
In
coordination chemistry
A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...
, denticity () refers to the number of
donor groups in a given
ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
that bind to the central metal atom in a
coordination complex
A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...
. 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 ('kappa'). For example, κ
6-
EDTA
Ethylenediaminetetraacetic acid (EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-solubl ...
describes an EDTA ligand that coordinates through 6 non-contiguous atoms.
Denticity is different from
hapticity because hapticity refers exclusively to ligands where the coordinating atoms are contiguous. In these cases the
η ('eta') notation is used.
Bridging ligands use the
μ ('mu') notation.
Classes
Polydentate ligands are
chelating agents 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 site
In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may includ ...
s of the ligand are unused. Such sites can be used to form a bond with another
chemical species.
* Bidentate (also called didentate) ligands bind with two atoms, an example being
ethylenediamine
Ethylenediamine (abbreviated as en when a ligand) is the organic compound with the formula C2H4(NH2)2. This colorless liquid with an ammonia-like odor is a basic amine. It is a widely used building block in chemical synthesis, with approximately ...
.
:
*
Tridentate ligands bind with three atoms, an example being
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 and
9-ane-S3 bind in a facial manner.
* Quadridentate or
tetradentate ligands bind with four donor atoms, an example being
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
Tris(2-aminoethyl)amine is the organic compound with the chemical formula, formula N(CH2CH2NH2)3. This colourless liquid is soluble in water and is highly basic, consisting of a tertiary amine center and three pendant primary amine groups. Tris(2- ...
, are more constrained, and on octahedra leave two cis sites (adjacent to each other). Many naturally occurring
macrocyclic ligands are tetradentative, an example being the
porphyrin in
heme
Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...
. On an octahedral metal these leave two vacant sites opposite each other.
* Quin(qui)dentate or
pentadentate ligands bind with five atoms, an example being ethylenediaminetriacetic acid.
* Sexidentate or
hexadentate ligands bind with six atoms, an example being
EDTA
Ethylenediaminetetraacetic acid (EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-solubl ...
(although it can bind in a tetradentate manner).
High denticity ligands

Larger ions, such as the
lanthanides, Ca
2+, and Ba
2+ 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 derived from
pentetic acid (. A related ligand is
1,4,7,10-tetraazacyclododecane-1,4,7,10-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. 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 are a quantitative measure to assess the thermodynamic stability of coordination complexes.
See also
*
Chelate
External links
EDTA chelation lecture notes. 2.4MB PDF - Slide 3 on denticity
References
{{reflist
Coordination chemistry
Chemical bonding
Chelating agents