# Denticity

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Number of atoms in a ligand that bond to the central atom of a coordination complex

Atom with
monodentate ligands

In [coordination chemistry](/source/Coordination_chemistry), **denticity** (from [Latin](/source/Latin_language) *dentis* 'tooth') refers to the number of [donor](/source/Electron_donor) groups in a given [ligand](/source/Ligand) that bind to the central metal atom in a [coordination complex](/source/Coordination_complex).[1][2] In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be **unidentate** or **monodentate**. Ligands with more than one bonded atom are called **multidentate** or **polydentate**. The denticity of a ligand is described with the Greek letter [κ](/source/Kappa_(letter)) ('kappa').[3] For example, κ6-[EDTA](/source/Ethylenediaminetetraacetic_acid#Coordination_chemistry_principles) describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Denticity is different from [hapticity](/source/Hapticity) because hapticity refers exclusively to ligands where the coordinating atoms are contiguous. In these cases the [η](/source/Eta_(letter)) ('eta') notation is used.[4] [Bridging ligands](/source/Bridging_ligand) use the [μ](/source/Mu_(letter)) ('mu') notation.[5][6]

## Classes

Relationship between "linear" bi-, tri- and tetradentate ligands (red) bound to an octahedral metal center.  The structures marked with * are chiral owing to the backbone of the tetradentate ligand.

Polydentate ligands are [chelating agents](/source/Chelating_agents)[7] and classified by their denticity. Some atoms cannot form the maximum possible number of bonds a ligand could make. In that case one or more [binding sites](/source/Binding_site) of the ligand are unused. Such sites can be used to form a bond with another [chemical species](/source/Chemical_species).

Structure of the pharmaceutical [Oxaliplatin](/source/Oxaliplatin), which features two different bidentate ligands.

- **Bidentate** (also called didentate) ligands bind with two atoms, an example being [ethylenediamine](/source/Ethylenediamine).

- [Tridentate ligands](/source/Tridentate_ligand) bind with three atoms, an example being [terpyridine](/source/Terpyridine). Tridentate ligands usually bind via two kinds of connectivity, called "mer" and "fac." "fac" stands for facial, the donor atoms are arranged on a triangle around one face of the octahedron. "mer" stands for meridian, where the donor atoms are stretched out around one half of the octahedron. Cyclic tridentate ligands such as [TACN](/source/1%2C4%2C7-Triazacyclononane) and [9-ane-S3](/source/1%2C4%2C7-Trithiacyclononane) bind in a facial manner.

- **Quadridentate** or [tetradentate ligands](/source/Tetradentate_ligand) bind with four donor atoms, an example being [triethylenetetramine](/source/Triethylenetetramine) (abbreviated trien). For different central metal geometries there can be different numbers of isomers depending on the ligand's topology and the geometry of the metal center. For octahedral metals, the linear tetradentate trien can bind via three geometries. Tripodal tetradentate ligands, e.g. [tris(2-aminoethyl)amine](/source/Tris(2-aminoethyl)amine), are more constrained, and on octahedra leave two cis sites (adjacent to each other). Many naturally occurring [macrocyclic](/source/Macrocycle) ligands are tetradentative, an example being the [porphyrin](/source/Porphyrin) in [heme](/source/Heme). On an octahedral metal these leave two vacant sites opposite each other.

- **Quinquidentate** or [pentadentate ligands](/source/Pentadentate_ligand) bind with five atoms, an example being [ethylenediaminetriacetic acid](/source/Ethylenediaminetriacetic_acid).

- **Sexidentate** or [hexadentate ligands](/source/Hexadentate_ligand) bind with six atoms, an example being [EDTA](/source/EDTA) (although it can bind in a tetradentate manner).

### High denticity ligands

 [Structure](/source/X-ray_crystallography) of the ytterbium complex of DTPA5-. Like related lanthanide complexes, the DTPA wraps around the metal ion as a octadentate (κ8-) ligand.  Water is also bound to Yb3+, giving it a [coordination number](/source/Coordination_number) of nine.[8] Color code: red = O, blue = N, green = Yb

Larger ions, such as the [lanthanides](/source/Lanthanide), Ca2+, and Ba2+ prefer coordination numbers greater than 6. For firmly binding these ions, ligands of denticity greater than six are often used. One example is the [triaminopentacarboxylate](/source/Aminopolycarboxylic_acid) derived from [pentetic acid](/source/Pentetic_acid) (HO2CH2N(CH2N(CH2CO2H)2)2. A related ligand is [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate](/source/1%2C4%2C7%2C10-tetraazacyclododecane-1%2C4%2C7%2C10-tetraacetate) (DOTA).

## Stability constants

In general, the stability of a metal complex correlates with the denticity of the ligands, which can be attributed to the [chelate effect](/source/Chelation#Chelate_effect). Polydentate ligands such as hexa- or octadentate ligands tend to bind metal ions more strongly than ligands of lower denticity, primarily due to entropic factors. [Stability constants](/source/Stability_constants_of_complexes) are a quantitative measure to assess the thermodynamic stability of coordination complexes.

## See also

- [Chelate](/source/Chelate)

## References

1. **[^](#cite_ref-1)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[denticity](https://goldbook.iupac.org/terms/view/D01594.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.D01594](https://doi.org/10.1351%2Fgoldbook.D01594)

1. **[^](#cite_ref-2)** von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. [ISBN](/source/ISBN_(identifier)) [047195599X](https://en.wikipedia.org/wiki/Special:BookSources/047195599X).

1. **[^](#cite_ref-3)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[κ (kappa) *in inorganic nomenclature*](https://goldbook.iupac.org/terms/view/K03366.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.K03366](https://doi.org/10.1351%2Fgoldbook.K03366)

1. **[^](#cite_ref-4)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[η (eta or hapto) in inorganic nomenclature](https://goldbook.iupac.org/terms/view/H01881.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.H01881](https://doi.org/10.1351%2Fgoldbook.H01881)

1. **[^](#cite_ref-5)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[bridging ligand](https://goldbook.iupac.org/terms/view/B00741.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.B00741](https://doi.org/10.1351%2Fgoldbook.B00741)

1. **[^](#cite_ref-6)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[µ- (mu) *in inorganic nomenclature*](https://goldbook.iupac.org/terms/view/M03659.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.M03659](https://doi.org/10.1351%2Fgoldbook.M03659)

1. **[^](#cite_ref-7)** [IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry), *[Compendium of Chemical Terminology](/source/IUPAC_books#Gold_Book)*, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "[chelation](https://goldbook.iupac.org/terms/view/C01012.html)". [doi](/source/Doi_(identifier)):[10.1351/goldbook.C01012](https://doi.org/10.1351%2Fgoldbook.C01012)

1. **[^](#cite_ref-8)** Hardcastle, Kenneth Irvin; Botta, Mauro; Fasano, Mauro; Digilio, Giuseppe (2000). "Experimental Evidence for a Second Coordination Sphere Water Molecule in the Hydration Structure of YbDTPA – Insights for a Re-Assessment of the Relaxivity Data of GdDTPA". *European Journal of Inorganic Chemistry* (5): 971. [doi](/source/Doi_(identifier)):[10.1002/(SICI)1099-0682(200005)2000:5<971::AID-EJIC971>3.3.CO;2-A](https://doi.org/10.1002%2F%28SICI%291099-0682%28200005%292000%3A5%3C971%3A%3AAID-EJIC971%3E3.3.CO%3B2-A).

## External links

- [EDTA chelation lecture notes.](https://web.archive.org/web/20100622050028/http://people.chem.byu.edu/dvd/chem223/lecture_notes/11_EDTA_Chelation.pdf/at_download/file) 2.4MB PDF - Slide 3 on denticity

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