Ligand

Cobalt complex
[HCo(CO)4] with five ligands

In coordination chemistry, a ligand is an ion or molecule (or functional group) that binds to a central metal atom to form a coordination complex. The bond between the metal and the ligand generally involves the donation of one or more pairs of electrons from the ligand. The nature of the metal-ligand bond ranges from the covalent bond to the ionic bond. Furthermore, the metal-ligand bond order can range from one to three. Ligands are considered Lewis bases, although rare instances of "ligands" Lewis acids..

Simple ligands, such as water or chloride anion, which have a single electron donor atom, only form a bond with the central atom and are therefore called monodentate ligands, while those with more than one donor atom They are called polydentate ligands or chelating agents. There is also a third type of ligands known generically as ambidentate ligands which are actually ligands that act as monodentates, but in two different ways depending on the metal.

Anions or molecules capable of acting as ligands must have atoms that have at least one pair of unshared valence electrons. These atoms are found in the upper right corner of the periodic table, and among them the most important are oxygen and nitrogen, later giving way to carbon, phosphorus, sulfur, chlorine, fluorine, etc.

Ligands can be classified in multiple ways, according to their chemical nature, denticity, oxidation state, charge, type of bond...

Types of ligands according to denticity

Depending on the number of atoms to bond to the central atom, we can distinguish between:

Monodentate ligands

Ligands of this type have a single anchor point to the coordination nucleus, hence the name monodentate, which means a single tooth. Usually these are small molecules, which have a single electron donor atom such as ammonia (NH
3), water (H
2O), or halide anions (X−
), alkoxide (RO−
), or I rent (R−
) among others.

When a complex of a metal cation is formed from the union with monodentate ligands, the solubility properties of the cation are notoriously altered, in general this is due to the fact that the complexation causes an increase in the size of the ion, which in turn In turn, it translates into a decrease in the attractive force between the cation and its counterions. This usually causes an increase in the solubility of the ion, or, better expressed, a decrease in its tendency to precipitate.

Complete EDTA Quelate with a Metal

Polydentate ligands or chelating agents

Ligands of this type are capable of establishing two or more simultaneous unions with the coordination nucleus, they can be bidentate, tridentate, tetradentate > etc These types of ligands are also often called "chelating agents" a name derived from the Greek word kela meaning "clamp" because the kind of spatial structure that is formed resembles a crab with the coordinating nucleus caught in its claws. Chelating agents are often used as precipitating agents, since by being able to establish two or more simultaneous unions they can also function as "bridges" between two or more coordination nuclei, facilitating the formation of enormous macromolecular aggregates that precipitate easily.

Among this type of compound we find, for example, phosphate anions (PO3−
4), carbonate (CO2−
3), oxalate (-OOC-COO-), ethylenediamine (NH
2-CH
2-CH
2-NH
2) and bipyridine. A polydentate ligand of enormous importance due to the number of applications it has is EDTA, EDTA has six binding sites.

Ambidentate ligands

This type of ligands could be considered a special case of polydentate ligands, because they have more than one atom capable of donating unshared pairs of electrons, however they are too small in size to be able to donate electrons with both atoms to at the same time, and instead they are linked in one way or another depending on the circumstances.

Within this group we find, for example, the anions thiocyanate (S=C=N-), nitrite (O=N-O-) and isothiocyanate (NC-S-)

Types of ligands according to hapticity

Hapticity of cyclopentadienyl ligands

Depending on the number of contiguous atoms of a ligand that is coordinated to a central atom, we can have different degrees of hapticity, denoted by the Greek letter η. When it is only linked through one atom, we are dealing with a monodentate ligand whose hapticity is equal to 1, and it is not denoted as ¹η. For example, butadiene can have hapticity η² or η⁴ depending on the number of carbon atoms being formed. bond to the central atom.

Types of ligands according to the type of bond

The ligands of coordination complexes can be of several types, depending on their external nature:

π-donor σ-donor ligands

They are very electronegative ligands, hard bases, with great electronic density around the nucleus. They are characterized by stabilizing mainly transition metals in high oxidation states. The metal in a high oxidation state has empty d orbitals, so it can accept the π interaction offered by the ligand, forming very stable complexes. Some examples of these ligands are halides, oxo, and in general highly electronegative elements.

Donor σ-ligands

With σ-donor capacity exclusively. They are characterized by stabilizing the most stable oxidation state of the first transition series. For the 2nd and 3rd series they are not very important, since they participate little in the stabilization or destabilization of the molecule. For example: ligand aquo (H₂O), ammonia (NH₃) and amines in general (NR₃)^{[citation required]}

Donor-σ and acceptor-π ligands

They are ligands that stabilize low oxidation states. Being low, the metal has lost few electrons, so it still has electrons in its valence d orbitals to give Π interaction to the ligand. The metal and ligand must have proper symmetry for stabilization. The most characteristic are carbon monoxide (CO), phosphines (PR₃), nitrogen (N₂) and cyanides.^{[ citation required]}

Complex π

They are a special case of σ-donor π-acceptor ligands. They stabilize low oxidation states, but the π interaction does not occur with the metal, but with a cloud of electron density π of the ligand, that is, with an excess electron density of the molecule, such as the cloud electron that is formed by having a molecule with π bonds. For example: ethylene and other olefins.^{[citation needed]}