For biochemical uses in particular see Ligand (biochemistry). Biochemistry is the study of the chemical processes in living Organisms It deals with the Structure and function of cellular components such as In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule

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 electrons. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by In Organic chemistry, functional groups are specific groups of Atoms within Molecules that are responsible for the characteristic Chemical reactions The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The metal-ligand bonding ranges from covalent 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 acidic "ligands. "^[1]

Metal and metalloids are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in high vacuum. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across 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 Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state Ligand selection is a critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry. Bioinorganic chemistry is a specialized field that spans the Chemistry of Metal -containing Molecules within biological systems Homogeneous catalysis is a Chemistry term which describes Catalysis where the Catalyst is in the same phase (ie Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places

Ligands are classified in many ways: their charge, size (bulk), the identity of the coordinating atom(s), their denticity. The size of a ligand is indicated by its cone angle. Ligand cone angle (also known as the Tolman cone angle) is a measure of the size of a Ligand.

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. The term complex in Chemistry is usually used to describe molecules or ensembles formed by the combination of Ligands and metal Ions. "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). An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The complex, along with its counter ions (if required), is called a coordination compound. A counter ion is an Ion, the presence of which allows the formation of an overall neutrally charged species The term complex in Chemistry is usually used to describe molecules or ensembles formed by the combination of Ligands and metal Ions.

Metal-EDTA complex, wherein the aminocarboxylate is a hexadentate chelating ligand. EDTA is a widely used abbreviation for the Chemical compound ethylenediaminetetraacetic acid (and many other names see table

Cobalt(III) complex containing six ammonia ligands, which are monodentate. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor The chloride is not a ligand.

Strong field and weak field ligands

Main article: Crystal field theory

In general, ligands are viewed as donating electrons to the central atom. Crystal field theory (CFT is a model that describes the Electronic structure of Transition metal compounds all of which can be considered coordination Bonding is often described using the formalisms of molecular orbital theory. In general, electron pairs) occupy the HOMO of the ligands. 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

Ligands and metal ions can be ordered in many ways, one ranking system focuses on ligand 'hardness' (see also hard soft acid base theory). The HSAB concept is an Acronym for 'hard and soft Acids and bases. 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 point of view, the HOMO of the ligand should have an energy that makes overlap with the LUMO of the metal preferential. In Chemistry, molecular orbital theory ( MO theory) is a method for determining molecular structure in which Electrons are not assigned to individual HOMO and LUMO are Acronyms for highest occupied Molecular orbital and lowest unoccupied Molecular orbital, respectively Metal ions bound to strong-field ligands follow the Aufbau principle, whereas complexes bound to weak-field ligands follow Hund's rule. The Aufbau principle (from the German Aufbau meaning "building up construction" also Aufbau rule or building-up principle) is In Atomic physics, Hund's rules refer to a simple set of rules used to determine which is the Term symbol that corresponds to the ground state of a multi- Electron

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). An octahedron (plural octahedra is a Polyhedron with eight faces Crystal field theory (CFT is a model that describes the Electronic structure of Transition metal compounds all of which can be considered coordination

3 orbitals of low energy: d_xy, d_xz and d_yz

2 of high energy: d_z² and d_x²−y²

The energy difference between these 2 sets of d-orbitals is called the splitting parameter, Δ_o. The magnitude of Δ_o is determined by the field-strength of the ligand: strong field ligands, by definition, increase Δ_o more than weak field ligands. Ligands can now be sorted according to the magnitude of Δ_o (see the table below). In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally This ordering of ligands is almost invariable for all metal ions and is called 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: d_z² and d_x²−y²

3 orbitals of high energy: d_xy, d_xz and d_yz

The energy difference between these 2 sets of d-orbitals is now called Δ_t. The magnitude of Δ_t is smaller than for Δ_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 is neither octahedral nor tetrahedral, the splitting becomes correspondingly more complex. The coordination number of an atom in a molecule or a crystal is the number of its nearest neighbours For the purposes of ranking ligands, however, the properties of the octahedral complexes and the resulting Δ_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) 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. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy 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 diagrams. Tanabe-Sugano diagrams are used in Coordination chemistry to predict absorptions in the UV and visible Electromagnetic spectrum of Coordination compounds

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. Electron density is the measure of the Probability of an Electron being present at a specific location 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 pairs on more than one atom. 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 Ligands that bind via more than one atom are often termed chelating. Chelation is the binding or complexation of a bi- or multidentate Ligand. 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 bidentate ligand is ethylenediamine, which is derived by the linking of two ammonia groups with an ethylene (-CH₂CH₂-) linker. Ethylenediamine (abbreviated as en when a Ligand) is the Organic compound with the formula C2H4(NH22 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. EDTA is a widely used abbreviation for the Chemical compound ethylenediaminetetraacetic acid (and many other names see table The number of atoms with which a polydentate ligand bind to the metal centre is called its denticity, symbolized κⁿ, where n indicates the number non-contiguous donor sites by which a ligand attaches to a metal. EDTA⁴⁻, when it is sexidentate, binds as a κ⁶-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. Chelate bite angle is a geometric parameter used to classify chelating ligands in inorganic and Organometallic chemistry.

Related to but distinct to from denticity is hapticity, symbolized η or eta. The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom Hapticity refers to the number of contiguous atoms in a ligand that are attached to a metal. Butadiene forms both η² and η⁴ complexes depending on the number of carbon atoms are bonded to the metal. To simplify matters, ηⁿ usually refers to unsaturated hydrocarbons and κⁿ 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. A monodentate ligand is a Ligand which forms only one bond with the central Atom, usually a Metal Ion. This enhanced stability, the chelate effect, is usually attributed to effects of entropy, which favors the displacement of many ligands by one polydentate ligand. Chelation is the binding or complexation of a bi- or multidentate Ligand. In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy 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. A heme ( American English) or haem ( British English) is a Prosthetic group that consists of an Iron atom contained in the center of Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 A porphyrin is a heterocyclic Macrocycle derived from four Pyrroline subunits interconnected via their α carbon atoms via Methine bridges (=CH- The very stable dimethylglyoximate complex of nickel is a synthetic macrocycle derived from the anion of dimethylglyoxime. Dimethylglyoxime is a Chemical compound described by the formula CH3C(NOHC(NOHCH3

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⁻, which can attach at either the sulfur atom or the nitrogen atom. Thiocyanate (also known as sulphocyanate or thiocyanide or rhodanide) is the anion − Such compounds give rise to linkage isomerism. Linkage isomerism is the existence of co-ordination compounds 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. A bridging Ligand is an Atom or a polyatomic entity that connects two or more Metal centres in a complex. Polyatomic ligands such as CO₂^2- are especially prone to bridge. In Chemistry, a carbonate is a salt or Ester of Carbonic acid. The bonding is complicated because polyatomic ligands are ambidentate and thus the capacity for many different linkage isomers. Linkage isomerism is the existence of co-ordination compounds that have the same composition differing with the connectivity of the metal to a Ligand. 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 bonds some ligands can bond to a metal center through the same atom but with a different number of lone pairs. In Chemistry a' metal ligand multiple bond' describes the interaction of certain Ligands with a metal with a Bond order greater than one 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 The bond order of the metal ligand bond can be in part distinguished through the metal ligand bond angle (M-X-R). 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 Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. 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. η¹-Nitric oxide can coordinate to a metal center in linear or bent manner.

Specialized ligand types

Noninnocent ligand

Noninnocent ligands bond with metals in such a manner that the distribution of electron density between the metal center and ligand is unclear. In Chemistry, a non-innocent ligand refers to a Ligand in a Metal complex where the oxidation state is unclear Describing the bonding of noninnocent ligands often involves writing multiple resonance forms which have partial contributions to the overall state. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component

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 polymers. Coordination polymer is the term given in Inorganic chemistry to a metal Coordination compound where a Ligand Bridges between metal

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 (Ph₂PCH₂) groups at the 1 and 11 positions. ^[2][3] The polycyclic framework suffers sterically clashing hydrogen centers. XANTHOS is a more reliable trans-spanning ligand. ^[2] 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. ^[2] TRANSDIP is based on a α-cyclodextrin. Cyclodextrins (sometimes called cycloamyloses make up a family of cyclic Oligosaccharides, composed of 5 or more α-D-glucopyranoside units linked 1->4 as in Amylose ^[4]

Common ligands

See nomenclature. The term complex in Chemistry is usually used to describe molecules or ensembles formed by the combination of Ligands and metal Ions.

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 halides and pseudohalides are important anionic ligands whereas ammonia, carbon monoxide, and water are particularly common charge-neutral ligands. A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Simple organic species are also very common, be they anionic (RO⁻ and RCO₂⁻) or neutral (R₂O, R₂S, R_3−xNH_x, and R₃P). An alkoxide is the Conjugate base of an Alcohol and therefore consists of an organic group bonded to a negatively charged Oxygen atom A carboxylate anion is an Ion with Negative charge that contains the group -COO&minus Ether is a class of Organic compounds which contain an ether group — an Oxygen Atom connected to two (substituted Alkyl A thioether (similar to Sulfide) is a functional group in Organic chemistry that has the structure R1-S-R2 as shown on right Amines are Organic compounds and Functional groups that contain a basic Nitrogen Atom with a Lone pair. Phosphine is the common name for phosphorus hydride (PH3 also known by the IUPAC name phosphane and occasionally phosphamine. The steric properties of some ligands are evaluated in terms of their cone angles. Ligand cone angle (also known as the Tolman cone angle) is a measure of the size of a Ligand.

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 silanes, hydrocarbons, and dihydrogen (see also: agostic interaction). Silane is a Chemical compound with Chemical formula Si[[hydrogen H]]4 In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Agostic interaction is a term in Organometallic chemistry, for the interaction of a coordinately-unsaturated Transition metal with a C-H bond, when the

In complexes of non-innocent ligands, the ligand is bonded to metals via conventional bonds, but the ligand is also redox-active. In Chemistry, a non-innocent ligand refers to a Ligand in a Metal complex where the oxidation state is unclear

Examples of common ligands (by field strength)

In the following table the ligands are sorted by field strength (weak field ligands first):

Note: The entries in the table are sorted by field strength, binding through the stated atom (i. An iodide Ion is an iodine atom with a &minus1 charge. Compounds with iodine in formal Oxidation state &minus1 are called iodides A monodentate ligand is a Ligand which forms only one bond with the central Atom, usually a Metal Ion. A bromide Ion is a Bromine atom with charge of −1 Compounds with bromine in formal Oxidation state −1 are called bromides The term sulfide ( sulphide in British English) refers to several types of Chemical compounds containing Sulfur in its lowest Oxidation Thiocyanate (also known as sulphocyanate or thiocyanide or rhodanide) is the anion − The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus In Inorganic chemistry, a nitrate is a salt of Nitric acid with an Ion composed of one Nitrogen and three Oxygen atoms Azide is the anion with the formula N3− It is the Conjugate base of Hydrazoic acid. Fluoride is the reduced form of Fluorine. Both organic and Inorganic compounds containing the element fluorine are considered fluorides In Chemistry, hydroxide is the most common name for the diatomic Anion OH− consisting of Oxygen and Hydrogen An oxalate (also ethanedioate) is a salt or Ester of Oxalic acid. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Isothiocyanate is the Chemical group - N = C = S, formed by substituting Sulfur for Oxygen in the Isocyanate Acetonitrile (ACN is the Chemical compound with formula CH3CN Pyridine is a Chemical compound with the formula C5[[Hydrogen H5]] N. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Ethylenediamine (abbreviated as en when a Ligand) is the Organic compound with the formula C2H4(NH22 22'-Bipyridine is a Chemical compound with the formula (C5H4N2 Phenanthroline is a Heterocyclic Organic compound. As a bidentate ligand in Coordination chemistry, commonly abbreviated "phen" it forms The nitrite Ion is NO2− The anion is bent being Isoelectronic with O3. The nitrite Ion is NO2− The anion is bent being Isoelectronic with O3. Triphenylphosphine (in Europe triphenylphosphane is a common Organophosphorus compound with the formula P(C6H53 - often abbreviated to A cyanide is any Chemical compound that contains the cyano group (C≡N which consists of a Carbon Atom triple-bonded to a Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. 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.

See also

• 2-Mercaptoindole
• Crystal field theory
• Ligand field theory
• Coordination chemistry
• Inorganic chemistry
• Radioligand
• Tanabe-Sugano diagram
• Spectrochemical series
• Scatchard equation

References