All Topics  
Ligand

 

 
   Email Print
   Bookmark   Link
 

 

Ligand
 
 
  Topics Ligand Topic Home

In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a ligand is either an atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
, ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
, or molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
 (see also: functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
) that bonds to a central metal, generally involving formal donation of one or more of its electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s. The metal-ligand bonding ranges from covalent
Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds....
 to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acid
Lewis acid

A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB.Gilbert N....
ic "ligands."

Metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
 and metalloid
Metalloid

is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal....
s are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in high vacuum.






Discussion
Ask a question about 'Ligand'
Start a new discussion about 'Ligand'
Answer questions from other users
Full Discussion Forum



Encyclopedia


In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a ligand is either an atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
, ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
, or molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
 (see also: functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
) that bonds to a central metal, generally involving formal donation of one or more of its electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s. The metal-ligand bonding ranges from covalent
Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds....
 to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acid
Lewis acid

A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB.Gilbert N....
ic "ligands."

Metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
 and metalloid
Metalloid

is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal....
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
Reactivity

Reactivity refers to the Reaction rate at which a chemical substance tends to undergo a chemical reaction in time. In pure chemical compounds, reactivity is regulated by the physical properties of the sample....
 of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
. Ligand selection is a critical consideration in many practical areas, including bioinorganic
Bioinorganic chemistry

Bioinorganic chemistry is a specialized field that spans the chemistry of metal-containing molecules within biological systems. This field is concerned with the control and use of metal ions in biochemical processes....
 and medicinal chemistry
Medicinal chemistry

Medicinal or pharmaceutical chemistry is a discipline at the intersection of chemistry and pharmacology involved with drug design, organic synthesis and developing pharmaceutical medication....
, homogeneous catalysis
Homogeneous catalysis

Homogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in the same phase as the reactants. It is the opposite to heterogeneous catalysis....
, and environmental chemistry
Environmental chemistry

Environmental chemistry is the science of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source....
.

Ligands are classified in many ways: their charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal (denticity
Denticity

Denticity refers to the number of atoms in a single ligand that bind to a central metal in a Complex . In most cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate ....
 or hapticity
Hapticity

The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordination chemistry to a central atom. Hapticity of a ligand is indicated by the Greek language character 'eta', ?....
). The size of a ligand is indicated by its cone angle
Ligand cone angle

Ligand cone angle is a measure of the size of a ligand. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone ....
.

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
Anionic

In chemistry, an anionic species is one that contains a full electric charge. These types of chemical compound can range in reactivity, but most are fairly reactive.Anionic is a chemical that is also used in things like washing detergent.......
. The complex, along with its counter ions (if required), is called a coordination compound.

Strong field and weak field ligands

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, 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 hard soft acid base theory
HSAB theory

The HSAB concept is an acronym for 'hard and soft acids and base s'. Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of chemical compound, chemical reaction mechanisms and pathways....
). 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 MO
Molecular orbital theory

In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual chemical bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule....
 point of view, the HOMO
HOMO/LUMO

HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The difference of the energies of the HOMO and LUMO, termed the band gap, can sometimes serve as a measure of the excited state of the molecule: the smaller the energy, the more easily it will be exc...
 of the ligand should have an energy that overlaps with the LUMO of the metal preferential. Metal ions bound to strong-field ligands follow the Aufbau principle
Aufbau principle

The Aufbau principle is used to determine the electron configuration of an atom, molecule or ion. The principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons....
, 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
Crystal field theory

Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered complex ....
). 3 orbitals of low energy: dxy, dxz and dyz 2 of high energy: dz2 and dx2−y2

The energy difference between these 2 sets of d-orbitals is called the splitting parameter, ?o. The magnitude of ?o is determined by the field-strength of the ligand: strong field ligands, by definition, increase ?o more than weak field ligands. Ligands can now be sorted according to the magnitude of ?o (see the table below
Ligand

In chemistry, a ligand is either an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons....
). This ordering of ligands is almost invariable for all metal ions and is called spectrochemical series
Spectrochemical series

A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group and its identity....
.

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: dz2 and dx2−y2 3 orbitals of high energy: dxy, dxz and dyz The energy difference between these 2 sets of d-orbitals is now called ?t. The magnitude of ?t is smaller than for ?o, 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
Coordination number

The coordination number of an atom in a molecule or a crystal is the integer number of its nearest Neighbourhood . This number is determined somewhat differently for molecules and for crystals....
 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 ?o 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
Color

Color or colour is the visual perception property corresponding in humans to the categories called red, yellow, blue and others....
) 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
Tanabe-Sugano diagram

Tanabe-Sugano diagrams are used in coordination chemistry to predict absorption s in the UV and visible electromagnetic spectrum of coordination compounds....
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
Electron density

Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds....
 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


Denticity

Denticity (represented by ?
?

or is a letter derived from the Latin alphabet. Both glyphs of the majuscule and Lower case forms of this letter are based on the rotated form of a minuscule e; a similar letter with identical minuscule is used in the Pan-Nigerian Alphabet, but has the capital form majuscule , based on a horizontally flipped majuscule E....
) refers to the number of times a ligand bonds to a metal through non-contiguous donor sites. Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pair
Lone pair

A lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons....
s on more than one atom. Ligands that bind via more than one atom are often termed chelating
Chelation

Chelation is the binding or complex of a bi- or multidentate ligand. These ligands, which are often organic compounds, are called chelants, chelators, chelating agents, or sequestration....
. A ligand that binds through two sites is classified as bidentate, and three sites as tridentate. The "bite angle
Bite angle

Chelate bite angle is a geometric parameter used to classify chelating ligands in inorganic and organometallic chemistry. Together with ligand cone angle, this parameter is relevant to diphosphine ligands, which are used in industrial processes such as hydroformylation and hydrocyanation....
" 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 (-CH2CH2-) linker. A classic example of a polydentate ligand is the hexadentate chelating agent EDTA
EDTA

EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid . EDTA is a polyamino carboxylic acid with the chemical formula [CH2N2]2....
, which is able to bond through six sites, completely surrounding some metals. The number of times a polydentate ligand bind to a metal centre is symbolized with "?n", where "n" indicates the number sites by which a ligand attaches to a metal. EDTA4−, when it is sexidentate, binds as a ?6-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
Bite angle

Chelate bite angle is a geometric parameter used to classify chelating ligands in inorganic and organometallic chemistry. Together with ligand cone angle, this parameter is relevant to diphosphine ligands, which are used in industrial processes such as hydroformylation and hydrocyanation....
.

Complexes of polydentate ligands are called chelate complexes. They tend to be more stable than complexes derived from monodentate
Monodentate

A monodentate ligand is a ligand which forms only one chemical bond with the central atom, usually a metal ion. A monodentate ligand is also sometimes called a "unidentate ligand" from the root words meaning "one tooth"....
 ligands. This enhanced stability, the chelate effect, is usually attributed to effects of entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
, 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
Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
 is a good example: the iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 atom is at the centre of a porphyrin
Porphyrin

Porphyrins are a group of chemical compounds of which many occur in nature, such as in green leaves and red blood cells, and in bio-inspired synthetic catalysts and devices....
 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
Dimethylglyoxime

Dimethylglyoxime is a chemical compound described by the formula CH3CCCH3. This colourless solid is the dioxime derivative of the diketone diacetyl ....
.

Hapticity

Hapticity (represented by ?
?

or is a letter derived from the Latin alphabet. Both glyphs of the majuscule and Lower case forms of this letter are based on the rotated form of a minuscule e; a similar letter with identical minuscule is used in the Pan-Nigerian Alphabet, but has the capital form majuscule , based on a horizontally flipped majuscule E....
) refers to the number contiguous atoms that comprise a donor site and attach to a metal center. Butadiene forms both ?2 and ?4 complexes depending on the number of carbon atoms that are bonded to the metal.

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
Thiocyanate

Thiocyanate is the anion, [SCN]-. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates....
, SCN−, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism
Linkage isomerism

Linkage isomerism is the existence of Complex s that have the same composition differing with the connectivity of the metal to a ligand....
. 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 CO22-
Carbonate

In chemistry, a carbonate is a salt or ester of carbonic acid....
 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. FeCl2, are polymers by virtue of the presence of multiple bridging ligands.

Metal ligand multiple bond

Metal ligand multiple bond
Metal ligand multiple bond

In Chemistry, a metal ligand multiple bond describes the interaction of certain ligands with a metal with a bond order greater than one. Complexes featuring multiply bonded ligands are of both scholarly and practical interest....
s some ligands can bond to a metal center through the same atom but with a different number of lone pair
Lone pair

A lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons....
s. The bond order
Bond order

Bond order is the number of bonds between a pair of atoms. For example in nitrogen N=N the bond order is 3, in acetylene H-C=C-H the bond order between the two carbon atoms is also 3 and the C-H bond order is 1....
 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. ?1-Nitric oxide can coordinate to a metal center in linear or bent manner.

Specialized ligand types


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 noninnocent ligands often involves writing multiple resonance form
Resonance (chemistry)

Resonance in chemistry is a key component of valence bond theory used to graphically represent and mathematically model certain types of molecular structures when no single, conventional Lewis structure can satisfactorily represent the observed structure or explain its properties....
s which have partial contributions to the overall state.

Trans-spanning ligand

Trans-spanning ligands are bidentate ligands that can span coordination positions on opposite sides of a coordination complex.

Common ligands

See nomenclature
Complex (chemistry)

In chemistry, a complex, also called a "coordination compound" or "metal complex", is a structure consisting of a central atom or molecule connected to surrounding atoms or molecules....
.


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
Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an chemical element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound....
s and pseudohalides are important anionic ligands whereas ammonia
Ammonia

Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
, carbon monoxide
Carbon monoxide

Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalent bond to one oxygen atom....
, and water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 are particularly common charge-neutral ligands. Simple organic species are also very common, be they anionic (RO−
Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO–, where R is the organic substituent....
 and RCO2−) or neutral (R2O
Ether

Ether is a class of organic compounds which contain an ether functional group ? an oxygen atom connected to two alkyl or aryl groups ? of general formula R?O?R....
, R2S
Thioether

A thioether is a functional group in organic chemistry that has the structure R1-S-R2 as shown on right. Like many other sulfur-containing compounds, Volatile organic compound thioethers characteristically have foul odors....
, R3−xNHx
Amine

Amines are organic compounds and functional groups that contain a base nitrogen atom with a lone pair. Amines are derivative s of ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups....
, and R3P
Phosphine

Phosphine is the common name for phosphorus trihydride , also known by the IUPAC name phosphane and, occasionally, phosphamine....
). The steric properties of some ligands are evaluated in terms of their cone angles.

Beyond the classical Lewis bases and anions, all unsaturated molecules are also ligands, utilizing their p-electrons in forming the coordinate bond. Also, metals can bind to the s bonds in for example silane
Silane

Silane is a chemical compound with chemical formula siliconhydrogen4. It is the silicon Analog of methane. At room temperature, silane is a gas, and is pyrophoric ? it undergoes spontaneous combustion in air, without the need for external ignition....
s, hydrocarbon
Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "pure" hydrocarbons, whereas other hydrocarbons with bonded com...
s, and dihydrogen (see also: agostic interaction
Agostic interaction

Agostic interaction is a term in organometallic_chemistry for the interaction of a coordinately-unsaturated transition metal with a carbon-hydrogen bonds, when the two electrons involved in the C-H bond enter the empty d orbital of a transition metal, resulting in a two electron three center bond....
).

In complexes of non-innocent ligand
Non-innocent ligand

In chemistry, a non-innocent ligand refers to a ligand in a metal complex where the oxidation state is unclear. Typically, complexes containing non-innocent ligands are redox active at mild Electric potential....
s, the ligand is bonded to metals via conventional bonds, but the ligand is also redox-active.

Examples of common ligands (by field strength)

In the following table the ligands are sorted by field strength (weak field ligands first):
Ligand formula (bonding atom(s) in bold) Charge Most common denticity Remark(s)
Iodide
Iodide

An iodide ion is an iodine with a −1 electric charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide....
 (iodo)		 I− monoanionic monodentate
Monodentate

A monodentate ligand is a ligand which forms only one chemical bond with the central atom, usually a metal ion. A monodentate ligand is also sometimes called a "unidentate ligand" from the root words meaning "one tooth"....
 		 
Bromide
Bromide

A bromide ion is a bromine atom with electric charge of -1.Compounds with bromine in formal oxidation state -1 are called bromides, and each individual chemical in this class can be called a bromide, as well....
 (bromo)		 Br− monoanionic monodentate  
Sulfide
Sulfide

The term sulfide refers to several types of chemical compounds containing sulfur in its lowest oxidation number of −2.Formally, "sulfide" is the dianion, S2−, which exists in strongly alkaline aqueous solutions formed from H2S or alkali metal salts such as Li2S, Na2S, and K2...
 (thio or less commonly "bridging thiolate")		 S2− dianionic monodentate (M=S), or bidentate bridging (M-S-M')  
Thiocyanate
Thiocyanate

Thiocyanate is the anion, [SCN]-. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates....
 (thiocyanato)		 S-CN− monoanionic monodentate ambidentate (see also isothiocyanate, below)
Chloride
Chloride

The chloride ion is formed when the chemical element chlorine picks up one electron to form an anion Cl−....
 (chloro)		 Cl− monoanionic monodentate also found bridging
Nitrate
Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates....
 (nitroso) 		O-NO2− monoanionic monodentate  
Azide
Azide

Azide is the anion with the formula N3-. It is the conjugate base of hydrazoic acid. N3- is a linear anion that is isoelectronic with carbon dioxide and nitrous oxide....
 (azido) 		N-N2− monoanionic monodentate  
Fluoride
Fluoride

Fluoride is the Redox form of fluorine. Both organic compounds and inorganic compounds containing the chemical element fluorine are considered fluorides....
 ( fluorido) 		F− monoanionic monodentate  
Hydroxide
Hydroxide

In chemistry, hydroxide is the name for the Diatomic molecule anion OH-, consisting of oxygen and hydrogen atoms, usually derived from the Dissociation of a base ....
 hydroxo		 O-H− monoanionic monodentate often found as a bridging ligand
Oxalate
Oxalate

An oxalate is the deprotonated, charged form of oxalic acid or an ester of oxalic acid. As a salt, the oxalate anion has the chemical formula C2O42- or 22-....
 (oxalato) 		[O-C(=O)-C(=O)-O]2− dianionic bidentate  
Water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 (aqua) 		H-O-H neutral monodentate monodentate
Nitrite
Nitrite

The nitrite ion is NO2-. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a Salt or an ester of nitrous acid....
 nitrito		 O-N-O− monoanionic monodentate ambidentate (see also nitro)
Isothiocyanate
Isothiocyanate

Isothiocyanate is the chemical group -nitrogen=carbon=sulfur, formed by substituting sulfur for oxygen in the isocyanate group. Allyl isothiocyanate is a chemical compound found in mustard oil that is responsible for its pungency....
 (isothiocyanato)		 N=C=S− monoanionic monodentate ambidentate (see also thiocyanate, above)
Acetonitrile
Acetonitrile

Acetonitrile is the chemical compound with chemical formula CH3CN. This colourless liquid is the simplest organic nitrile and is widely used as a solvent....
 (acetonitrilo) 		CH3CN neutral monodentate  
Pyridine
Pyridine

Pyridine is a simple and important heterocyclic aromatic organic compound with the formula CarbonHydrogenNitrogen. This colorless liquid with a distinctive fish-like odor is structurally related to benzene, wherein one CH group in the six-membered ring is replaced by a nitrogen atom....
 (pyridino) 		C5H5N neutral monodentate  
Ammonia
Ammonia

Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
 (ammine or less commonly "ammino")		 NH3 neutral monodentate  
Ethylenediamine en neutral bidentate  
2,2'-Bipyridine
2,2'-Bipyridine

2,2'-Bipyridine is a chemical compound with the formula 2. This colorless solid, commonly abbreviated bipy , is a widely used isomer of bipyridine....
 		 bipy neutral bidentate easily reduced to its (radical) anion or even to its dianion
1,10-Phenanthroline
Phenanthroline

Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, commonly abbreviated "phen," it forms strong complexes with most metal ions....
 		 phen neutral bidentate  
Nitrite
Nitrite

The nitrite ion is NO2-. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a Salt or an ester of nitrous acid....
 (nitro)		 N-O2− monoanionic monodentate ambidentate (see also nitrito)
Triphenylphosphine
Triphenylphosphine

Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PhosphorusPhenyl group or Ph3P....
 		 PPh3 neutral monodentate  
Cyanide
Cyanide

A cyanide is any chemical compound that contains the nitrile , which consists of a carbon atom chemical bond to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN-....
 (cyano)		 CN− monoanionic monodentate can bridge between metals (both metals bound to C, or one to C and one to N)
Carbon monoxide
Carbon monoxide

Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalent bond to one oxygen atom....
 carbonyl 		CO 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.

Ligand formula (bonding atom(s) in bold) Charge Most common denticity Remark(s)
Acetonitrile
Acetonitrile

Acetonitrile is the chemical compound with chemical formula CH3CN. This colourless liquid is the simplest organic nitrile and is widely used as a solvent....
 		 MeCN neutral monodentate  
Acetylacetonate
Acetylacetone

Acetylacetone is an organic compound with molecular formula C5H8O2. This diketone is formally named 2,4-pentanedione....
 (Acac)		 CH3-C(O)-CH-C(O)-CH3 monoanionic bidentate In general bidentate, bound through both oxygens, but sometimes bound through the central carbon only,
see also analogous ketimine analogues
Alkene
Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
s 		R2C=CR2 neutral compounds with a C-C double bond
Benzene
Benzene

Benzene, or benzol, is an organic compound chemical compound and a known carcinogen with the molecular formula Carbon6Hydrogen6....
 		 C6H6 neutral and other arenes
1,2-Bis(diphenylphosphino)ethane
1,2-Bis(diphenylphosphino)ethane

1,2-Bisethane is a commonly used ligand in coordination chemistry. Dppe is almost invariably chelation, although there are examples of unidentate and of bridging behavior....
 (dppe) 		Ph2PC2H4PPh2 neutral bidentate  
1,1-Bis(diphenylphosphino)methane
1,1-Bis(diphenylphosphino)methane

1,1-Bismethane , is a compound with the formula CH22. Dppm, a white, crystalline powder, is used in inorganic and organometallic chemistry as a ligand....
 (dppm) 		C25H22P2 neutral Can bond to 2 metal atoms at once, forming dimers
Corrole
Corrole

A corrole is an aromaticity organic chemical, whose structure is in the form of the corrin ring which is also present in vitamin B12. The ring consists of nineteen carbon atoms, with four nitrogen atoms in the core of the molecule....
s 		tetradentate  
Crown ether
Crown ether

Crown ethers are heterocycle chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-....
s 		neutral primarily for alkali and alkaline earth metal cations
2,2,2-crypt
Cryptand

Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J....
 		 hexadentate primarily for alkali and alkaline earth metal cations
Cryptates neutral  
Cyclopentadienyl
Cyclopentadienyl complex

A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadiene moieties, cyclopentadienyl complexes are classified into the following three categories: a) p-complexes, b) s-complexes, and c) ionic complexes....
 		 [C5H5]− monoanionic  
Diethylenetriamine (dien) C4H13N3 neutral tridentate related to TACN, but not constrained to facial complexation
Dimethylglyoximate
Dimethylglyoxime

Dimethylglyoxime is a chemical compound described by the formula CH3CCCH3. This colourless solid is the dioxime derivative of the diketone diacetyl ....
 (dmgH−) 		monoanionic  
Ethylenediaminetetraacetate
EDTA

EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid . EDTA is a polyamino carboxylic acid with the chemical formula [CH2N2]2....
 (EDTA) 		tetra-anionic hexadentate actual ligand is the tetra-anion
Ethylenediaminetriacetate trianionic pentadentate actual ligand is the trianion
glycinate
Glycine

Glycine is the organic compound with the chemical formula NH2CH2COOH. It is the smallest of the 20 amino acids commonly found in proteins, coded by codons GGU, GGC, GGA and GGG....
 		 bidentate other a-amino acid anions are comparable (but chiral)
Heme
Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
 		 dianionic tetradentate macrocyclic ligand
Nitrosyl
Nitrosyl

Nitrosyls are molecules with the general formula RNO, where R represents an unspecified substituent. A common example is nitrosyl chloride, NOCl .Nitrosyl also refers to the discrete molecule nitric oxide, NO....
 		 NO+ cationic bent (1e) and linear (3e) bonding mode
Pyrazine
Pyrazine

Pyrazine is a heterocyclic aromatic organic compound.Pyrazine is a symmetrical molecule with point group D2h. It is found in folic acid in the form of pterin....
 		 N2C4H4 neutral ditopic sometimes bridging
Scorpionate ligand
Scorpionate ligand

The term Scorpionate ligand refers to a tridentate ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the trishydroborates or Tp ligands....
 		 tridentate  
Sulfite
Sulfite

Sulfites are chemical compound that contain the sulfite ion sulfuroxygen32- ....
 		 monoanionic monodentate ambidentate
2,2',5',2-Terpyridine
Terpyridine

In chemistry, terpyridine is a polypyridine compound in which three pyridine molecules are bound with a single chemical bond. Its molecular formula is C15H11N3....
 (terpy) 		neutral tridentate meridional bonding only
Thiocyanate
Thiocyanate

Thiocyanate is the anion, [SCN]-. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates....
 		 monoanionic monodentate ambidentate, sometimes bridging
Triazacyclononane (tacn) (C2H4)3(NR)3 neutral tridentate macrocyclic ligand
see also the N,N',N"-trimethylated analogue
Tricyclohexylphosphine (C6H11)3P or (PCy3) neutral monodentate  
Triethylenetetramine (trien) neutral tetradentate  
Trimethylphosphine
Trimethylphosphine

Trimethylphosphine is the chemical compound with the formula P3, commonly abbreviated PMe3. This colorless liquid has a strongly unpleasant odour, which characteristic of alkylphosphines....
 		 PMe3 neutral monodentate  
Tri(o-tolyl)phosphine P(o-tolyl)3 neutral monodentate  
Tris(2-aminoethyl)amine
Tris(2-aminoethyl)amine

Trisamine is the organic compound with the chemical formula N3. This colourless liquid is soluble in water and is highly basic, consisting of a tertiary amine center and three pendant primary amine groups....
 (tren) 		(NH2CH2CH2)3N neutral tetradentate  
Tris(2-diphenylphosphineethyl)amine (np3) neutral tetradentate  
Terpyridine
Terpyridine

In chemistry, terpyridine is a polypyridine compound in which three pyridine molecules are bound with a single chemical bond. Its molecular formula is C15H11N3....
 		 C15H11N3 neutral tridentate  
Tropylium C7H7+ cationic  


See also

  • 2-Mercaptoindole
    2-Mercaptoindole

    2-Mercaptoindole is a bicyclic heterocycle containing a thiol group. It is a popular ligand and building block for more complex structures....
  • Crystal field theory
    Crystal field theory

    Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered complex ....
  • Ligand field theory
    Ligand field theory

    Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes....
  • Coordination chemistry
  • Inorganic chemistry
    Inorganic chemistry

    Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry....
  • Radioligand
    Radioligand

    A radioligand is a radioactive biochemical substance that is used for diagnosis or for research-oriented study of the receptor systems of the body....
  • Tanabe-Sugano diagram
    Tanabe-Sugano diagram

    Tanabe-Sugano diagrams are used in coordination chemistry to predict absorption s in the UV and visible electromagnetic spectrum of coordination compounds....
  • Spectrochemical series
    Spectrochemical series

    A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group and its identity....
  • Scatchard equation
    Scatchard equation

    The Scatchard equation is an equation for calculating the Dissociation_constant#Protein-Ligand_binding constant of a ligand with a protein. The Scatchard equation is given by...