In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a Lewis base). Fewer examples exist where a molecule can be described as a ligand that accepts electrons from a Lewis base (hence, the ligand acts as a Lewis acid). Chemistry - the study of atoms, made of nuclei (conglomeration of center particles) and electrons (outer particles), and the structures they form. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... “Multivalent” redirects here. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... In organic chemistry, functional groups are specific groups of atoms within molecules, that are responsible for the characteristic chemical reactions of those molecules. ... e- redirects here. ... A coordinate covalent bond (also known as dative bond) is a description of covalent bonding in many kinds of compounds. ... Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. ... A Lewis base is any molecule or ion that can form a new covalent bond by donating a pair of electrons. ... In chemistry, a Lewis acid can accept a pair of electrons and form a coordinate covalent bond, after the American chemist Gilbert Lewis. ...

Most commonly the central atom is a metal or metalloid in inorganic chemistry. But in organic chemistry ligands are also used to protect functional groups (e.g. borane BH₃ as ligand for the protection of phosphine PH₃), or to stabilize reactive compounds (tetrahydrofuran THF as a ligand for BH₃ to make BH₃ easier to handle). The molecule resulting from the coordination of a ligand (or an array of ligands) to a central atom is termed a complex. Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily loses electrons to form positive ions (cations) and has metallic bonds between metal atoms. ... Together with the metals and nonmetals, the metalloids (in Greek metallon = metal and eidos = sort) form one of the three categories of chemical elements as classified by ionization and bonding properties. ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... Organic chemistry is a specific discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of chemical compounds consisting of primarily carbon and hydrogen, which may contain any number of other elements, including nitrogen, oxygen, halogens as well... A borane is an inorganic chemical compound of boron and hydrogen. ... Phosphine is the common name for phosphorus hydride (PH3), also known by the IUPAC name phosphane and, occasionally, phosphamine. ... For other uses of THF, see THF (disambiguation) Tetrahydrofuran is a heterocyclic organic compound. ... Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate. ...

Factors that characterize the ligands are their charge, size (bulk), and of course the nature of the constituent atoms.

The ligands in a complex:

• stabilize the central atom, and
• dictate the reactivity of the central atom.

Reactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. ...

Ligands in metal complexes

The constitution of metal complexes has been described by Alfred Werner, who developed the basis for modern coordination chemistry. The ligands that are directly bonded to the metal (that is, share electrons), are called "inner sphere" ligands. If the inner-sphere ligands do not balance the charge of the central atom (the oxidation number), this may be done by simple ionic bonding with another set of counter ions (the "outer-sphere" ligands). The complex of the metal with the inner sphere ligands is then called a complex ion (which can be either cationic or anionic). The complex, along with its counter ions, is called a coordination compound. The size of a ligand is indicated by its cone angle. A complex in chemistry and biochemistry is a reversible association of molecules, atoms, or ions through weak non-covalent chemical bonds. ... Alfred Werner (December 12, 1866 - November 15, 1919) was a German Nobel prize-winning chemist. ... complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... The oxidation number of an element in a molecule or complex is the charge that it would have if all the ligands (basically, atoms that donate electrons) were removed along with the electron pairs that were shared with the central atom[1]. It is used in the inorganic nomenclature of... Electron configurations of lithium and fluorine. ... A counter ion is an ion, the presence of which allows the formation of an overall neutrally charged species. ... complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... In chemistry, a cationic species is one that contains a full positive charge. ... In chemistry, an anionic species is one that contains a full negative charge. ... A counter ion is an ion, the presence of which allows the formation of an overall neutrally charged species. ... complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... Ligand cone angle is a measure of the size of a ligand. ...

Donation and back-donation

In general, ligands donate electron density to the (electron deficient) central atom - that is, they overlap between the highest occupied molecular orbital (HOMO) of the ligand with the lowest unoccupied molecular orbital (LUMO) of the central atom. The ligand thus acts as a Lewis base by donating electron density (in general electron pairs) to the central atom, acting as a Lewis acid. In some cases ligands donate only one electron from a singly occupied orbital (the donating atom in these ligands is a radical). Electron density is the measure of the probability of an electron being present at a specific location. ... A compound that is electron deficient has too few valence electrons for the connections between atoms to be described as covalent bonds. ... HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. ... HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. ... HOMO and LUMO are acronyms for Highest Occupied Molecular Orbitals and Lowest Unoccupied Molecular Orbitals respectively. ... A Lewis base is any molecule or ion that can form a new covalent bond by donating a pair of electrons. ... A lone pair is an electron pair without bonding or sharing with other atoms. ... In chemistry, radicals (often referred to as free radicals) are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. ...

Some metal centers in combination with certain ligands (e.g. carbon monoxide (CO)) can be further stabilised by donating 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, with the chemical formula CO, is a colorless, odorless, and tasteless gas. ... Electron density is the measure of the probability of an electron being present at a specific location. ... π backbonding, also called π backdonation, is a concept from chemistry, that refers to the flow of electrons from an atomic orbital on one atom to a π anti-bonding orbital that belongs to other atoms. ...

Strong field vs. weak field ligands

Ligands and metal ions can be ordered by their 'hardness' (see also hard soft acid base theory). Certain metal ions have a preference for 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. Metal ions bound to strong-field ligands follow the Aufbau principle, whereas complexes bound to weak-field ligands follow Hund's rule (see crystal field theory). also known as HSAB theory HSAB is the acronym for hard and soft acids and bases The HSAB concept is widely used in chemistry for explaining stability of compounds, reaction paths etc. ... In quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. ... HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. ... Electron atomic and molecular orbitals The Aufbau principle (also Aufbau rule or building-up principle), is used to determine the electron configuration of an atom, molecule or ion. ... Hunds rule is a principle of physical chemistry which states that before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron. ... Crystal field theory is used to describe the electronic structure of transition metal complexes. ...

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 is used to describe the electronic structure of transition metal complexes. ...

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). This ordering of ligands is almost invariable for all metal ions and is called spectrochemical series. In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a... A spectrochemical series is a list of ligands based on ligand strength. ...

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. For the purposes of ranking ligands, however, the properties of the octahedral complexes and the resulting Δ_o has been of primary interest. In chemistry, the coordination number (c. ...

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 diagrams. Color is an important part of the visual arts. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... Tanabe-Sugano diagrams are used in coordination chemistry to predict absorptions in the UV and visible electromagnetic spectrum of coordination compounds. ...

Denticity

Some ligand molecules are able to bind to the metal ion through multiple sites because they have free lone pairs on more than one atom. Ligands that bind to more than one site are termed chelating (from the Greek for claw). For example, a ligand binding through two sites is bidentate and three sites is tridentate. The bite angle refers to the angle between the two bonds of a bidentate chelate. A classic example of a polydentate ligand is EDTA. It is able to bond through six sites, completely surrounding some metals. Polydentate ligands are commonly formed by joining organic molecules together. The number of atoms with which a polydentate ligand bind to the metal centre is called its denticity (symbol κ). κ indicates the number non-contiguous donor sites by which a ligand attaches to a metal. Hapticity (η) and denticity are often confused. Hapticity refers to contiguous atoms that are attached to a metal. Ethylene forms η² complexes. Ethylenediamine forms κ² complexes. Cyclopentadienyl is typically bonded in η⁵ mode because each bonded carbon is connected directly. EDTA^4- on the other hand, when it is sexidentate, is κ⁶ mode, the amines and the carboxylate oxygen atoms are not connected directly. To simplify matters, ηⁿ tends to refer to unsaturated hydrocarbons and κⁿ tends to describe polydentate amine and carboxylate ligands. A lone pair is an electron pair without bonding or sharing with other atoms. ... Chelation (from Greek χηλή, chelè, meaning claw) is the process of reversible binding (complexation) of a ligand - the chelant, chelator, chelating agent, sequestering agent, or complexing agent - to a metal ion, forming a metal complex, the chelate. ... EDTA is a popular acronym for the chemical compound ethylenediamine tetraacetic acid (and many other names, see table). ... // η-bonding The Greek letter η (eta) is used to denote the number of atoms of a ligand that bind to a metal center. ... --141. ...

Complexes of polydentate ligands are called chelate complexes. They tend to be more stable than monodentate complexes, as it is necessary to break all of the bonds to the central atom for the ligand to be displaced. This increased stability or inertness is called the chelate effect. Entropy favors the displacement of many ligands by one polydentate ligand. The increase in the total number of molecules in solution is favorable. Monodentate ligand is a ligand which forms only one bond with the central atom, e. ... It has been suggested that this article or section be merged into Chelation. ... Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ...

Related to the chelate effect is the macrocyclic effect. A macrocyclic ligand is any large cyclic ligand which 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. Structure of Heme b 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. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... Core porphyrin structure 3D representation A porphyrin is a heterocyclic macrocycle made from 4 pyrrole subunits linked on opposite sides (α position) through 4 methine bridges (=CH-). The macrocycle, therefore, is more aromatic than the related corrins, chlorins (2,3-dihydroporphyrin) and bacteriochlorins (2,3,12,13-tetrahydroporphyrin). ... This page is a candidate to be copied to Wiktionary. ...

Unlike polydentate ligands, ambidentate ligands can attach to the central atom in two places but not both. A good example of this is thiocyanide, SCN^-, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism. Thiocyanate (also known as sulphocyanate or thiocyanide) is a functional group consisting of sulfur, carbon, and nitrogen: S=C=N- It has a charge of 1-. It is analogous to the cyanate ion, with oxygen replaced by sulfur. ... Linkage isomerism is a concept in coordination chemistry. ...

Common ligands

See 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 halides and pseudohalides are important anionic ligands whereas ammonia, carbon monoxide, and water are particularly important charge-neutral ligands. Simple organic compounds are also very important, be they anionic (RO^- and RCO₂^-) or neutral (R₂O, R₂S, R_3-xNH_x, and R₃P). 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). Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate. ... 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 electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ... 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 electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ... Ammonia is a compound with the formula NH3. ... Carbon monoxide, with the chemical formula CO, is a colorless, odorless, and tasteless gas. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Silane is a chemical compound with chemical formula SiH4. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... The word agostic was first coined for organometallic chemistry by Malcolm Green and Maurice Brookhart to describe a weak bonding interaction between a transition metal and a C-H bond on a carbon atom that is held close to the metal center through an additional Ï€- or σ-bond. ...

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.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). An iodide ion is an iodine atom with a −1 (negative one) charge. ... Monodentate ligand is a ligand which forms only one bond with the central atom, e. ... A bromide is a phrase, or person who uses phrases, which have been used and repeated so many times as to become either insincere in their meaning, or seem like an attempt at trying to explain the obvious. ... In chemistry, a sulfide (sulphide in British and Canadian English) is a combination of sulfur with an oxidation number of -2, with another chemical element or a radical thereof. ... Thiocyanate (also known as sulphocyanate or thiocyanide) is both an anion SCN- and, in organic chemistry, a functional group: An example of the anion is found in potassium thiocyanate, KSCN. Thiocyanate is analogous to the cyanate ion, OCN-, wherein oxygen is replaced by sulfur. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. ... An electrostatic potential map of the nitrate ion. ... An azide is the N3- anion, the anion of hydrazoic acid or a reactive group in organic chemistry where a carbon substituent is attached as RN3. ... Fluoride is the ionic form of fluorine. ... Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: OH− It has a charge of −1. ... An oxalate (called also: ethanedioate) is a salt or ester of oxalic acid. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Isothiocyanate is the chemical group -N=C=S, formed by substituting sulfur for oxygen in the isocyanate group. ... Acetonitrile is an organic molecule, often used as a solvent, with the chemical formula of CH3CN. Also known as methyl cyanide, it is the simplest of the organic nitriles. ... Pyridine is a chemical compound with the formula C5H5N. It is a liquid with a distinctively putrid odour. ... Ammonia is a compound with the formula NH3. ... Ethylene diamine (EDA), or 1,2-diaminoethane, is an organic compound from the amines group. ... Bipyridine is one of the simplest polypyridine compounds. ... Phenanthroline is a heterocyclic organic compound. ... // Definition The nitrite ion is NO2−. A nitrite compound is one that contains this group, either an ionic compound, or an analogous covalent one. ... Triphenylphosphine (in Europe: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. ... The cyanide ion, CN−. From the top: 1. ... Carbon monoxide, with the chemical formula CO, is a colorless, odorless, and tasteless gas. ...

Other general encountered ligands (alphabetical)

In this table other common ligands are listed in alphabetical order.

R-phrases , S-phrases , , , Flash point 34 °C Autoignition temperature 340 °C Explosive limits 2. ... The chemical structure of ethylene, the simplest alkene. ... Benzene, also known as benzol, is an organic chemical compound with the formula C6H6. ... Dppe is the acronym for 1,2-bis(diphenylphosphino)ethane. ... Corrole is an aromatic ring structure compound similar as porphyrin and vitamin B12. ... Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. ... Cryptands (trade name Kryptofix) are a group of cyclic or polycyclic multidentate ligands that are used in the synthesis of other molecules and also have significant biological importance. ... Cryptands (trade name Kryptofix) are a group of chemicals that are used in the synthesis of other molecules. ... A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, abbreviated as Cp). ... Diethylene triamine (DETA) is a yellow hygroscopic liquid, soluble in water and hydrocarbons. ... This page is a candidate to be copied to Wiktionary. ... EDTA is a popular acronym for the chemical compound ethylenediamine tetraacetic acid (and many other names, see table). ... For the plant, see Glycine (plant). ... Structure of Heme b 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. ... The nitrosyl group is NO, a nitrogen atom plus an oxygen atom. ... Scorpionate ligands, are representatives of tridentate ligands that are more precisely members of the polypyrazolylborate ligands. ... Sulfites (also sulphite) are compounds that contain the sulfite ion SO32−. They are often used as preservatives in wines (to prevent spoilage and oxidation), dried fruits, and dried potato products. ... In chemistry, terpyridine (2,2;62-terpyridine) is a polypyridine compound in which three pyridine molecules are bound with a single bond. ... Thiocyanate (also known as sulphocyanate or thiocyanide) is both an anion SCN- and, in organic chemistry, a functional group: An example of the anion is found in potassium thiocyanate, KSCN. Thiocyanate is analogous to the cyanate ion, OCN-, wherein oxygen is replaced by sulfur. ... 1,4,7-Triazacyclononane, known as TACN which is pronounced tack-en, is a cyclic organic compound with the formula C6H12(NH)3. ... Chemical structure of triethylenetetramine. ... In chemistry, terpyridine (2,2;62-terpyridine) is a polypyridine compound in which three pyridine molecules are bound with a single bond. ...

See also

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