Editing Ligand (section)

==Polydentate and polyhapto ligand motifs and nomenclature==
===Denticity===
{{Main|Denticity|chelate}}

Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have [[lone pair]]s on more than one atom. Such ligands are polydentate.<ref>{{Greenwood&Earnshaw2nd|page=906}}</ref> Ligands that bind via more than one atom are often termed ''[[chelation|chelating]]''. A ligand that binds through two sites is classified as ''[[bidentate]]'', and three sites as ''[[tridentate]]''. The "[[bite angle]]" refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers. A classic bidentate ligand is [[ethylenediamine]], which is derived by the linking of two ammonia groups with an ethylene (−CH<sub>2</sub>CH<sub>2</sub>−) linker. A classic example of a polydentate ligand is the [[hexadentate]] chelating agent [[EDTA]], which is able to bond through six sites, completely surrounding some metals. The number of times a polydentate ligand binds to a metal centre is symbolized by "''κ<sup>n</sup>''", where ''n'' indicates the number of sites by which a ligand attaches to a metal. EDTA<sup>4−</sup>, when it is hexidentate, binds as a ''κ''<sup>6</sup>-ligand, the amines and the carboxylate oxygen atoms are not contiguous. In practice, the n value of a ligand is not indicated explicitly but rather assumed. The binding affinity of a chelating system depends on the chelating angle or [[bite angle]].

Denticity (represented by ''[[Kappa|κ]]'') is nomenclature that described to the number of noncontiguous atoms of a ligand bonded to a metal.  This descriptor is often omitted because the denticity of a ligand is often obvious.  The complex [[tris(ethylenediamine)cobalt(III)]] could be described as [Co(κ<sup>2</sup>-en)<sub>3</sub>]<sup>3+</sup>.

Complexes of polydentate ligands are called ''chelate'' complexes. They tend to be more stable than complexes derived from [[Denticity|monodentate]] ligands. This enhanced stability, called the [[Chelation#Chelate effect|''chelate effect'']], is usually attributed to effects of [[entropy]], which favors the displacement of many ligands by one polydentate ligand.

Related to the chelate effect is the [[Stability constants of complexes#The macrocyclic effect|macrocyclic effect]]. A macrocyclic ligand is any large ligand that at least partially surrounds the central atom and bonds to it, leaving the central atom at the centre of a large ring. The more rigid and the higher its denticity, the more inert will be the macrocyclic complex. [[Heme]] is an example, in which the [[iron]] atom is at the centre of a [[porphyrin]] macrocycle, bound to four nitrogen atoms of the tetrapyrrole macrocycle. The very stable [[Nickel bis(dimethylglyoximate)|dimethylglyoximate complex of nickel]] is a synthetic macrocycle derived from [[dimethylglyoxime]].

=== Hapticity ===
{{Main|Hapticity}}

'''Hapticity''' (represented by Greek letter ''[[Eta (letter)|η]]'') refers to the number of ''contiguous'' atoms that comprise a donor site and attach to a metal center. The ''η-notation'' applies when multiple atoms are coordinated. For example, ''η''<sup>2</sup> is a ligand that coordinates through two contiguous atoms. [[Butadiene]] forms both ''η''<sup>2</sup> and ''η''<sup>4</sup> complexes depending on the number of carbon atoms that are bonded to the metal.<ref>{{Cite journal |last=Chemistry (IUPAC) |first=The International Union of Pure and Applied |title=IUPAC - η (eta or hapto) (H01881) |url=https://goldbook.iupac.org/terms/view/H01881 |access-date=2023-11-08 |website=goldbook.iupac.org |doi=10.1351/goldbook.h01881|doi-access=free }}</ref><ref>{{Cite journal |last=Chemistry (IUPAC) |first=The International Union of Pure and Applied |title=IUPAC - denticity (D01594) |url=https://goldbook.iupac.org/terms/view/D01594 |access-date=2023-11-08 |website=goldbook.iupac.org |doi=10.1351/goldbook.d01594|doi-access=free }}</ref><ref>{{Cite book |last=Hartwig |first=John Frederick |title=Organotransition metal chemistry: from bonding to catalysis |date=2010 |publisher=University science books |isbn=978-1-891389-53-5 |location=Sausalito (Calif.)}}</ref>