What is bidentate ligand

The macrocyclic effect follows the same principle as the chelate effect, but the effect is further enhanced by the cyclic conformation of the ligand. Macrocyclic ligands are not only multi-dentate, but because they are covalently constrained to their cyclic form, they allow less conformational freedom.

The ligand is said to be "pre-organized" for binding, and there is little entropy penalty for wrapping it around the metal ion. Monodentate Ligands The term "monodentate" can be translated as "one tooth," referring to the ligand binding to the center through only one atom. 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. 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. Inorganic Chemistry: A Modern Introduction. Amer Library Assn. Bowker, R. Inorganic Chemistry Concepts.

Porterfield, William. Inorganic chemistry. In terms of entropy, which deals with the distribution of energy within a system, it is generally thought that bringing two molecules together a bidentate ligand and a metal complex costs less than bringing three molecules together two monodentate ligands and a metal complex.

That's because individual molecules are free to move around, tumble and vibrate independently. Once they come together, they have to do all these things together. Since these different types of motion represent different ways of distributing energy, if the system becomes more restricted, energy can't be distributed in as many states.

Compared to two separate donors, bidentate donation is entropically favoured. A ligand could be monodentate, meaning it binds through a lone pair on a single atom. It could be bidentate, meaning it binds through lone pairs on two different atoms.

It could even be tridentate, with three atoms bearing their own lones pairs, tetradentate, and so on. Table CC4. Examples of polydentate ligands. For example, in ethylenediamine or 1,2-diaminoethane, NH 2 CH 2 CH 2 NH 2 , the two nitrogen atoms can be bound to the metal at the same time, although none of the other atoms in between would be directly attached to the metal.

I n each of the following cases,. In the following cases, the ligand has slipped, so that it isn't binding as tightly as it possibly could. In each case,. 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. With dmpe, the bite angle appears larger in the picture than the one for dmpm, and in reality it is larger, although not quite as large as it appears here.

Two different ligands that bind with two different bite angles will have different influences on the complex that forms. In fact, chemists often use these differences to "tune" the behaviour of transition metals that are used as catalysts for important properties.

They might add similar ligands with different bite angles to see which one best promotes the desired catalytic reaction. Many factors can influence the bite angle, including structural features of the bidentate ligand itself, the metal, and other ligands bound to the metal.

However, a particular ligand will usually have a normal range of bite angles that it will be able to adopt under different circumstances.

Certain ligands may have natural bite angles that work better in some cases than in others.