ligands

In the presence of ligands or chelating groups, a metal ion bonds with them, resulting in complex ions / coordination compounds. In these complexes, a central atom, usually a transition metal, is surrounded by a cluster of ions or neutral molecules. Ligands are neutral molecules or ions that form a chemical connection with the central metal atom or ion. The core atom is a Lewis acid, and the ligands are Lewis bases . At least one donor atom with an electron pair is present in these ligands, which is used to form covalent bonds with the core atom.

Alfred Stock initially coined the term ligand concerning silicon chemistry in 1916. The term ligand originates from the latin word ligare, which means “to bind.” Anions, cations, and neutral substances can all be used as ligands. Ligands are further classified as monodentate, bidentate, tridentate, and so on, where the concept of teeth (dent) is introduced, resulting in ideas such as bite angle and hapticity. 

Coordination number

In a complex or coordination compound or a crystal, the Ligancy, also known as the coordination number, stands for the number of ions, atoms, or molecules that a central ion or atom has as its nearest neighbors. In the coordination complexes [Mo(CN)8]4- and [Sr(H2O)8]2+, the metal atom has coordination number 8, 7 in the complex [ZrF7]3-, 4 in the complexes [Cu(CN)4]3- and [Ni(CN)4]4-, 2 in the complexes [Ag(NH3)2]+ and [AuCl2]. In complexes, coordination numbers ranging from 2 to 9 have been recorded, more significant coordination numbers, though rare, have also been observed. The coordination number of an ion or atom is not always the same; for example, Al3+ has coordination number 4 in [AlCl4]- but 6 in [AlF6]3-. It varies in accordance with the complex it is present in.

Denticity

The number of donor sites of a given ligand is defined as denticity. This peculiar yet enticing feature comes from the concepts of ligancy and has a very predominant importance in studying behavioral aspects of a coordination compound and thus plays an essential role in learning the legacy of the compound. Many ligands can secure metal ions at several locations, mainly due to lone pairs of electrons on multiple atoms. Chelating ligands are those ligands that bind to more than one atom; monodentate ligands bind to one site; bidentate ligands combine to two sites; while tridentate ligands attach to three different sites. The angle created by the two bonds of a bidentate chelate is known as the “bite angle.” These ligands are usually created by joining donor groups together by the help of organic linkers.

(I) Monodentate ligands

The term “monodentate” means “one tooth,” referring to the ligand that binds to the center with only one atom, i.e. having ligancy 1. Chloride ions (‘color as a ligand), water ( ‘aqua’ as a ligand), hydroxide ions (‘hydroxo’ as a ligand), and ammonia(‘ammine’ as a ligand) are all instances of monodentate ligands.

(II) Bidentate ligands

Bidentate ligands have two donor atoms and have ligancy 2, allowing them to attach to a central metal atom or ion from two different directions. Ethylenediamine (en) and the oxalate ion are two examples of bidentate ligands (ox). The nitrogen (blue) atoms on the margins of the diagram below each have two free electrons that can attach to an intermediate metal atom or ion.

(III) Polydentate ligands

The number of atoms employed to connect to a core metal atom/ ion has different features across polydentate ligands, they are unique. EDTA contains six donor atoms with different sets of electron pairs connecting to a central metal atom/ ion. The legacy of a metal ion is the number of bonds it has. Each bond contributes to the coordination number in polydentate ligands. The ligancy  of two bidentate ligands is four. The hexadentate ligand ethylenediaminetetraacetic acid (EDTA) can attach to a metal via several “teeth.” Ambidentate ligands, unlike polydentate ligands, can connect to the central atom in two different ways. Thiocyanate, SCN, is an excellent illustration of this, as it can connect to either the sulfur or nitrogen atom.

Hapticity

Hapticity is defined as the number of the same ligands attached to a central metal ion. The coordination of a ligand to a metal center through an unbroken and contiguous series of atoms is called hapticity. Furthermore, if the ligand coordinates through numerous non-contiguous atoms, this is referred to as denticity (rather than hapticity).

Chelation

Chelation is a chemical process that occurs when a polydentate ligand forms a ring with a metal ion. A chelating agent is a chelate produced by this method, and the polydentate ligand is a chelating agent. Chelating ligands have a higher affinity for metal ions than monodentate (single tooth) ligands, as the name indicates. Many more chelating compounds may be discovered in the inorganic laboratory, including ethylenediamine and ethylenediaminetetraacetic acid.Also chelating ligands provide more stability to the atom.

Chelate effect   

The chelate effect refers to chelating ligands’ increased affinity for a metal ion compared to a collection of similar non-chelating (monodentate) ligands’ association for the same metal; the ligancy is also considered in this concept.

Bridging ligand

A bridging ligand is one that joins more than two atoms, usually metal ions, in coordination chemistry. It does not matter if that ligand is atomic or polyatomic and so does the ligand. Because virtually any complex organic compound can act as a bridging ligand, the word is mainly applied to small ligands like pseudohalides or ligands expressly engineered to connect two metals.

Conclusion

The peculiar yet beautiful features of a chemical compound to its affinity towards binding and reacting to a specific compound are of paramount virtues as it lets the world of chemistry grow into multifaceted terms and helps it cater to newer understandings.