An example of a bidentate ligand is bis dimethylphosphino propane. It can bind to a metal via two donor atoms at once: it uses one lone pair on each phosphorus atom. More examples of bidentate ligands are shown below. They all have at least two different atoms with lone pairs. In some cases, there are additional atoms with lone pairs, but only two of them are able to face the metal at one time. Oxalate and glycinate would act as bidentate donors, donating up to two sets of lone pairs at the same time.
Bidentate binding allows a ligand to bind more tightly. Tridentate ligands, which bind through three donors, can bind even more tightly, and so on. This phenomenon is generally called the "chelate effect". This term comes from the Greek chelos, meaning "crab". A crab does not have any teeth at all, but it does have two claws for tightly holding onto something. A very simple analogy is that, if you are holding something with two hands rather than one, you are not as likely to drop it.
The chemical reasons for the chelate effect involve relative enthalpy and entropy changes upon binding a multidentate ligand. Changing the size and electronic properties of ligands can be used to control catalysis of the central ion and stabilize unusual coordination sites.
Different ligand structural arrangements result from the coordination number. Most structures follow the pattern as if the central atom were in the middle and the corners of that shape are the locations of the ligands. These shapes are defined by orbital overlap between ligand and metal orbitals and ligand-ligand repulsions, which tend to lead to certain regular geometries.
However, there are many cases that deviate from regular geometry. For example, ligands of different sizes and with different electronic effects often result in irregular bond lengths. Boundless vets and curates high-quality, openly licensed content from around the Internet.
This particular resource used the following sources:. Skip to main content. Transition Metals. Search for:. Ethylenediamine en is a bidentate ligand that forms a five-membered ring in coordinating to a metal ion M. Polydentate Ligands Polydentate ligands range in the number of atoms used to bond to a central metal atom or ion.
Images courtesy of Wikimedia Commons. Chelation Chelation is a process in which a polydentate ligand bonds to a metal ion, forming a ring. The Chelate Effect The chelate effect is the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating monodentate ligands for the same metal.
References Petrucci, Harwood, Herring, Madura. Prentice Hall. New Jersey, Cox, Tony. Instant notes in inorganic chemistry. Libraries, Association, Robert Williams, and J. Bringing chemistry to life. Now, just because a ligand could bind through multiple atoms does not mean that it always binds that way.
That's often true with the acetate ligand, for example, because the four-membered ring that forms when it binds through both oxygen atoms is a little too strained. Consequently, there are examples of binding through both oxygen atoms, and there are also examples of binding through only one.
Sometimes, acetate uses one oxygen to bind to one metal atom and the other oxygen to bind to a second metal atom, forming a bridge. Usually, if a ligand is capable of chelation, assume it binds that way. However, there may be cases in which you are asked specifically to draw it binding in one way or another. In the following cases, the ligand has slipped so that it isn't binding as tightly as it possibly could.
In each case,. In this case, the acetate gets the complex to 18 electrons by binding twice. See the examples below. The two molecules on the right are fac and mer isomers of diethylenetriaminetrichlorocobalt III. More details on isomers are provided in a more advanced course.
There are more subtle aspects of chelation. For example, two different bidentate ligands may not necessarily bind to the metal in exactly the same way. In the drawing below, it's apparent that the three bidentate phosphine ligands, bis dimethylphosphino methane, bis dimethylphosphino ethane, and bis dimethylphosphino propane, do not all bind the metal with the same geometry.
In each case, the metal forms a different angle with the two phosphines. The term "bite angle" is frequently used to describe how different bidentate ligands will attach to metals at different angles.
In the picture, the P-Pd-P angle appears to be about 90 degrees when dmpm is bound; in reality it is even smaller.
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