{{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 [[Electron donor|donor]] groups in a given [[ligand]] that bind to the central metal atom in a [[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 '''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 [[kappa (letter)|κ]] ('kappa').<ref>{{GoldBookRef |title=κ (kappa) ''in inorganic nomenclature'' |file=K03366 }}</ref> For example, κ<sup>6</sup>-[[Ethylenediaminetetraacetic_acid#Coordination_chemistry_principles|EDTA]] 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 [[eta (letter)|η]] ('eta') notation is used.<ref>{{GoldBookRef |title=η (eta or hapto) in inorganic nomenclature |file=H01881 }}</ref> [[Bridging ligand]]s use the [[mu (letter)|μ]] ('mu') notation.<ref>{{GoldBookRef |title=bridging ligand |file=B00741}}</ref><ref>{{GoldBookRef |title=µ- (mu) ''in inorganic nomenclature'' |file=M03659 }}</ref>

== Classes == [[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.]] 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 [[binding site]]s of the ligand are unused. Such sites can be used to form a bond with another [[chemical species]]. [[Image:Oxaliplatin.svg|thumb|220px|right|Structure of the pharmaceutical [[Oxaliplatin]], which features two different bidentate ligands.]] * '''Bidentate''' (also called didentate) ligands bind with two atoms, an example being [[ethylenediamine]]. * [[Tridentate ligand]]s 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 [[1,4,7-Triazacyclononane|TACN]] and [[1,4,7-Trithiacyclononane|9-ane-S3]] bind in a facial manner. * '''Quadridentate''' or [[tetradentate ligand]]s 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]], are more constrained, and on octahedra leave two cis sites (adjacent to each other). Many naturally occurring [[macrocycle|macrocyclic]] ligands are tetradentative, an example being the [[porphyrin]] in [[heme]]. On an octahedral metal these leave two vacant sites opposite each other. * '''Quinquidentate''' or [[pentadentate ligand]]s bind with five atoms, an example being [[ethylenediaminetriacetic acid]]. * '''Sexidentate''' or [[hexadentate ligand]]s bind with six atoms, an example being [[EDTA]] (although it can bind in a tetradentate manner).

===High denticity ligands=== [[File:CSD CIF KOLGIB.png|thumb| [[X-ray crystallography|Structure]] of the ytterbium complex of DTPA<sup>5-</sup>. Like related lanthanide complexes, the DTPA wraps around the metal ion as a octadentate (κ<sup>8</sup>-) ligand. Water is also bound to Yb<sup>3+</sup>, giving it a [[coordination number]] of nine.<ref>{{cite journal |doi=10.1002/(SICI)1099-0682(200005)2000:5<971::AID-EJIC971>3.3.CO;2-A |title=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 |date=2000 |last1=Hardcastle |first1=Kenneth Irvin |last2=Botta |first2=Mauro |last3=Fasano |first3=Mauro |last4=Digilio |first4=Giuseppe |journal=European Journal of Inorganic Chemistry |issue=5 |page=971 }}</ref> Color code: red = O, blue = N, green = Yb|left]] Larger ions, such as the [[lanthanide]]s, Ca<sup>2+</sup>, and Ba<sup>2+</sup> prefer coordination numbers greater than 6. For firmly binding these ions, ligands of denticity greater than six are often used. One example is the [[aminopolycarboxylic acid|triaminopentacarboxylate]] derived from [[pentetic acid]] ({{chem2|HO2CH2N(CH2N(CH2CO2H)2)2}}. 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 [[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. [[Stability constants of complexes|Stability constants]] are a quantitative measure to assess the thermodynamic stability of coordination complexes.

{{clear}} ==See also== * [[Chelate]]

==References== {{reflist}}

==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

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