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{{Short description|Ion or molecule bound to a metal atom}}
{{About|ligands in inorganic chemistry|ligands in biochemistry|Ligand (biochemistry)|other uses|Ligand (disambiguation)}}
{{Use dmy dates|date=June 2022}}
[[File:HCo(CO)4-3D-balls.png|thumb|200px|Cobalt complex [[cobalt tetracarbonyl hydride|HCo(CO)<sub>4</sub>]] with five ligands]]

In [[coordination chemistry]], a '''ligand'''{{refn|group=lower-alpha|The word ''ligand'' comes from Latin ''[[wikt:ligare#Latin|ligare]]'', to bind/tie. It is pronounced either {{IPAc-en|ˈ|l|aɪ|g|ə|n|d}} or {{IPAc-en|ˈ|l|ɪ|g|ə|n|d}}; both are very common.}} 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 [[electron pair]]s, often through [[Lewis acids and bases|Lewis bases]].<ref>Burdge, J., & Overby, J. (2020). ''Chemistry – Atoms first'' (4th ed.). New York: McGraw Hill. ISBN 978-1260571349</ref> The nature of metal–ligand bonding can range from [[covalent bond|covalent]] to [[ionic bond|ionic]]. Furthermore, the metal–ligand [[bond order]] can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve [[Lewis acids and bases|Lewis acid]]ic "ligands".<ref>{{cite book |last=Cotton |first=Frank Albert |url=https://archive.org/details/advancedinorgani0000unse/page/1355/mode/2up |title=Advanced Inorganic Chemistry |author2=Geoffrey Wilkinson |author3=Carlos A. Murillo |publisher=Wiley-Interscience |year=1999 |isbn=978-0471199571 |pages=1355 |url-access=registration}}</ref><ref>{{cite book |title= Inorganic Chemistry |publisher= Prentice Hall |last= Miessler |first= Gary L. |author2= Paul J. Fischer |author3= Donald Arthur Tarr |year= 2013 |pages= 696 |isbn= 978-0321811059}}</ref>

Metals and metalloids are bound to ligands in almost all circumstances, although gaseous "naked" metal ions can be generated in a high vacuum. Ligands in a complex dictate the [[reactivity (chemistry)|reactivity]] of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and [[redox]]. Ligand selection requires critical consideration in many practical areas, including [[bioinorganic chemistry|bioinorganic]] and [[medicinal chemistry]], [[homogeneous catalysis]], and [[environmental chemistry]].

Ligands are classified in many ways, including: charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal ([[denticity]] or [[hapticity]]). The size of a ligand is indicated by its [[ligand cone angle|cone angle]].

==History==
The composition of [[coordination complex]]es have been known since the early 1800s, such as [[Prussian blue]] and [[Copper(II) sulfate|copper vitriol]]. The key breakthrough occurred when [[Alfred Werner]] reconciled formulas and [[isomer]]s. He showed, among other things, that the formulas of many cobalt(III) and chromium(III) compounds can be understood if the metal has six ligands in an [[octahedral geometry]]. The first to use the term "ligand" were [[Alfred Werner]] and Carl Somiesky, in relation to silicon chemistry. The theory allows one to understand the difference between coordinated and ionic chloride in the cobalt [[ammine]] chlorides and to explain many of the previously inexplicable isomers. He resolved the first coordination complex called [[hexol]] into optical isomers, overthrowing the theory that [[chirality (chemistry)|chirality]] was necessarily associated with carbon compounds.<ref>{{Cite journal
| doi= 10.1021/ic040042e
| volume= 43
| issue= 20
| pages= 6249–6254
| last= Jackson
| first= W. Gregory
|author2=Josephine A. McKeon |author3=Silvia Cortez
| title= Alfred Werner's Inorganic Counterparts of Racemic and Mesomeric Tartaric Acid: A Milestone Revisited
| journal= Inorganic Chemistry
| date= 1 October 2004
| pmid= 15446870
}}</ref><ref>{{Cite journal|last=Bowman-James|first=Kristin|author-link=Kristin Bowman-James|year=2005|title=Alfred Werner Revisited: The Coordination Chemistry of Anions|journal=Accounts of Chemical Research|volume=38|issue=8|pages=671–678|doi=10.1021/ar040071t|pmid=16104690}}</ref>

==Strong field and weak field ligands==
{{Main|Crystal field theory}}

In general, ligands are viewed as electron donors and the metals as electron acceptors, i.e., respectively, [[Lewis base]]s and [[Lewis acid]]s. This description has been semi-quantified in many ways, e.g. [[ECW model]]. Bonding is often described using the formalisms of molecular orbital theory.<ref>{{cite book |title=Basic Principles of Ligand Field Theory|author=Hans Ludwig Schläfer and Günter Gliemann|year=1969|publisher=Wiley-Interscience|isbn=0471761001|place=London}}</ref><ref>{{cite book|title=Inorganic Chemistry|edition=5|first1=Gary|last1=Miessler|first2=Paul J.|last2=Fischer|first3=Donald A.|last3=Tarr|year=2014| publisher=Pearson|isbn=978-0321811059}}</ref>

Ligands and metal ions can be ordered in many ways; one ranking system focuses on ligand 'hardness' (see also [[HSAB theory|hard/soft acid/base theory]]). Metal ions preferentially bind certain ligands. In general, 'hard' metal ions prefer weak field ligands, whereas 'soft' metal ions prefer strong field ligands. According to the molecular orbital theory, the HOMO (Highest Occupied Molecular Orbital) of the ligand should have an energy that overlaps with the LUMO (Lowest Unoccupied Molecular Orbital) of the metal preferential. Metal ions bound to strong-field ligands follow the [[Aufbau principle]], whereas complexes bound to weak-field ligands follow [[Hund's rule]].

Binding of the metal with the ligands results in a set of molecular orbitals, where the metal can be identified with a new HOMO and LUMO (the orbitals defining the properties and reactivity of the resulting complex) and a certain ordering of the 5 d-orbitals (which may be filled, or partially filled with electrons). In an [[octahedral]] environment, the 5 otherwise degenerate d-orbitals split in sets of 3 and 2 orbitals (for a more in-depth explanation, see [[crystal field theory]]):

*3 orbitals of low energy: d''<sub>xy</sub>'', d''<sub>xz</sub>'' and d''<sub>yz</sub>'' and
*2 orbitals of high energy: d<sub>''z''<sup>2</sup></sub> and d<sub>''x''<sup>2</sup>−''y''<sup>2</sup></sub>.

The energy difference between these 2 sets of d-orbitals is called the splitting parameter, Δ<sub>o</sub>. The magnitude of Δ<sub>o</sub> is determined by the field-strength of the ligand: strong field ligands, by definition, increase Δ<sub>o</sub> more than weak field ligands. Ligands can now be sorted according to the magnitude of Δ<sub>o</sub> (see the table [[#Examples of common ligands (by field strength)|below]]). This ordering of ligands is almost invariable for all metal ions and is called [[spectrochemical series]].

For complexes with a tetrahedral surrounding, the d-orbitals again split into two sets, but this time in reverse order:
*2 orbitals of low energy: d<sub>''z''<sup>2</sup></sub> and d<sub>''x''<sup>2</sup>−''y''<sup>2</sup></sub> and
*3 orbitals of high energy: d<sub>''xy''</sub>, d<sub>''xz''</sub> and d<sub>''yz''</sub>.
The energy difference between these 2 sets of d-orbitals is now called Δ<sub>t</sub>. The magnitude of Δ<sub>t</sub> is smaller than for Δ<sub>o</sub>, because in a tetrahedral complex only 4 ligands influence the d-orbitals, whereas in an octahedral complex the d-orbitals are influenced by 6 ligands. When the [[coordination number]] is neither octahedral nor tetrahedral, the splitting becomes correspondingly more complex. For the purposes of ranking ligands, however, the properties of the octahedral complexes and the resulting Δ<sub>o</sub> has been of primary interest.

The arrangement of the d-orbitals on the central atom (as determined by the 'strength' of the ligand), has a strong effect on virtually all the properties of the resulting complexes. E.g., the energy differences in the d-orbitals has a strong effect in the optical absorption spectra of metal complexes. It turns out that valence electrons occupying orbitals with significant 3 d-orbital character absorb in the 400–800&nbsp;nm region of the [[spectrum]] (UV–visible range). The absorption of light (what we perceive as the [[color]]) by these electrons (that is, excitation of electrons from one orbital to another orbital under influence of light) can be correlated to the [[ground state]] of the metal complex, which reflects the bonding properties of the ligands. The relative change in (relative) energy of the d-orbitals as a function of the field-strength of the ligands is described in [[Tanabe–Sugano diagram]]s.

In cases where the ligand has low energy LUMO, such orbitals also participate in the bonding. The metal–ligand bond can be further stabilised by a formal donation of [[electron density]] back to the ligand in a process known as ''[[back-bonding]].'' In this case a filled, central-atom-based orbital donates density into the LUMO of the (coordinated) ligand. Carbon monoxide is the preeminent example a ligand that engages metals via back-donation. Complementarily, ligands with low-energy filled orbitals of pi-symmetry can serve as pi-donor.

[[File:Metal-EDTA.svg|thumb|200px|Metal–[[EDTA]] complex, wherein the aminocarboxylate is a hexadentate (chelating) ligand ]]
[[File:CoA6Cl3.svg|thumb|200px|Cobalt(III) complex containing six [[ammonia]] ligands, which are monodentate. The chloride is not a ligand.]]

==Classification of ligands as L and X==
{{Main|Covalent bond classification method}}
Ligands are classified according to the number of electrons that they "donate" to the metal. L ligands are [[Lewis base]]s. L ligands are represented by [[amine]]s, [[phosphine]]s, [[carbon monoxide|CO]], N<sub>2</sub>, and [[alkene]]s.  Examples of L ligands extend to include [[dihydrogen]] and hydrocarbons that interact by [[agostic interaction]]s. X ligands are halides and [[pseudohalide]]s. X ligands typically are derived from anionic precursors such as chloride but includes ligands where salts of anion do not really exist such as hydride and alkyl.<ref>{{Cite journal |last=Rasmussen |first=Seth C. |date=2015-03-05 |title=The 18-electron rule and electron counting in transition metal compounds: theory and application |journal=ChemTexts |language=en |volume=1 |issue=1 |article-number=10 |doi=10.1007/s40828-015-0010-4 |issn=2199-3793|doi-access=free |bibcode=2015ChTxt...1...10R }}</ref><ref>{{cite book|title=Organometallics|author=C. Elschenbroich|publisher=VCH|year=2006|isbn=  978-3-527-29390-2}}</ref>

Especially in the area of [[organometallic chemistry]], ligands are classified according to the "CBC Method" for Covalent Bond Classification, as popularized by [[Malcolm Green (chemist)|M. L. H. Green]] and "is based on the notion that there are three basic types [of ligands]... represented by the symbols L, X, and Z, which correspond respectively to 2-electron, 1-electron and 0-electron neutral ligands."<ref>{{Cite journal
| doi= 10.1016/0022-328X(95)00508-N
| issn= 0022-328X
| volume= 500
| issue= 1–2
| pages= 127–148
| last= Green
| first= M. L. H.
| title= A new approach to the formal classification of covalent compounds of the elements
| journal= Journal of Organometallic Chemistry
| date= 20 September 1995
}}</ref><ref>[http://www.columbia.edu/cu/chemistry/groups/parkin/mlxz.htm "mlxz plots – Columbia University"], Columbia University, New York.</ref>

==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 |url-access=subscription }}</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>

==Ligand motifs==
{{More citations needed section|date=January 2021}}

===Trans-spanning ligands===
{{Main|Trans-spanning ligand}}

In coordination chemistry, a trans-spanning ligand is a bidentate ligand that can span coordination positions on opposite sides of a coordination complex. This means that the two attachment points of the ligand are located on opposite sides of the metal center, forming a linear or near-linear arrangement. The trans-spanning ligand bridges the two coordination positions, providing stability to the complex.<ref>von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. {{ISBN|047195599X}}.</ref>

===Ambidentate ligand===
{{Main|Linkage isomerism}}

In contrast to polydentate ligands, ambidentate ligands can attach to the central atom in either one of two (or more) places, but not both. An example is [[thiocyanate]], SCN<sup>−</sup>, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to [[linkage isomerism]].

Polydentate and ambidentate are therefore two different types of polyfunctional ligands (ligands with more than one [[functional group]]) which can bond to a metal center through different ligand atoms to form various isomers. Polydentate ligands can bond through one atom AND another (or several others) at the same time, whereas ambidentate ligands bond through one atom OR another. Proteins are complex examples of polyfunctional ligands, usually polydentate.

===Bridging ligand===
{{Main|Bridging ligand}}

A bridging ligand links two or more metal centers. Virtually all inorganic solids with simple formulas are [[coordination polymer]]s, consisting of metal ion centres linked by bridging ligands. This group of materials includes all anhydrous binary metal ion halides and pseudohalides. Bridging ligands can also persist in solution. Polyatomic ligands such as [[carbonate]] are ambidentate and thus are found to often bind to two or three metals simultaneously. Atoms that bridge metals are sometimes indicated with the prefix "[[mu (letter)|''μ'']]". Most inorganic solids are polymers by virtue of the presence of multiple bridging ligands. Bridging ligands, capable of coordinating multiple metal ions, have been attracting considerable interest because of their potential use as building blocks for the fabrication of functional multimetallic assemblies.<ref>Sauvage,
J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994, 94, 993-1019</ref> Ligands such as halides which may not be bridging in solution can become bridging in the solid state due to the energetic favorability of filling the metal's first coordination sphere in the solid state. While in solution, open coordination sites in the coordination sphere of a metal complex are often filled by molecules of coordinating solvents such as water, acetonitrile, or tetrahydrofuran. 

===Binucleating ligand===
{{Main|Binucleating ligand}}

Binucleating ligands bind two metal ions.<ref>Gavrilova, A. L.; Bosnich, B., "Principles of Mononucleating and Binucleating Ligand Design", Chem. Rev. 2004, volume 104, 349–383. {{doi|10.1021/cr020604g}}</ref> Usually binucleating ligands feature bridging ligands, such as phenoxide, pyrazolate, or pyrazine, as well as other donor groups that bind to only one of the two metal ions.

===Metal–ligand multiple bond===
{{Main|Metal–ligand multiple bond}}

Some ligands can bond to a metal center through the same atom but with a different number of [[lone pair]]s. The [[bond order]] of the metal ligand bond can be in part distinguished through the metal ligand [[bond angle]] (M−X−R). This bond angle is often referred to as being linear or bent with further discussion concerning the degree to which the angle is bent. For example, an imido ligand in the ionic form has three lone pairs. One lone pair is used as a sigma X donor, the other two lone pairs are available as L-type pi donors. If both lone pairs are used in pi bonds then the M−N−R geometry is linear. However, if one or both these lone pairs is nonbonding then the M−N−R bond is bent and the extent of the bend speaks to how much pi bonding there may be. ''η''<sup>1</sup>-Nitric oxide can coordinate to a metal center in linear or bent manner.

===Spectator ligand===
{{Main|Spectator ligand}}

A spectator ligand is a tightly coordinating polydentate ligand that does not participate in chemical reactions but removes active sites on a metal. Spectator ligands influence the reactivity of the metal center to which they are bound.

===Bulky ligands===
{{Main|Ligand cone angle}}

Bulky ligands are used to control the steric properties of a metal center. They are used for many reasons, both practical and academic. On the practical side, they influence the selectivity of metal catalysts, e.g., in [[hydroformylation]]. Of academic interest, bulky ligands stabilize unusual coordination sites, e.g., reactive coligands or low coordination numbers. Often bulky ligands are employed to simulate the steric protection afforded by proteins to metal-containing active sites. Of course excessive steric bulk can prevent the coordination of certain ligands.
[[File:1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (aka IMes).png|thumb|left|220px|The [[N-heterocyclic carbene|''N''-heterocyclic carbene]] ligand called [[IMes]] is a bulky ligand by virtue of the pair of mesityl groups.]]

===Chiral ligands===
{{Main|Chiral ligand}}

Chiral ligands are useful for inducing asymmetry within the coordination sphere. Often the ligand is employed as an optically pure group. In some cases, such as secondary amines, the asymmetry arises upon coordination. Chiral ligands are used in [[homogeneous catalysis]], such as [[asymmetric hydrogenation]].

===Hemilabile ligands===
{{Main|Hemilability}}
Hemilabile ligands contain at least two electronically different coordinating groups and form complexes where one of these is easily displaced from the metal center while the other remains firmly bound, a behaviour which has been found to increase the reactivity of catalysts when compared to the use of more traditional ligands.

===Non-innocent ligand===
{{Main|Non-innocent ligand}}

Non-innocent ligands bond with metals in such a manner that the distribution of electron density between the metal center and ligand is unclear. Describing the bonding of non-innocent ligands often involves writing multiple [[resonance (chemistry)|resonance form]]s that have partial contributions to the overall state.

==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 || ||
|}

==Ligand exchange==
A '''ligand exchange''' (also called '''ligand substitution''') is a  [[chemical reaction]] in which a ligand in a compound is replaced by another. Two general mechanisms are recognized: [[associative substitution]] or by [[dissociative substitution]].
[[File:Ligand_association.jpg|left|thumb|A generalized example of ligand association]]
[[Associative substitution]] closely resembles the [[SN2 reaction|S<sub>N</sub>2]] mechanism in organic chemistry. A typically smaller ligand can attach to an unsaturated complex followed by loss of another ligand.  Typically, the rate of the substitution is first order in entering ligand L and the unsaturated complex.<ref name=":1">{{Cite book |last=Wilkins |first=Ralph G. |title=Kinetics and mechanism of reactions of transition metal complexes |date=1991 |publisher=VCH |isbn=978-1-56081-125-1 |edition=2. thoroughly rev. |location=Weinheim}}</ref>
[[File:Ligand_dissociation_figure.jpg|left|thumb|A generalized example of ligand dissociation]]

[[Dissociative substitution]] is common for octahedral complexes. This pathway closely resembles the [[SN1 reaction|S<sub>N</sub>1]] mechanism in organic chemistry.  The identity of the entering ligand does not affect the rate.<ref name=":1"/>

==Ligand–protein binding database==
BioLiP<ref>[http://zhanglab.ccmb.med.umich.edu/BioLiP/ BioLiP]</ref> is a comprehensive ligand–protein interaction database, with the 3D structure of the ligand–protein interactions taken from the [[Protein Data Bank]]. MANORAA is a webserver for analyzing conserved and differential molecular interaction of the ligand in complex with protein structure homologs from the Protein Data Bank. It provides the linkage to protein targets such as its location in the biochemical pathways, SNPs and protein/RNA baseline expression in target organ.<ref name="tanramluk">{{cite journal |vauthors=Tanramluk D, Naripiyakul L, Akavipat R, Gong S, Charoensawan V |title= MANORAA (Mapping Analogous Nuclei Onto Residue And Affinity) for identifying protein-ligand fragment interaction, pathways and SNPs |journal= Nucleic Acids Research |volume= 44 |issue= W1 |pages= W514-21 |year=2016 |doi= 10.1093/nar/gkw314 |pmid= 27131358 |pmc=4987895}}</ref>

==See also==
* [[Bridging carbonyl]]
* [[Crystal field theory]]
* [[DNA binding ligand]]
* [[Inorganic chemistry]]
* [[Josiphos ligands]]
* [[Ligand dependent pathway]]
* [[Ligand field theory]]
* [[Ligand isomerism]]
* [[Spectrochemical series]]

== Explanatory notes ==
{{Notelist}}

==References==
{{Reflist}}

==External links==
{{Wiktionary}}
{{Commons category}}
* See the modeling of ligand–receptor–ligand binding in Vu-Quoc, L., [http://clesm.mae.ufl.edu/wiki.pub/index.php/Configuration_integral_%28statistical_mechanics%29 Configuration integral (statistical mechanics)], 2008. This wiki site is down; see [https://web.archive.org/web/20120428193950/http://clesm.mae.ufl.edu/wiki.pub/index.php/Configuration_integral_%28statistical_mechanics%29 this article in the Internet Archive from 2012 April 28].

{{Authority control}}

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