Denticity

Denticity can be defined as the number of atoms in a single ligand that connect to a central atom. It has been observed that in multiple scenarios, just one atom in the ligand links to the metal, leading in a monodentate ligand with a denticity of one. More than one atom is bonded in polydentate or multidentate ligands. Denticity is derived from the Latin word dentis, which means tooth. The ligand is thought to bite the metal at one or more connection points. Denticity contrasts from hapticity in that the central metal is connected to the electrons of a bond or covalently linked sequence of links rather than being segregated to a single ligand atom.

Talking about the denticity study material, it differs from hapticity in that hapticity refers solely to ligands with continuous coordinating atoms. The (‘eta’) notation is used in these circumstances. The (‘mu’) notation is used to represent bridging ligands.

Classes of Denticity

Chelating compounds known as polydentate ligands are characterised according to their denticity. Some atoms cannot form the maximum number of bonds that a ligand can make. In this instance, one or more of the ligand’s binding sites are left unoccupied and can be used to build a chemical interaction with another species.

Ethylenediamine is an example of a bivalent (also known as didentate) ligand that binds with two atoms.

The ethylenediamine molecule contains a total of 12 atom(s). There are 8 Hydrogen atom(s), 2 Carbon atom(s) and 2 Nitrogen atom(s). A chemical formula of ethylenediamine can therefore be written as:

The chemical formula of Ethylenediamine is C2H8N2.

Terpyridine is an example of a tridentate ligand, which binds with three atoms. The two types of connection used by tridentate ligands are known as “mer” and “fac.” The donor atoms are grouped in a triangle around one face of the octahedron, hence the name “fac.”  The donor atoms tend to spread out around one-half of the octahedron, which is known as a meridian. TACN and 9-ane-S3 are cyclic tridentate ligands that bind facially.

Triethylenetetramine, for example, is a tetradentate ligand that binds with four donor atoms (abbreviated trien). Depending on the topology of the ligand and the geometry of the metal centre, different numbers of isomers can exist for different central metal geometries. The linear tetradentate trien may bind to octahedral metals in three different geometries. 

Tripodal tetradentate ligands, such as tris(2-aminoethyl) amine, are more restricted, leaving two cis sites on octahedra (adjacent to each other). The porphyrin in heme is an example of a naturally occurring tetradentate macrocyclic ligand. These leave two empty spots opposite each other on an octahedral metal.

Ethylenediaminetriacetic acid is an example of a pentadentate ligand, which binds with five atoms.

EDTA is an example of a hexadentate ligand, which binds with six atoms (although it can bind in a tetradentate manner).

Stability constants

The chelate effect can be linked to the fact that the durability of a metal complex correlates with the denticity of the ligands. Due to entropic considerations, polydentate ligands such as hexadentate or octadentate ligands bind metal ions more strongly than ligands with lower denticity. Stability constants are a numerical measure of a coordination complex’s thermodynamic stability.

Chelation effect

In the study material notes on denticity, we will also learn about chelation. It refers to the bonding between ions or other substances and the core atom. It requires the development of one or more coordination bonds between the ligands and the single central metal atom. The complex chemical that results has a ring-like structure.

• A chelating ligand can form a ring-like structure with the central atom.
• Chelating ligands include all polydentate ligands.
• Chelated complexes are more stable than monodentate ligand complexes because dissociation of chelated complexes entails the breaking of two bonds rather than one.
• Because it forms a three-membered ring, NH2NH2 cannot operate as a chelating ligand.

Based on bonding interaction

Ligands are divided into two groups based on the bonding contact between the ligand and the central atom.

1. Classical or simple donor ligands: These ligands only provide the centre atom with a single pair of electrons.

Example: O₂-, OH-, F-, NH₂-, NH₃, N₃-, etc.

2. Non-classical ligands: These ligands contribute to the central atom’s lone pair of electrons and accept the central atom’s electron cloud in their low-lying empty orbitals. Synergic bonding is the term for this return donation of electrons.

Things to remember

• Coordination compounds comprise two parts: the core atom and the ligands that surround it.
• Lewis bases are ligands.
• The centre atom forms Lewis acid.
• Ligands are categorised based on three characteristics: charge, denticity and bonding interaction.
• Ligands are classified as neutral, positive or negative based on their charge.
• Ligands are classified into six categories based on their denticity: monodentate, bidentate, tridentate, tetradentate, pentadentate and hexadentate.
• Ligands are classified into two types based on the bonding contact between the ligand and the central atom: classical and non-classical.

Conclusion

The denticity of the ligands directly impacts the stability of a metal ion complex. Under the denticity study material, you will find that the chelate effect occurs when ligands with a greater form of denticity are stabler than those with lower denticity. Chelating chemicals’ stability can be linked to better entropy shifts, as most chelating compounds can form stable ring structures.