Complex (chemistry) - AbsoluteAstronomy.com

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a coordination complex or metal complex, is an atom or ion (usually metallic), bonded

Chemical bond

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

to a surrounding array of molecules or anions, that are in turn known as ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s or complexing agents. Many metal-containing compounds consist of coordination complexes.

Nomenclature and terminology

Coordination complexes are so pervasive that the structure and reactions are described in many ways, sometimes confusingly. The atom within a ligand that is bonded to the central atom or ion is called the donor atom. A typical complex is bound to several donor atoms, which can be the same or different. Polydentate (multiple bonded) ligands consist of several donor atoms, several of which are bound to the central atom or ion. These complexes are called chelate complexes, the formation of such complexes is called chelation, complexation, and coordination.

The central atom or ion, together with all ligands comprise the coordination sphere

Coordination sphere

In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around.Molecules that are attached noncovalently to the ligands are called the second coordination sphere....

. The central atoms or ion and the donor atoms comprise the first coordination sphere.

Coordination refers to the "coordinate covalent bonds" (dipolar bonds) between the ligands and the central atom. Originally, a complex implied a reversible association of molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s, atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s, or ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s through such weak chemical bond

Chemical bond

s. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.

History

Coordination complexes were known - although not understood in any sense - since the beginning of chemistry, e.g. Prussian blue

Prussian blue

Prussian blue is a dark blue pigment with the idealized formula Fe718. Another name for the color Prussian blue is Berlin blue or, in painting, Parisian blue. Turnbull's blue is the same substance but is made from different reagents....

and copper vitriol. The key breakthrough occurred when Alfred Werner

Alfred Werner

Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...

proposed in 1893 that Co(III) bears six ligands in an octahedral geometry. His theory allows one to understand the difference between coordinated and ionic in a compound, for example chloride in the cobalt ammine

Ammine

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelled with a single "m"...

chlorides and to explain many of the previously inexplicable isomers.

In 1914, Werner resolved the first coordination complex, called hexol

Hexol

Hexol is a cobalt compound that was first prepared by Alfred Werner in 1914 and represented the first non-carbon-containing chiral compound. The salt with the molecular formula of[Co3]3 was prepared starting from cobalt sulfate....

, into optical isomers, overthrowing the theory that chirality

Chirality (chemistry)

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

was necessarily associated with carbon compounds.

Structures

The ions or molecules surrounding the central atom are called ligands

Ligand

. Ligands are generally bound to the central atom by a coordinate covalent bond

Coordinate covalent bond

A dipolar bond, also known as dative covalent bond or coordinate bond is a kind of 2-centre, 2-electron covalent bond in which the two electrons derive from the same atom. Typically, a dipolar bond is formed when a Lewis base donates a pair of electrons to a Lewis acid. This description of bonding...

(donating electrons from a lone electron pair

Lone pair

In chemistry, 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...

into an empty metal orbital), and are said to be coordinated to the atom. There are also organic ligands such as alkene

Alkene

In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

s whose pi bonds can coordinate to empty metal orbitals. An example is ethene in the complex known as Zeise's salt

Zeise's salt

Zeise's salt, potassium trichloroplatinate, is the chemical compound with the formula KPtCl3]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry.-Preparation:This compound is commercially...

, K⁺[PtCl₃(C₂H₄)]^-.

Geometry

In coordination chemistry, a structure is first described by its coordination number

Coordination number

In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....

, the number of ligands attached to the metal (more specifically, the number of donor atoms). Usually one can count the ligands attached, but sometimes even the counting can become ambiguous. Coordination numbers are normally between two and nine, but large numbers of ligands are not uncommon for the lanthanides and actinides. The number of bonds depends on the size, charge, and electron configuration

Electron configuration

In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...

of the metal ion and the ligands. Metal ions may have more than one coordination number.

Typically the chemistry of complexes is dominated by interactions between s and p molecular orbital

Molecular orbital

In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...

s of the ligands and the d orbitals of the metal ions. The s, p, and d orbitals of the metal can accommodate 18 electrons (see 18-Electron rule

18-Electron rule

The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...

). The maximum coordination number for a certain metal is thus related to the electronic configuration of the metal ion (to be more specific, the number of empty orbitals) and to the ratio of the size of the ligands and the metal ion. Large metals and small ligands lead to high coordination numbers, e.g. [Mo(CN)₈]^4-. Small metals with large ligands lead to low coordination numbers, e.g. Pt[P(CMe₃)]₂. Due to their large size, lanthanide

Lanthanide

The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...

s, actinide

Actinide

The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...

s, and early transition metals tend to have high coordination numbers.

Different ligand structural arrangements result from the coordination number. Most structures follow the points-on-a-sphere pattern (or, as if the central atom were in the middle of a polyhedron

Polyhedron

In elementary geometry a polyhedron is a geometric solid in three dimensions with flat faces and straight edges...

where the corners of that shape are the locations of the ligands), where orbital overlap (between ligand and metal orbitals) and ligand-ligand repulsions tend to lead to certain regular geometries. The most observed geometries are listed below, but there are many cases that deviate from a regular geometry, e.g. due to the use of ligands of different types (which results in irregular bond lengths; the coordination atoms do not follow a points-on-a-sphere pattern), due to the size of ligands, or due to electronic effect

Electronic effect

An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry of a...

s (see, e.g., Jahn-Teller distortion):

Linear
Linear molecular geometry
In chemistry, the Linear molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º. Linear organic molecules, e.g. acetylene, are often described by invoking sp orbital hybridization for the carbon centers. Many linear molecules exist, prominent...

for two-coordination
Trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...

for three-coordination
Tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

or square planar
Square planar
The square planar molecular geometry in chemistry describes the stereochemistry that is adopted by certain chemical compounds...

for four-coordination
Trigonal bipyramidal
Trigonal bipyramid molecular geometry
In chemistry a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular dipyramid...

or square pyramid
Square pyramidal molecular geometry
In molecular geometry, square based pyramidal geometry describes the shape of certain compounds with the formula ML5 where L is a ligand. If the ligand atoms were connected, the resulting shape would be that of a pyramid with a square base. The geometry is common for certain main group compounds...

al for five-coordination
Octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

(orthogonal) or trigonal prismatic
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...

for six-coordination
Pentagonal bipyramidal for seven-coordination
Square antiprism
Square antiprism
In geometry, the square antiprism is the second in an infinite set of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps...

atic for eight-coordination
Tri-capped trigonal prismatic
Triaugmented triangular prism
In geometry, the triaugmented triangular prism is one of the Johnson solids . As the name suggests, it can be constructed by attaching square pyramids to each of the three equatorial faces of the triangular prism...

(Triaugmented triangular prism) for nine coordination.

Some exceptions and provisions should be noted:

The idealized descriptions of 5-, 7-, 8-, and 9- coordination are often indistinct geometrically from alternative structures with slightly different L-M-L (ligand-metal-ligand) angles. The classic example of this is the difference between square pyramidal and trigonal bipyramidal structures.
Due to special electronic effects such as (second-order) Jahn-Teller stabilization, certain geometries are stabilized relative to the other possibilities, e.g. for some compounds the trigonal prismatic geometry is stabilized relative to octahedral structures for six-coordination.

Isomerism

The arrangement of the ligands is fixed for a given complex, but in some cases it is mutable by a reaction that forms another stable isomer

Isomer

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

.

There exist many kinds of isomerism in coordination complexes, just as in many other compounds.

Stereoisomerism

Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms , but that differ only in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections...

occurs with the same bonds in different orientations relative to one another. Stereoisomerism can be further classified into:

Cis-trans isomerism and facial-meridional isomerism

Cis-trans isomerism occurs in octahedral and square planar

Plane (mathematics)

In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...

complexes (but not tetrahedral). When two ligands are mutually adjacent they are said to be cis, when
opposite each other, trans. When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac. In a fac isomer, any two identical ligands are adjacent or cis to each other. If these three ligands and the metal ion are in one plane, the isomer is said to be meridional, or mer. A mer isomer can be considered as a combination of a trans and a cis, since it contains both trans and cis pairs of identical ligands.

Optical isomerism

Optical isomerism occurs when the mirror image of a compound is not superimposable with the original compound. It is so called because such isomers are optically active, that is, they rotate the plane of polarized light. The symbol Λ (lambda) is used as a prefix to describe the left-handed propeller twist formed by three bidentate ligands, as shown. Likewise, the symbol Δ (delta) is used as a prefix for the right-handed propeller twist.

Structural isomerism

Structural isomerism, or constitutional isomerism , is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for constitutional isomers.Three categories of constitutional isomers...

occurs when the bonds are themselves different. Linkage isomerism is only one of several types of structural isomerism in coordination complexes (as well as other classes of chemical compounds). Linkage isomerism occurs with ambidentate ligands that can bind in more than one place. For example, NO₂ is an ambidentate ligand: It can bind to a metal at either the N atom or an O atom.

Electronic properties

Many of the properties of metal complexes are dictated by their electronic structures. The electronic structure can be described by a relatively ionic model that ascribes formal charges to the metals and ligands. This approach is the essence of 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 coordination complexes. CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes,...

(CFT). Crystal field theory, introduced by Hans Bethe

Hans Bethe

Hans Albrecht Bethe was a German-American nuclear physicist, and Nobel laureate in physics for his work on the theory of stellar nucleosynthesis. A versatile theoretical physicist, Bethe also made important contributions to quantum electrodynamics, nuclear physics, solid-state physics and...

in 1929, gives a quantum mechanically

Quantum mechanics

Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

based attempt at understanding complexes. But crystal field theory treats all interactions in a complex as ionic and assumes that the ligands can be approximated by negative point charges.

More sophisticated models embrace covalency, and this approach is described by 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. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...

(LFT) and Molecular orbital theory

Molecular orbital theory

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

(MO). Ligand field theory, introduced in 1935 and built from molecular orbital theory, can handle a broader range of complexes and can explain complexes in which the interactions are covalent. The chemical applications of group theory

Group theory

In mathematics and abstract algebra, group theory studies the algebraic structures known as groups.The concept of a group is central to abstract algebra: other well-known algebraic structures, such as rings, fields, and vector spaces can all be seen as groups endowed with additional operations and...

can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry based solutions to the formal equations.

Chemists tend to employ the simplest model required to predict the properties of interest; for this reason, CFT has been a favorite for the discussions when possible. MO and LF theories are more complicated, but provide a more realistic perspective.

The electronic configuration of the complexes gives them some important properties:

Color

Metal complexes often have spectacular colors caused by electronic transitions by the absorption of light. For this reason they are often applied as pigments. Most transitions that are related to colored metal complexes are either d-d transitions or charge transfer bands. In a d-d transition, an electron in a d orbital on the metal is excited by a photon to another d orbital of higher energy. A charge transfer band entails promotion of an electron from a metal-based orbital into an empty ligand-based orbital (Metal-to-Ligand Charge Transfer

Charge transfer complex

A charge-transfer complex or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic attraction provides a...

or MLCT). The converse also occurs: excitation of an electron in a ligand-based orbital into an empty metal-based orbital (Ligand to Metal Charge Transfer

Charge transfer complex

or LMCT). These phenomena can be observed with the aid of electronic spectroscopy; also known as UV-Vis. For simple compounds with high symmetry, the d-d transitions can be assigned using Tanabe-Sugano diagram

Tanabe-Sugano diagram

Tanabe-Sugano diagrams are used in coordination chemistry to predict absorptions in the UV and visible electromagnetic spectrum of coordination compounds. The results from a Tanabe-Sugano diagram analysis of a metal complex can also be compared to experimental spectroscopic data...

s. These assignments are gaining increased support with computational chemistry

Computational chemistry

Computational chemistry is a branch of chemistry that uses principles of computer science to assist in solving chemical problems. It uses the results of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids...

Colours of Various Example Coordination Complexes
	Fe^II	Fe^III	Co^II	Cu^II	Al^III	Cr^III
Hydrated Ion	Pale green	Yellow/brown	Pink	Blue	Colourless	Green
OH^–, dilute	Dark green	Brown	Blue/green	Blue	White	Green
OH^–, concentrated	Dark green	Brown	Blue/green	Blue	Colourless	Green
NH₃, dilute	Dark green	Brown	Blue/green	Blue	White	Green
NH₃, concentrated	Dark green	Brown	Straw coloured	Deep blue	White	Green
CO₃^2–	Dark green	Brown	Pink	Blue/green

Magnetism

Metal complexes that have unpaired electrons are magnetic. Considering only monometallic complexes, unpaired electrons arise because the complex has an odd number of electrons or because electron pairing is destabilized. Thus, monomeric Ti(III) species have one "d-electron" and must be (para)magnetic

Paramagnetism

Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...

, regardless of the geometry or the nature of the ligands. Ti(II), with two d-electrons, forms some complexes that have two unpaired electrons and others with none. This effect is illustrated by the compounds TiX₂[(CH₃)₂PCH₂CH₂P(CH₃)₂]₂: when X = Cl, the complex is paramagnetic (high-spin configuration), whereas when X=CH₃, it is diamagnetic (low-spin configuration). It is important to realize that ligands provide an important means of adjusting the ground state properties.

In bi- and polymetallic complexes, in which the individual centers have an odd number of electrons or that are high-spin, the situation is more complicated. If there is interaction (either direct or through ligand) between the two (or more) metal centers, the electrons may couple (antiferromagnetic coupling

Antiferromagnetism

In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...

, resulting in a diamagnetic compound), or they may enhance each other (ferromagnetic coupling

Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...

). When there is no interaction, the two (or more) individual metal centers behave as if in two separate molecules.

Reactivity

Complexes show a variety of possible reactivities:

Electron transfers

A common reaction between coordination complexes involving ligands are inner
Inner sphere electron transfer
Inner sphere or bonded electron transfer proceeds via a covalent linkage between the two redox partners, the oxidant and the reductant. In Inner Sphere electron transfer , a ligand bridges the two metal redox centers during the electron transfer event. Inner sphere reactions are inhibited by...

and outer sphere electron transfer
Outer sphere electron transfer
Outer sphere refers to an electron transfer event that occurs between chemical species that remain separate intact before, during, and after the ET event. In contrast, for inner sphere electron transfer the participating redox sites undergoing ET become connected by a chemical bridge...

s. They are two different mechanisms of electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...

redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

reactions, largely defined by the late Henry Taube
Henry Taube
Henry Taube, Ph.D, M.Sc, B.Sc, FRSC was a Canadian-born American chemist noted for having been awarded the 1983 Nobel Prize in Chemistry for "his work in the mechanisms of electron-transfer reactions, especially in metal complexes." He was the first Canadian-born chemist to win the Nobel Prize...

. In an inner sphere reaction, a ligand with two lone electron pairs acts as a bridging ligand
Bridging ligand
A bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...

, a ligand to which both coordination centres can bond. Through this, electrons are transferred from one centre to another.
(Degenerate) ligand exchange

One important indicator of reactivity is the rate of degenerate exchange of ligands. For example, the rate of interchange of coordinate water in [M(H₂O)₆]ⁿ⁺ complexes varies over 20 orders of magnitude. Complexes where the ligands are released and rebound rapidly are classified as labile. Such labile complexes can be quite stable thermodynamically. Typical labile metal complexes either have low-charge (Na⁺), electrons in d-orbitals that are antibonding with respect to the ligands (Zn²⁺), or lack covalency (Ln³⁺, where Ln is any lanthanide). The lability of a metal complex also depends on the high-spin vs. low-spin configurations when such is possible. Thus, high-spin Fe(II) and Co(III) form labile complexes, whereas low-spin analogues are inert. Cr(III) can exist only in the low-spin state (quartet), which is inert because of its high formal oxidation state, absence of electrons in orbitals that are M-L antibonding, plus some "ligand field stabilization" associated with the d3 configuration.
Associative processes

Complexes that have unfilled or half-filled orbitals often show the capability to react with substrates. Most substrates have a singlet ground-state; that is, they have lone electron pairs (e.g., water, amines, ethers), so these substrates need an empty orbital to be able to react with a metal centre. Some substrates (e.g., molecular oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

) have a triplet ground state, which results that metals with half-filled orbitals have a tendency to react with such substrates (it must be said that the dioxygen-molecule also has lone pairs, so it is also capable to react as a 'normal' Lewis base).

If the ligands around the metal are carefully chosen, the metal can aid in (stoichiometric or catalytic) transformations of molecules or be used as a sensor.

Classification

Metal complexes, also known as coordination compounds, include all metal compounds, aside from metal vapors, plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

s, and alloy

Alloy

An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

s. The study of "coordination chemistry" is the study of "inorganic chemistry" of all alkali

Alkali metal

The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

and alkaline earth metal

Alkaline earth metal

The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...

s, transition metal

Transition metal

The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...

s, lanthanide

Lanthanide

The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...

s, actinides, and metalloid

Metalloid

Metalloid is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, each element can usually be classified as a metal or a nonmetal. However, some elements with intermediate or mixed properties can be harder to characterize...

s. Thus, coordination chemistry is the chemistry of the majority of the periodic table. Metals and metal ions exist, in the condensed phases at least, only surrounded by ligands.

The areas of coordination chemistry can be classified according to the nature of the ligands, in broad terms:

Classical (or "Werner
Alfred Werner
Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...

Complexes"): Ligands in classical coordination chemistry bind to metals, almost exclusively, via their "lone pair
Lone pair
In chemistry, 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" of electrons residing on the main group atoms of the ligand. Typical ligands are H₂O, NH₃, Cl⁻
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

, CN⁻
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

, en⁻

Examples: [Co(EDTA

EDTA

Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...

)]⁻, [Co(NH₃)₆]Cl₃

Cobalt(III) hexammine chloride

Hexamminecobalt chloride is the chemical compound with the formula [Co6]Cl3. This coordination compound is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. This salt consists of [Co6]3+ trications with three Cl− anions...

, [Fe(C₂O₄)₃]K₃

Potassium ferrioxalate

Potassium ferrioxalate, also known as potassium oxalatoferrate, is a chemical compound with the formula K3[Fe3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion,...

Organometallic Chemistry: Ligands are organic (alkenes, alkynes, alkyls) as well as "organic-like" ligands such as phosphines, hydride, and CO.

Example: (C₅H₅)Fe(CO)₂CH₃

Cyclopentadienyliron dicarbonyl dimer

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula 2Fe24, also abbreviated Cp2Fe24. It is called Fp2 or "fip dimer." It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but...

Bioinorganic Chemistry: Ligands are those provided by nature, especially including the side chains of amino acids, and many cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

s such as porphyrin
Porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...

s.

Example: hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

Many natural ligands are "classical" especially including water.

Cluster Chemistry: Ligands are all of the above also include other metals as ligands.

Example Ru₃(CO)₁₂

In some cases there are combinations of different fields:

Example: [Fe₄S₄(Scysteinyl)₄]²⁻

Iron-sulfur protein

Iron-sulfur proteins are proteins characterized by the presence of iron-sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states...

, in which a cluster is embedded in a biologically active species.

Mineralogy

Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...

, materials science

Materials science

Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...

, and solid state chemistry - as they apply to metal ions - are subsets of coordination chemistry in the sense that the metals are surrounded by ligands. In many cases these ligands are oxides or sulfides, but the metals are coordinated nonetheless, and the principles and guidelines discussed below apply. In hydrate

Water of crystallization

In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...

s, at least some of the ligands are water molecules. It is true that the focus of mineralogy, materials science, and solid state chemistry differs from the usual focus of coordination or inorganic chemistry. The former are concerned primarily with polymeric structures, properties arising from a collective effects of many highly interconnected metals. In contrast, coordination chemistry focuses on reactivity and properties of complexes containing individual metal atoms or small ensembles of metal atoms.

Older classifications of isomerism

Traditional classifications of the kinds of isomer have become archaic with the advent of modern structural chemistry. In the older literature, one encounters:

Ionisation isomerism describes the possible isomers arising from the exchange between the outer sphere and inner sphere. This classification relies on an archaic classification of the inner and outer sphere. In this classification, the "outer sphere ligands," when ions in solution, may be switched with "inner sphere ligands" to produce an isomer.
Solvation isomerism occurs when an inner sphere ligand is replaced by a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

molecule. This classification is obsolete because it considers solvents as being distinct from other ligands. Some of the problems are discussed under water of crystallization
Water of crystallization
In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...

.

Naming complexes

The basic procedure for naming a complex:

When naming a complex ion, the ligands are named before the metal ion.
Write the names of the ligands in alphabetical order. (Numerical prefixes do not affect the order.)
- Multiple occurring monodentate ligands receive a prefix according to the number of occurrences: di-, tri-, tetra-, penta-, or hexa. Polydentate ligands (e.g., ethylenediamine, oxalate) receive bis-, tris-, tetrakis-, etc.
- Anions end in ido. This replaces the final 'e' when the anion ends with '-ate', e.g. sulfate becomes sulfato. It replaces 'ide': cyanide becomes cyanido.
- Neutral ligands are given their usual name, with some exceptions: NH₃ becomes ammine
  Ammine
  In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelled with a single "m"...
  
  ; H₂O becomes aqua or aquo; CO becomes carbonyl; NO becomes nitrosyl.
Write the name of the central atom/ion. If the complex is an anion, the central atom's name will end in -ate, and its Latin name will be used if available (except for mercury).
If the central atom's oxidation state needs to be specified (when it is one of several possible, or zero), write it as a Roman numeral (or 0) in parentheses.
Name cation then anion as separate words (if applicable, as in last example)

Examples:

[NiCl₄]²⁻ → tetrachloridonickelate(II) ion

[CuNH₃Cl₅]³⁻ → amminepentachloridocuprate(II) ion

[Cd(en)₂(CN)₂] → dicyanidobis(ethylenediamine)cadmium(II)

[Co(NH₃)₅Cl]SO₄ → pentaamminechloridocobalt(III) sulfate

The coordination number of ligands attached to more than one metal (bridging ligands) is indicated by a subscript to the Greek symbol μ placed before the ligand name. Thus the dimer of aluminium trichloride is described by Al₂Cl₄(μ₂-Cl)₂.

Application of coordination compounds

They are used in photography, i.e., AgBr forms a soluble complex with sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...

in photography.
K[Ag(CN)₂] is used for electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...

of silver, and K[Au(CN)₂]is used for gold plating
Gold plating
Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver , by chemical or electrochemical plating...

.
Some ligands oxidise Co²⁺ to Co³⁺ ion.
Ethylenediaminetetraacetic acid (EDTA) is used for estimation of Ca²⁺ and Mg²⁺in hard water
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...

.
Silver and gold are extracted by treating zinc with their cyanide complexes.

External links

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