View source for Chemistry:Denticity

{{Short description|Number of atoms in a ligand that bond to the central atom of a coordination complex}}
[[File:Hexaaquasodium-3D-balls.png|thumb|220px|Atom with<br />monodentate ligands]]

In coordination chemistry, '''denticity''' ({{ety|la|dentis|tooth}}) refers to the number of [[Chemistry:Electron donor|donor]] groups in a given [[Chemistry:Ligand|ligand]] that bind to the central metal atom in a [[Chemistry:Coordination complex|coordination complex]].<ref>{{GoldBookRef | file = D01594 | title = denticity}}</ref><ref>von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. {{ISBN|047195599X}}.</ref>  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 '''monodentate''' (sometimes called '''unidentate'''). Ligands with more than one bonded atom are called '''polydentate''' or '''multidentate'''. The denticity of a ligand is described with the Greek letter κ ('kappa').<ref>{{GoldBookRef |title=κ (kappa) ''in inorganic nomenclature'' |file=K03366 }}</ref>  For example, κ<sup>6</sup>-[[Chemistry:Ethylenediaminetetraacetic acid#Coordination chemistry principles|EDTA]] describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Denticity is different from [[Chemistry:Hapticity|hapticity]] because hapticity refers exclusively to ligands where the coordinating atoms are contiguous.  In these cases the η ('eta') notation is used.<ref>{{GoldBookRef |title=η (eta or hapto) in inorganic nomenclature |file=H01881 }}</ref>  [[Chemistry:Bridging ligand|Bridging ligand]]s use the μ ('mu') notation.<ref>{{GoldBookRef |title=bridging ligand |file=B00741}}</ref><ref>{{GoldBookRef |title=µ- (mu) ''in inorganic nomenclature'' |file=M03659 }}</ref>

== Classes ==

Polydentate ligands are chelating agents<ref>{{GoldBookRef | file = C01012 | title = chelation}}</ref> 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 [[Biology:Binding site|binding site]]s of the ligand are unused. Such sites can be used to form a bond with another [[Chemistry:Chemical species|chemical species]].
* Bidentate (also called didentate) ligands bind with two atoms, an example being [[Chemistry:Ethylenediamine|ethylenediamine]].
:[[Image:Oxaliplatin.svg|thumb|220px|right|Structure of the pharmaceutical [[Chemistry:Oxaliplatin|Oxaliplatin]], which features two different bidentate ligands.]]
* [[Chemistry:Tridentate ligand|Tridentate ligand]]s bind with three atoms, an example being [[Chemistry:Terpyridine|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 [[Chemistry:1,4,7-Triazacyclononane|TACN]] and [[Chemistry:1,4,7-Trithiacyclononane|9-ane-S3]] bind in a facial manner.
* [[Chemistry:Tetradentate ligand|Tetradentate ligand]]s bind with four donor atoms, an example being [[Chemistry:Triethylenetetramine|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. [[Chemistry:Tris(2-aminoethyl)amine|tris(2-aminoethyl)amine]], are more constrained, and on octahedra leave two cis sites (adjacent to each other).  Many naturally occurring [[Chemistry:Macrocycle|macrocyclic]] ligands are tetradentative, an example being the [[Biology:Porphyrin|porphyrin]] in [[Biology:Heme|heme]]. On an octahedral metal these leave two vacant sites opposite each other.
* Pentadentate ligands bind with five atoms, an example being ethylenediaminetriacetic acid.
* [[Chemistry:Hexadentate ligand|Hexadentate ligand]]s bind with six atoms, an example being [[Chemistry:EDTA|EDTA]] (although it can bind in a tetradentate manner).
* Ligands of denticity greater than 6 are well known.  The ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) and diethylene triamine pentaacetate (DTPA) are octadentate.  They are particularly useful for binding lanthanide ions, which typically have coordination numbers greater than 6.
[[Image:Linear2-4Chelate.png|thumb|center|460px|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.]]

==Stability constants==
In general, the stability of a metal complex correlates with the denticity of the ligands, which can be attributed to the [[Chemistry:Chelation#Chelate effect|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. [[Chemistry:Stability constants of complexes|Stability constants]] are a quantitative measure to assess the thermodynamic stability of coordination complexes.

==See also==
* Chelate

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

==References==
{{reflist}}

[[Category:Coordination chemistry]]
[[Category:Chemical bonding]]
[[Category:Chelating agents]]

{{Sourceattribution|Denticity}}