In chemistry, a ligand is an atom, molecule or ion, be it monatomic or polyatomic, that donates an unshared pair of electrons (loose pair of electrons) to form a dative or coordinated covalent bond with a neutral atom or central cation . The compound thus formed is often called a coordination complex.
Depending on the electrical charge of the ligand and the central atom or ion, the coordinate complex may or may not have a net electrical charge. If it is neutral it is usually considered as a coordination compound , and if it is ionic it is called a complex ion . Also, any salt formed by a complex ion with a suitable counterion is called a “complex salt” .
Ligands can have very diverse structures and compositions. However, every ligand must have at least one atom that has a free or unshared pair of valence electrons available for coordinate bond formation. This atom (or these atoms, since some ligands have more than one) is called the donor atom since it is the one that contributes the electrons of the dative covalent bond.
Ligands as Lewis bases
As can be seen from their definition, ligands are actually Lewis bases , since they are electron-rich species that possess lone pairs of electrons and have the ability to donate lone electrons to a Lewis acid. For this reason, chemical reactions between a ligand and a central atom (almost always a metallic element) or a cation are nothing more than acid-base reactions.
As in many cases, there are several ways to classify ligands. The most used criteria are:
- The number of atoms that make them up.
- The electric charge.
- The number of electron pair donor atoms.
- Depending on the type of atomic or molecular orbital in which the donated electrons are found.
Classification according to the number of atoms that make them up
As their name indicates, they are those that are formed by only one atom. These are usually monatomic anions such as the fluoride (F – ) or chloride (Cl – ) ion.
They are the ligands that are formed by two or more atoms. They are by far the most common and include ligands such as the water molecule (H 2 O), molecular oxygen (O 2 ), hydroxide ion (OH – ) etc.
Classification according to its electrical charge
They are ligands that have no net electrical charge. That is, they are molecular species that have groups with atoms such as O, N, S, P or some of the halogens.
Examples of neutral ligands
Water ( H2O ) Ammonia ( NH3 ) The ethers (RO-R’) Amines (R 3 N) Phosphine (PH 3 ) Thioethers (RS-R’) Carbon monoxide or carbonyl (CO)
Anionic or negatively charged ligands
Many ligands are groups that have an excess of electrons and therefore have a net negative charge. These anions are very common ligands and are characterized by having a negative charge generally on a very electronegative atom such as O, N or a halogen, which is in turn the donor atom. They can have one or more negative charges.
Examples of Anionic or Negatively Charged Ligands
Chloride ion (Cl – ) Fluoride ion (F – ) Bromide ion (Br – ) Iodide ion (I – ) Hydroxide ion (OH – ) Cyanide ion (CN – ) Carbonate ion (CO 3 2- ) Alkoxides (RO – )
Classification according to the number of donor atoms of electron pairs (hapticity)
Some ligands that can only bind to a metal center through one coordinate bond, while others through 2 or more. This gives rise to the following types of ligands:
They are the ligands that only have a donor atom, so they can only form a coordinated covalent bond with the metal center.
Examples of monodentate ligands
Water (H2O ) Ammonia ( NH3 ) The ethers (RO-R’) Chloride ion (Cl-) Phosphine (PH 3 ) Thioethers (RS-R’) Amines (R 3 N) Hydroxide ion (OH-)
Polydentate ligands or chelating agents
Many ligands have more than one donor atom, and their structure allows the ligand to bind to the metal center with more than one coordinate bond. In the final structure, the ligand surrounds the central atom like a mouth biting into it, with the donor atoms acting like teeth (hence the name polydentate). The complexes that are formed by the union of a polydentate ligand with a metallic center are called chelates, so the ligand is also called a chelating agent (the one that forms a chelate).
Some chelating agents have two donor atoms, in which case they are called bidentate ligands (the prefix bi- means 2).
Those with three donor atoms are called tridentate ligands, those with four tetradentate, and so on.
Examples of polydentate ligands
Ethylenediamine (H 2 N-CH 2 -CH 2 -NH 2 ) ethylenediaminetetraacetic acid (EDTA) The heme porphyrin ring in hemoglobin crown ethers
These are ligands that have two or more donor atoms but whose structure does not allow both atoms to be attached to the same metal center simultaneously. In these cases, two different complexes can be formed with the same metal center depending on which of the two “sides” of the ligand binds to the metal. In these cases, the same ligand is given different names depending on which of the atoms is the true donor.
Examples of Ambidentate Ligands
Cyanide ion or cyano ligand with C as donor (–CN – ) Isocyanide ion or isocyan ligand with N as donor (–NC – ) Thiocyanate ion or thiocyanate ligand with S as donor (–SCN – ) Isothiocyanate ion or isothiocyan ligand with N as donor (–NCS – ) Nitro with N as a donor (–NO 2 – ) Nitrite with O as a donor (–ONO – )
Lastly, we have ligands that can simultaneously bind to more than one metal center, either through two separate donor atoms, or through the same donor atom when it has more than one unshared pair of electrons. This last case is particularly common in ligands that have O, S or some halogen atoms, or in the cases of amides that have a negative nitrogen with two pairs of free electrons.
By simultaneously binding to two metals, these ligands form a bridge between the two centers and that is where their name comes from.
Examples of Bridging Ligands
Hydroxide ion (OH – ) Oxide ion (O 2 2- ) Amido ion (NH 2 2- ) Cyanide ion (CN – ) Carbon monoxide or carbonyl (CO) Chloride ion (Cl – )
Classification according to the type of atomic or molecular orbital involved
Donor ligands σ (sigma)
They are ligands that only have one pair of free electrons and donate it through a σ covalent bond. They generally stabilize cations with low oxidation states. Examples of these ligands are ammonia and amines.
Donor ligands π (pi)
These ligands bind to the metal center via the π electron cloud. These include olefins and aromatics.
σ and π donor ligands
They are ligands with very electronegative elements and high electronic density, that is, they are hard Lewis bases. By binding to the metal center, they are able to stabilize high oxidation states in which the metal has all or most of its d orbitals empty, allowing the ligand to donate electron density through both π and σ bonds. Typical examples are the halogens.
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