related
5. Molecular docking simulation: multiple ligand simultaneous docking http://www.youtube.com/watch?v=oeqJo9xYviY
6. Identification of Ligands for Protein Purification http://www.youtube.com/watch?v=429j3Ikxxy0
7. In Scopus. "ScienceDirect - Analytical Biochemistry : LIGAND: A Versatile Computerized Approach for Characterization of Ligand-binding Systems." ScienceDirect - Home. Web. 02 June 2010. <http://www.sciencedirect.com/science...b01ccf162fea5e>.
8. "Systematic Evolution of Ligands by Exponential Enrichment: RNA Ligands to Bacteriophage T4 DNA Polymerase -- Tuerk and Gold 249 (4968): 505 -- Science." Science/AAAS | Scientific Research, News and Career Information. Web. 02 June 2010. <http://www.sciencemag.org/cgi/conten...i;249/4968/505>.
9. "Differential Ligand Activation of Estrogen Receptors ER{alpha} and ER at AP1 Sites -- Paech Et Al. 277 (5331): 1508 -- Science." Science/AAAS | Scientific Research, News and Career Information. Web. 02 June 2010. <http://www.sciencemag.org/cgi/conten.../277/5331/1508>.
10. Green, M.L.H. A new approach to the formal classification of covalent compounds of the elements. (1995) Journal of Organometallic Chemistry