Saturday 22 March 2014
Ligands
Table of Contents
- 1. Monodentate Ligands
- 2. Bidentate Ligands
- 3. Polydentate Ligands
- 4. Chelation
- 5. Ligand Nomenclature
- 6. Color and Magnetism
- 7. Spectrochemical Series
- 8. Problems
- 9. Answers
- 10. References
- 11. Outside Sources
- 12. Contributors
Ligands are ions or neutral molecules that bond to a central metal atom or ion. Ligands act as Lewis bases (electron donors), and the central atom acts as a Lewis acid (electron 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
- 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 (SO42-) becomes sulfato.
- 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.
- 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).
- 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.
Color and Magnetism
Color changes observed in solutions are caused by changes that occur in the ligands attatched to the metal ion or atom. While these changes occur, the oxidation state of the metal ion remains constant.
Spectrochemical Series
The spectrochemical series is the arrangement of ligands based on how large or small of a field splitting energy they create between d-orbitals in coordination complexes. In most cases, the spectrochemical series is arranged from left to right, where the left-side ligands create the smallest d-orbital splittings (strong field ligands) and the right-side ligands create the least amount of d-orbital splitting (weak field ligands). Read more about this in these two modules: Spectrochemical Series, Spectrochemical Series.
I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- < C2O42- < H2O < NCS- < CH3CN < NH3 < en < bipy < phen < NO2- < PPh3 < CN-
