Editing Ligand (section)

==Common ligands==
{{See also|Complex (chemistry)#Naming complexes}}
{{More citations needed section|date=January 2021}}

Virtually every molecule and every ion can serve as a ligand for (or "coordinate to") metals. Monodentate ligands include virtually all anions and all simple Lewis bases. Thus, the [[halide]]s and [[pseudohalide]]s are important anionic ligands whereas [[ammonia]], [[carbon monoxide]], and [[properties of water|water]] are particularly common charge-neutral ligands. Simple organic species are also very common, be they anionic ([[alkoxide|RO<sup>−</sup>]] and [[Carboxylate|{{chem|RCO|2|−}}]]) or neutral ([[Ether|R<sub>2</sub>O]], [[Thioether|R<sub>2</sub>S]], [[amine|R<sub>3−''x''</sub>NH<sub>''x''</sub>]], and [[phosphine|R<sub>3</sub>P]]). The steric properties of some ligands are evaluated in terms of their [[cone angle]]s.

Beyond the classical Lewis bases and anions, all unsaturated molecules are also ligands, utilizing their pi electrons in forming the coordinate bond. Also, metals can bind to the σ bonds in for example [[silane]]s, [[hydrocarbon]]s, and [[dihydrogen]] (see also: [[Agostic interaction]]).

In complexes of [[non-innocent ligand]]s, the ligand is bonded to metals via conventional bonds, but the ligand is also redox-active.

===Examples of common ligands (by field strength)===
<!-- This section is linked from [[Ligand]] -->
In the following table the ligands are sorted by field strength{{citation needed|date=July 2022}} (weak field ligands first):

{| class="wikitable" style="margin: 1em auto 1em auto 1em auto 1em auto"
! Ligand || formula (bonding atom(s) in bold) || Charge || Most common denticity || Remark(s)
|-
| [[Iodide]] (iodo)|| '''I'''<sup>−</sup> || [[wikt:monoanionic|monoanionic]] || [[denticity|monodentate]] ||
|-
| [[Bromide]] (bromido)|| '''Br'''<sup>−</sup> || monoanionic || monodentate ||
|-
| [[Sulfide]] (thio or less commonly "bridging thiolate")|| '''S'''<sup>2−</sup> || dianionic || monodentate (M=S), or bidentate bridging (M−S−M') ||
|-
| [[Thiocyanate]] (''S''-thiocyanato)|| '''S'''−CN<sup>−</sup> || monoanionic || monodentate || ambidentate (see also isothiocyanate, below)
|-
| [[Chloride]] (chlorido)|| '''Cl'''<sup>−</sup> || monoanionic || monodentate || also found bridging
|-
| [[Nitrate]] (nitrato) || '''O'''−{{chem|NO|2|−}} || monoanionic || monodentate ||
|-
| [[Azide]] (azido) || '''N'''−{{chem|N|2|−}} || monoanionic || monodentate || Very Toxic
|-
| [[Fluoride]] (fluoro) || '''F'''<sup>−</sup> || monoanionic || monodentate ||
|-
| [[Hydroxide]] (hydroxido)|| '''O'''−H<sup>−</sup> || monoanionic || monodentate || often found as a bridging ligand
|-
| [[Oxalate]] (oxalato) || ['''O'''−CO−CO−'''O''']<sup>2−</sup> || dianionic || bidentate ||
|-
| [[Properties of water|Water]] (aqua) || '''O'''−H<sub>2</sub> || neutral || monodentate ||
|-
| [[Nitrite]] (nitrito)|| '''O'''−N−O<sup>−</sup> || monoanionic || monodentate || ambidentate (see also nitro)
|-
| [[Isothiocyanate]] (isothiocyanato)|| '''N'''=C=S<sup>−</sup> || monoanionic || monodentate || ambidentate (see also thiocyanate, above)
|-
| [[Acetonitrile]] (acetonitrilo) || CH<sub>3</sub>C'''N''' || neutral || monodentate ||
|-
| [[Pyridine]] (py) || C<sub>5</sub>H<sub>5</sub>'''N''' || neutral || monodentate ||
|-
| [[Ammonia]] (ammine or less commonly "ammino")|| '''N'''H<sub>3</sub> || neutral || monodentate ||
|-
| [[Ethylenediamine]] (en) || '''N'''H<sub>2</sub>−CH<sub>2</sub>−CH<sub>2</sub>−'''N'''H<sub>2</sub> || neutral || bidentate ||
|-
| [[2,2'-Bipyridine]] (bipy) || '''N'''C<sub>5</sub>H<sub>4</sub>−C<sub>5</sub>H<sub>4</sub>'''N'''|| neutral || bidentate || easily reduced to its (radical) anion or even to its dianion
|-
| [[Phenanthroline|1,10-Phenanthroline]] (phen) || C<sub>12</sub>H<sub>8</sub>'''N<sub>2</sub>''' || neutral || bidentate ||
|-
| [[Nitrite]] (nitro)|| '''N'''−{{chem|O|2|−}} || monoanionic || monodentate || ambidentate (see also nitrito)
|-
| [[Triphenylphosphine]] || '''P'''−(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub> || neutral || monodentate ||
|-
| [[Cyanide]] (cyano)|| '''C'''≡N<sup>−</sup><br/>'''N'''≡C<sup>−</sup> || monoanionic || monodentate || can bridge between metals (both metals bound to C, or one to C and one to N)
|-
| [[Carbon monoxide]] (carbonyl) || –'''C'''O, others || neutral || monodentate || can bridge between metals (both metals bound to C)
|}

The entries in the table are sorted by field strength, binding through the stated atom (i.e. as a terminal ligand). The 'strength' of the ligand changes when the ligand binds in an alternative binding mode (e.g., when it bridges between metals) or when the conformation of the ligand gets distorted (e.g., a linear ligand that is forced through steric interactions to bind in a nonlinear fashion).

===Other generally encountered ligands (alphabetical)===
In this table other common ligands are listed in alphabetical order.

{| class="wikitable sortable"
! Ligand !! Formula (bonding atom(s) in bold) !! Charge !! Most common denticity !! Remark(s)
|-
| [[Acetylacetone|Acetylacetonate]] (acac)|| CH<sub>3</sub>−C'''O'''−CH<sub>2</sub>−C'''O'''−CH<sub>3</sub> || monoanionic || bidentate || In general bidentate, bound through both oxygens, but sometimes bound through the central carbon only,<br/> see also analogous ketimine analogues
|-
| [[Alkene]]s || R<sub>2</sub>'''C=C'''R<sub>2</sub> || neutral || || compounds with a C−C double bond
|-
| [[Aminopolycarboxylic acid]]s (APCAs) || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| [[BAPTA]] (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| [[Benzene]] || '''C<sub>6</sub>'''H<sub>6</sub> || neutral || || and other arenes
|-
| [[1,2-Bis(diphenylphosphino)ethane]] (dppe) || (C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>'''P'''−C<sub>2</sub>H<sub>4</sub>−'''P'''(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub> || neutral || bidentate ||
|-
| [[1,1-Bis(diphenylphosphino)methane]] (dppm) || (C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>'''P'''−CH<sub>2</sub>−'''P'''(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub> || neutral || || Can bond to two metal atoms at once, forming dimers
|-
| [[Corrole]]s || || || tetradentate ||
|-
| [[Crown ether]]s || || neutral || || primarily for alkali and alkaline earth metal cations
|-
| [[Cryptand|2,2,2-cryptand]] || || || hexadentate || primarily for alkali and alkaline earth metal cations
|-
| [[Cryptate]]s || || neutral || ||
|-
| [[Cyclopentadienyl complex|Cyclopentadienyl]] (Cp) || {{chem|'''C'''|'''5'''|H|5|−}} || monoanionic || || Although monoanionic, by the nature of its occupied molecular orbitals, it is capable of acting as a tridentate ligand.
|-
| [[Diethylenetriamine]] (dien) || C<sub>4</sub>H<sub>13</sub>'''N<sub>3</sub>''' || neutral || tridentate || related to TACN, but not constrained to facial complexation
|-
| [[Dimethylglyoxime|Dimethylglyoximate]] (dmgH<sup>−</sup>) || || monoanionic || ||
|-
| [[DOTA (chelator)|1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid]] (DOTA) || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| Diethylenetriaminepentaacetic acid (DTPA) ([[pentetic acid]]) || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| [[Ethylenediaminetetraacetic acid]] (EDTA) (edta<sup>4−</sup>) || (<sup>−</sup>O'''O'''C−CH<sub>2</sub>)<sub>2</sub>'''N'''−C<sub>2</sub>H<sub>4</sub>−'''N'''(CH<sub>2</sub>-C'''O'''O<sup>−</sup>)<sub>2</sub> || tetraanionic || hexadentate ||
|-
| Ethylenediaminetriacetate || <sup>−</sup>O'''O'''C−CH<sub>2</sub>'''N'''H−C<sub>2</sub>H<sub>4</sub>−'''N'''(CH<sub>2</sub>-C'''O'''O<sup>−</sup>)<sub>2</sub> || trianionic || pentadentate ||
|-
| [[EGTA (chemical)|Ethyleneglycolbis(oxyethylenenitrilo)tetraacetate]] (egta<sup>4−</sup>) || (<sup>−</sup>O'''O'''C−CH<sub>2</sub>)<sub>2</sub>'''N'''−C<sub>2</sub>H<sub>4</sub>−'''O'''−C<sub>2</sub>H<sub>4</sub>−'''O'''−C<sub>2</sub>H<sub>4</sub>−'''N'''(CH<sub>2</sub>−C'''O'''O<sup>−</sup>)<sub>2</sub> || tetraanionic || octodentate ||
|-
| [[Fura-2]] || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| [[Glycine|Glycinate]] (glycinato) || '''N'''H<sub>2</sub>CH<sub>2</sub>C'''O'''O<sup>−</sup> || monoanionic || bidentate || other α-amino acid anions are comparable (but chiral)
|-
| [[Heme]] || || dianionic || tetradentate || macrocyclic ligand
|-
| [[Iminodiacetic acid]] (IDA) || &nbsp; || &nbsp; || tridentate || Used extensively to make [[radioactive tracer|radiotracers]] for [[scintigraphy]] by complexing the metastable radionuclide [[technetium-99m]]. For example, in [[cholescintigraphy]], HIDA, BrIDA, PIPIDA, and DISIDA are used
|-
| [[Nicotianamine]] || &nbsp; || &nbsp; || &nbsp; || Ubiquitous in higher [[plant]]s
|-
| [[Nitrosyl]] || '''N'''O<sup>+</sup> || cationic || || bent (1e<sup>−</sup>) and linear (3e<sup>−</sup>) bonding mode
|-
| [[Nitrilotriacetic acid]] (NTA) || &nbsp; || &nbsp; || &nbsp; || &nbsp;
|-
| [[Oxo (ligand)|Oxo]] || '''O'''<sup>2−</sup> || dianion || monodentate || sometimes bridging
|-
| [[Pyrazine]] || '''N'''<sub>2</sub>C<sub>4</sub>H<sub>4</sub> || neutral || ditopic || sometimes bridging
|-
| [[Scorpionate ligand]] || || || tridentate ||
|-
| [[Sulfite]] || '''O'''−{{chem|SO|2|2−}}<br/>'''S'''−{{chem|O|3|2−}} || monoanionic || monodentate || ambidentate
|-
| [[Terpyridine|2,2';6',2″-Terpyridine]] (terpy) || '''N'''C<sub>5</sub>H<sub>4</sub>−C<sub>5</sub>H<sub>3</sub>'''N'''−C<sub>5</sub>H<sub>4</sub>'''N''' || neutral || tridentate || meridional bonding only
|-
| [[Triazacyclononane]] (tacn) || (C<sub>2</sub>H<sub>4</sub>)<sub>3</sub>('''N'''R)<sub>3</sub> || neutral || tridentate || macrocyclic ligand<br/> see also the ''N'',''N''′,''N''″-trimethylated analogue
|-
| [[Tricyclohexylphosphine]] || '''P'''(C<sub>6</sub>H<sub>11</sub>)<sub>3</sub> or '''P'''Cy<sub>3</sub> || neutral || monodentate ||
|-
| [[Triethylenetetramine]] (trien) || C<sub>6</sub>H<sub>18</sub>'''N<sub>4</sub>''' || neutral || tetradentate ||
|-
| [[Trimethylphosphine]] || '''P'''(CH<sub>3</sub>)<sub>3</sub> || neutral || monodentate ||
|-
| [[Tris(o-tolyl)phosphine]] || '''P'''(''o''-tolyl)<sub>3</sub> || neutral || monodentate ||
|-
| [[Tris(2-aminoethyl)amine]] (tren) || ('''N'''H<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>'''N''' || neutral || tetradentate ||
|-
| Tris(2-diphenylphosphineethyl)amine (np<sub>3</sub>) || || neutral || tetradentate ||
|-
|[[Tropylium]] || {{chem|'''C'''|'''7'''|H|7|+}} || cationic || ||
|-
|[[Carbon dioxide]] || –'''C'''O<sub>2</sub>, others || neutral || || see [[metal carbon dioxide complex]]
|-
|[[Phosphorus trifluoride]] (trifluorophosphorus) || –'''P'''F<sub>3</sub> || neutral || ||
|}