Ligand
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{{Dablink|For other uses, see [[Ligand (disambiguation)]]. For biochemical uses in particular see [[Ligand (biochemistry)]].}}
In [[chemistry]], a '''ligand''' is either an [[atom]], [[ion]], or [[molecule]] (see also: [[functional group]]) that bonds to a central metal, generally involving formal donation of one or more of its [[electron]]s. The metal-ligand bonding ranges from [[covalent bond|covalent]] to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as [[Lewis base]]s, although rare cases are known involving [[Lewis acid]]ic "ligands."<ref>Cotton, F. A. and Wilkinson, G., ''Advanced Inorganic Chemistry'', John Wiley and Sons: New York, 1988. ISBN 9780471199575</ref>
[[Metal]] and [[metalloid]]s are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in high vacuum. Ligands in a complex dictate the [[reactivity]] of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and [[redox]]. Ligand selection is a critical consideration in many practical areas, including [[bioinorganic chemistry|bioinorganic]] and [[medicinal chemistry]], [[homogeneous catalysis]], and [[environmental chemistry]].
Ligands are classified in many ways: their charge, size (bulk), the identity of the coordinating atom(s), and their denticity. The size of a ligand is indicated by its [[Ligand cone angle|cone angle]].
==Inner- vs out-sphere ligands==
In [[coordination chemistry]], the ligands that are directly bonded to the metal (that is, share electrons), are sometimes called "inner sphere" ligands. "Outer-sphere" ligands are not directly attached to the metal, but are bonded, generally weakly, to the first coordination shell, affecting the inner sphere in subtle ways. The complex of the metal with the inner sphere ligands is then called a coordination complex, which can be neutral, cationic, or [[anionic]]). The complex, along with its [[counter ion]]s (if required), is called a [[coordination compound]].
[[Image:Medta.png|thumb|left|120px|Metal-[[EDTA]] complex, wherein the aminocarboxylate is a hexadentate chelating ligand. ]]
[[Image:CoA6Cl3.png|thumb|left|120px|Cobalt(III) complex containing six [[ammonia]] ligands, which are monodentate. The chloride is not a ligand.]]
== Strong field and weak field ligands ==
{{main|Crystal field theory}}
In general, ligands are viewed as donating electrons to the central atom. Bonding is often described using the formalisms of molecular orbital theory. In general, [[Lone electron pair|electron pairs]] occupy the HOMO of the ligands.
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. From a [[Molecular orbital theory|MO]] point of view, the [[HOMO/LUMO|HOMO]] of the ligand should have an energy that makes overlap with the LUMO 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 2 and 3 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>''
::2 of high energy: ''d''<sub>''z''<sup>2</sup></sub> and ''d''<sub>''x''<sup>2</sup><nowiki>−</nowiki>''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 [[Ligand#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><nowiki>−</nowiki>''y''<sup>2</sup></sub>
::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 3d-orbital character absorb in the 400-800 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.
==Polydentate and polyhapto ligand motifs and nomenclature==
Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have [[lone pair]]s on more than one atom. 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. The classic ''bi''dentate 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 ''poly''dentate ligand is the hexadentate chelating agent [[EDTA]], which is able to bond through six sites, completely surrounding some metals. The number of atoms with which a polydentate ligand bind to the metal centre is called its denticity, symbolized κ<sup>n</sup>, where n indicates the number non-contiguous donor sites by which a ligand attaches to a metal. EDTA<sup>4<nowiki>−</nowiki></sup>, when it is sexidentate, 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]].
Related to but distinct to from denticity is [[hapticity]], symbolized η or eta. Hapticity refers to the number of ''contiguous'' atoms in a ligand that are attached to a metal. [[Butadiene]] forms both η<sup>2</sup> and η<sup>4</sup> complexes depending on the number of carbon atoms are bonded to the metal. To simplify matters, η<sup>n</sup> usually refers to unsaturated hydrocarbons and κ<sup>n</sup> usually to describe polydentate amine and carboxylate ligands.
Complexes of polydentate ligands are called ''chelate'' complexes. They tend to be more stable than complexes derived from [[monodentate]] ligands. This enhanced stability, the [[chelate effect]], is usually attributed to effects of [[entropy]], which favors the displacement of many ligands by one polydentate ligand. When the chelating ligand forms a large ring 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 a good example: the [[iron]] atom is at the centre of a [[porphyrin]] macrocycle, being bound to four nitrogen atoms of the tetrapyrrole macrocycle. The very stable dimethylglyoximate complex of nickel is a synthetic macrocycle derived from the anion of [[dimethylglyoxime]].
== Single atom bonding motifs ==
=== Ambidentate ligand ===
Unlike polydentate ligands, ambidentate ligands can attach to the central atom in two places but not both. A good example of this is [[thiocyanate]], SCN<sup><nowiki>−</nowiki></sup>, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to [[linkage isomerism]]. Polyfunctional ligands, see especially proteins, can bond to a metal center through different ligand atoms to form various isomers.
=== Bridging ligand ===
[[Bridging ligand]] link two or more metal centers. Polyatomic ligands such as [[Carbonate|CO<sub>2</sub><sup>2-</sup>]] are especially prone to bridge. The bonding is complicated because polyatomic ligands are ambidentate and thus the capacity for many different [[linkage isomers]]. Atoms that bridge metals are sometimes indicated with prefix of "μ" (mu). Most inorganic solids, e.g. FeCl<sub>2</sub>, are polymers by virtue of the presence of multiple bridging ligands.
=== Metal ligand multiple bond ===
[[Metal ligand multiple bond]]s 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 non-bonding 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.
==Specialized ligand types==
=== Noninnocent ligand ===
[[Noninnocent ligand]]s 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 noninnocent ligands often involves writing multiple [[Resonance (chemistry)|resonance form]]s which have partial contributions to the overall state.
=== Trans spanning ligand ===
Trans spanning ligands are bidentate ligands that can span opposite sites of a complex with square-planar geometry. A wide variety of ligands that chelate in the cis fashion already exist, but very few can link opposite verices on a coordination polyhedron. Early attempts to generate trans-spanning bidentate ligands relied on polymethylene chains to link the donor functionalities, but such ligands lead to [[coordination polymer]]s.
A diphosphane linked with pentamethylene was claimed to span across a square planare complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(I1) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph<sub>2</sub>PCH<sub>2</sub>) groups at the 1 and 11 positions.<ref>N. J. DeStefano, D. K. Johnson, R. M. Lane, L. M. Venanzi "Transition-Metal Complexes with Bidentate Ligands Spanning trans-Positions. I. The synthesis of 2,11-bis(diphenylphosphinomethyl)benzo[c]-phenanthrene, a ligand promoting the formation of square planar complexes" Helvetica Chimica Acta 1976, volume 59, pp. 2674-2682. {{DOI|10.1002/hlca.19760590806}}</ref><ref>Mochida, J. A. Mattern, J. C. Bailar Jr., J. (1975) “Stereochemistry of Complex Inorganic Compounds.XXXV. A Complex Containing a Ligand That Spans Trans Positions” J. Am. Chem. Soc., volume 97, 3021–3026. {{DOI|10.1021/ja00844a017}}.</ref> The polycyclic framework suffers sterically clashing hydrogen centers. XANTHOS is a more reliable trans-spanning ligand.<sup>[2]</sup> without the steric problems associated with TRANSPHOS. SPANPHOS is comparable to XANTHOS.
Subsequent to the reports on SPANPHOS and related ligands was a genuine trans-spanning ligand reported, one that would form neither bimetallic nor oligomeric complexes with certain transition metals, and strictly function as a trans-chelator This ligand, TRANSDIP, represented the first trans-spanning ligand to give exclusively chelating complexes, even when reacted with d8 metal ion halides.<sup>[2]</sup> TRANSDIP is based on a α-[[cyclodextrin]].<ref>L. Poorters, D. Armspach, M. Dominique, L. Toupet, S. Choua, P. Turek (2007) “Synthesis and properties of TRANSDIP, a rigid chelator built upon a cyclodextrin cavity: is TRANSDIP an authentic trans - spanning ligand?” Chemistry, A European Journal, volume 13, pp. 9448-9461. {{DOI|10.1002/chem.200700831}}</ref>
==Common ligands==
:''See [[Complex (chemistry)#Naming complexes|nomenclature]].''
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 [[water]] are particularly common charge-neutral ligands. Simple organic species are also very common, be they anionic ([[alkoxide|RO<sup><nowiki>−</nowiki></sup>]] and [[Carboxylate|RCO<sub>2</sub><sup><nowiki>−</nowiki></sup>]]) or neutral ([[Ether|R<sub>2</sub>O]], [[Thioether|R<sub>2</sub>S]], [[amine|R<sub>3<nowiki>−</nowiki>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 π-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 (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><nowiki>−</nowiki></sup> || monoanionic || [[monodentate]] ||
|-
| [[Bromide]] bromo|| '''Br'''<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate ||
|-
| [[Sulfide]] thio or bridging thiolate|| '''S'''<sup>2<nowiki>−</nowiki></sup> || dianionic || monodentate (M=S), or bidentate bridging (M-S-M') ||
|-
| [[Thiocyanate]] thiocyanato|| '''S'''-CN<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || ambidentate (see also isothiocyanate, below)
|-
| [[Chloride]] chloro|| '''Cl'''<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || also found bridging
|-
| [[Nitrate]] || '''O'''-NO<sub>2</sub><sup><nowiki>−</nowiki></sup> || monoanionic || monodentate ||
|-
| [[Azide]] || '''N'''-N<sub>2</sub><sup><nowiki>−</nowiki></sup> || monoanionic || monodentate ||
|-
| [[Fluoride]] fluoro|| '''F'''<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate ||
|-
| [[Hydroxide]] hydroxo|| '''O'''-H<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || often found as a bridging ligand
|-
| [[Oxalate]] || ['''O'''-C(=O)-C(=O)'''-O''']<sup>2<nowiki>−</nowiki></sup> || dianionic || bidentate ||
|-
| [[Water]] aqua || H-'''O'''-H || neutral || monodentate || monodentate
|-
| [[Isothiocyanate]] isothiocyanato|| '''N'''=C=S<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || ambidentate (see also thiocyanate, above)
|-
| [[Acetonitrile]] || CH<sub>3</sub>C'''N''' || neutral || monodentate ||
|-
| [[Pyridine]] || C<sub>5</sub>H<sub>5</sub>'''N''' || neutral || monodentate ||
|-
| [[Ammonia]] ammine|| '''N'''H<sub>3</sub> || neutral || monodentate ||
|-
| [[Ethylenediamine]] || en || neutral || bidentate ||
|-
| [[2,2'-Bipyridine]] || bipy || neutral || bidentate || easily reduced to its (radical) anion or even to its dianion
|-
| 1,10-[[Phenanthroline]] || phen || neutral || bidentate ||
|-
| [[Nitrite]] nitro || '''N'''-O<sub>2</sub><sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || ambidentate (see also nitrito)
|-
| [[Nitrite]] nitrito|| '''O'''-N-O<sup><nowiki>−</nowiki></sup> || monoanionic || monodentate || ambidentate (see also nitro)
|-
| [[Triphenylphosphine]] || '''P'''Ph<sub>3</sub> || neutral || monodentate ||
|-
| [[Cyanide]] cyano|| '''C'''N<sup><nowiki>−</nowiki></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 || neutral || monodentate || can bridge between metals (both metals bound to C)
|}
Note: 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 non-linear fashion).
=== Other general encountered ligands (alphabetical) ===
In this table other common ligands are listed in alphabetical order.
{| class="wikitable" style="margin: 1em auto 1em auto 1em auto 1em auto 1em auto"
! Ligand || formula (bonding atom(s) in bold) || Charge || Most common denticity || Remark(s)
|-
| [[Acetylacetone|Acetylacetonate]] (Acac)|| CH<sub>3</sub>-C('''O''')-CH-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
|-
| [[Benzene]] || '''C'''<sub>6</sub>H<sub>6</sub> || neutral || || and other arenes
|-
| [[1,2-Bis(diphenylphosphino)ethane]] (dppe) || Ph<sub>2</sub>'''P'''C<sub>2</sub>H<sub>4</sub>'''P'''Ph<sub>2</sub> || neutral || bidentate ||
|-
| [[Corrole]]s || || || tetradentate ||
|-
| [[Crown ether]]s || || neutral || || primarily for alkali and alkaline earth metal cations
|-
| [[Cryptand|2,2,2-crypt]] || || || hexadentate || primarily for alkali and alkaline earth metal cations
|-
| [[Cryptate]]s || || neutral || ||
|-
| [[Cyclopentadienyl complex|Cyclopentadienyl]] || [C<sub>5</sub>H<sub>5</sub>]<sup><nowiki>−</nowiki></sup> || monoanionic || ||
|-
| [[Diethylenetriamine]] (dien) || || neutral || tridentate || related to TACN, but not constrained to facial complexation
|-
| [[Dimethylglyoxime|Dimethylglyoximate]] (dmgH<sup><nowiki>−</nowiki></sup>) || || monoanionic || ||
|-
| [[EDTA|Ethylenediaminetetraacetate]] (EDTA) || || tetra-anionic || hexadentate || actual ligand is the tetra-anion
|-
| Ethylenediaminetriacetate || || trianionic || pentadentate || actual ligand is the trianion
|-
| [[Glycine|glycinate]] || || || bidentate || other α-amino acid anions are comparable (but chiral)
|-
| [[Heme]] || || dianionic || tetradentate || macrocyclic ligand
|-
| [[Nitrosyl]] || '''N'''O<sup>+</sup> || cationic || || bent (1e) and linear (3e) bonding mode
|-
| [[Scorpionate ligand]] || || || tridentate ||
|-
| [[Sulfite]] || || monoanionic || monodentate || ambidentate
|-
| 2,2',5',2''-[[Terpyridine]] (terpy) || || neutral || tridentate || meridional bonding only
|-
| [[Thiocyanate]] || || monoanionic || monodentate || ambidentate, sometimes bridging
|-
| [[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 || (C<sub>6</sub>H<sub>11</sub>)<sub>3</sub>P or (PCy<sub>3</sub>) || neutral || monodentate ||
|-
| [[Triethylenetetramine]] (trien) || || neutral || tetradentate ||
|-
| Tri(''o''-tolyl)phosphine || P(''o''-tolyl)<sub>3</sub> || neutral || monodentate ||
|-
|-
| [[Tris(2-aminoethyl)amine]] (tren) || || neutral || tetradentate ||
|-
| Tris(2-diphenylphosphineethyl)amine (np<sub>3</sub>) || || neutral || tetradentate ||
|-
|[[Terpyridine]] || || neutral || tridentate ||
|}
==See also==
*[[2-Mercaptoindole]]
*[[Crystal field theory]]
*[[Ligand field theory]]
*[[Coordination chemistry]]
*[[Inorganic chemistry]]
*[[Radioligand]]
*[[Tanabe-Sugano diagram]]
*[[Spectrochemical series]]
*[[Scatchard equation]]
==References==
{{reflist}}
[[Category:Coordination chemistry]]
[[Category:Chemical bonding]]
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