Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate.

A complex in chemistry originally implied a reversible association of molecules, atoms, or ions through weak non-covalent chemical bonds. As applied to coordination chemistry, this original meaning has changed. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are covalent and quite strong. The combination of the metal (ion) and the ligands is called coordination chemistry. I, the creator of this image, hereby release it into the public domain. ... Chemistry (derived from alchemy) is the science of matter at or near the atomic scale. ... In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. ... Properties In chemistry and physics, an atom (Greek άτομον meaning indivisible) is the smallest possible particle of a chemical element that retains its chemical properties. ... // An ion is an atom, group of atoms, or subatomic particle with a net electric charge. ... Covalently bonded hydrogen and carbon in a molecule of methane. ... A chemical bond is the physical phenomenon of chemical substances being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons or electrostatic forces. ...

Metal complexes

Metal complexes, also known as coordination compounds, include all metal compounds, aside from metal vapors, plasmas, and alloys. The study of "coordination chemistry" is the study of "inorganic chemistry" of all alkali and alkaline earth metals, transition metals, lanthanides, actinides, and metalloids. Thus, coordination chemistry is the chemistry of the majority of the periodic table. Metals and metal ions only exist, in the condensed phases at least, surrounded by ligands. It has been suggested that this article or section be merged with Ligand (biochemistry). ...

The ions or molecules surrounding the metal are called ligands. Ligands are generally bound to a metal ion by a coordinate covalent bond, and are thus said to be coordinated to the ion. The areas of coordination chemistry can be classified according to the nature of the ligands, broadly speaking: It has been suggested that this article or section be merged with Ligand (biochemistry). ... A coordinate covalent bond (also known as dative covalent bond) is a special type of covalent bond in which the shared electrons come from one of the atoms only. ...

• Classical (or "Werner Complexes): Ligands in classical coordination chemistry bind to metals, almost exclusively, via their "lone pairs" of electrons residing on the main group atoms of the ligand. Typical ligands are H₂O, NH₃, Cl^-, CN^-

Example: [Fe(EDTA)]^-]

• 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

• Bioinorganic Chemistry: Ligands are those provided by nature, especially including the side chains of amino acids, and many cofactors such as porphyrins.

Example: hemoglobin

Many natural ligands are "classical" especially including water.

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

Example Ru₃(CO)₁₂

• Some examples are combinations of different fields:

Example: [Fe₄S₄(Scysteinyl)₄]^2-, in which a cluster is embedded in a biologically active species.

Minerology, materials science, 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. It is true that the focus of minerology, materials science, and solid state chemistry differs from the usual focus of a coordination or inorganic chemistry. The former are primarily concerned with polymeric structures, properties arising from a collective effects of many highly interconnected metals. In contrast, the usual focus of coordination chemistry is on reactivity and properties of individual metal complexes or small ensembles. Alfred Werner (December 12, 1866 - November 15, 1919) was a German Nobel prize-winning chemist. ... A lone pair is an electron pair without bonding or sharing with other atoms. ... 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 are also called chlorides. ... A cyanide is any chemical compound that contains the cyano group C≡N, with the carbon atom triple-bonded to the nitrogen atom. ... EDTA Metal-EDTA chelate This is a computer generated image of EDTAs 3-dimensional structure. ... Cyclopentadienyliron dicarbonyl dimer is an organometallic complex with the formula (C5H5)2Fe2(CO)4, also abbreviated Cp2Fe2(CO)4. ... A cofactor is the following: In mathematics a cofactor is the minor of an element of a square matrix. ... A porphyrin is a heterocyclic macrocycle made from 3 pyrrole subunits and one pyrroline subunit, and linked on opposite sides through 4 methine bridges. ... 3-dimensional structure of hemoglobin. ... Iron-sulfur proteins are proteins characterized by the presence of polymetallic systems (iron-sulfur clusters) containing sulfide ions, in which the iron ions have variable oxidation states. ...

Structure of coordination compounds

Geometry

Structure in complex chemistry begins with a focus on "coordination number", the number of ligands attached to the metal. 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 uncommon. The number of bonds depends on the size, charge, and electron configuration of the metal ion. Most metal ions may have more than one coordination number. Electron atomic and molecular orbitals In atomic physics and quantum chemistry, the electron configuration is the arrangement of electrons in an atom, molecule or other body. ...

Typically the chemistry of complexes is dominated by interactions between s and p molecular orbitals of the ligands and the d (or f) orbitals of the metal ions. The s, p and d orbitals of the metal give the possibility to allocate 18 electrons (see 18-Electron rule; for f-block elements this extends to 32 electrons). The maximum coordination number for a certain metal is thus related to the electronic configuration of the metal ion (more specifically, 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, lanthanides, actinides, and early transition metals tend to have high coordination numbers. Electron atomic and molecular orbitals In quantum chemistry (electronic structure theory), the molecular electronic states, i. ... The valence shells of a transition metal can accommodate 18 electrons: 2 each in the five d orbitals, or 10 total; 2 each in the 3 p orbitals, or 6 total; and finally 2 in the s orbital (see Electron counting). ... The lanthanide series comprises the 15 elements from lanthanum to lutetium on the periodic table, with atomic numbers 57 through 71. ... The actinide series encompasses the 15 chemical elements that lie between actinium and lawrencium on the periodic table with atomic numbers 89 - 103. ...

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 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 (although many exceptions exist): This article is about the geometric shape. ...

• Linear for two-coordination,
• Trigonal planar for three-coordination,
• Tetrahedral or [[Plane (mathematics)|square planar] for four-coordination
• Trigonal bipyramidal or square planar - pyramidal for five-coordination,
• octahedral for six-coordination,
• Pentagonal bipyramidal for seven-coordination,
• Square antiprism for eight-coordination, and
• Tri-capped trigonal prismatic for nine coordination.

Some exceptions and provisions should be noted: The word line derives from the Latin lingui, meaning flax plant from which linen is produced; at one time, a stretched linen thread was the most reliable way to determine a straight line. ... A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ... The octahedral molecular geometry is a part of coordination chemistry and describes a molecular geometry in which atoms or ligands are arranged around a central atom with 4 of them in the same plane as the central atom at the corners of a square (basal positions) and two more at...

• The idealized descriptions of 5-, 7-, 8-, and 9- coordination are often indistinct geometrically from alternative structures with slightly different L-M-L 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. In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ...

There exists many kinds types of isomerism in coordination complexes, just as in many other compounds. Stereoisomerism occurs with the same bonds in different orientations relative to one another. Structural isomerism occurs when the bonds are themselves different. In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ... Main article: stereochemistry Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ... Structural isomerism is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders (as opposed to stereoisomerism). ...

Stereoisomerism can be further classified into geometric isomerism and optical isomerism. The latter occurs when compounds have mirror images of themselves. It is so called because such isomers are optically active, that is, they rotate plane-polarised light. The former occurs in octahedral and square planar complexes (but not tetrahedral). When two ligands are oppositie each other they are said to be trans, when mutually adjacent, cis. When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac. If these three ligands are coplanar, the isomer is said to be meridional, or mer. For example, in an octahedral compound with three of one ligand and three of another, there are two geometric isomers: the mer in which each set of three same ligands is in a meridian and the fac in which each set of three is on a face of the octahedron. In chemistry, geometric isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Optical isomerism is a form of isomerism (specifically stereoisomerism) where the two different isomers are the same in every way except being non-superposable mirror images of each other. ... When polarized light is passed through a substance containing chiral molecules (or nonchiral molecules arranged asymmetrically), the direction of polarization can be changed. ...

Linkage isomerism is only one of several types of structural isomerism in coordination complexes (as well as other classes of chemical compounds). Linkage isomerism, as described above, occurs with ambidentate ligands which can bind in more than one place.

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.
• Solvate isomerism occurs when an inner sphere ligand is replaced by a solvent 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.

A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... Water of crystallization is water that is tightly associated with crystalline metal salts, and remains after drying in a fixed proportion to the salt. ...

Electronic structure of coordination compounds

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 and does not focus on covalency. This approach is the essence of Crystal field theory (CFT). Crystal field theory, introduced by Hans Bethe in 1929, gives a quantum mechanically 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. Crystal field theory is used to describe the electronic structure of transition metal complexes. ... Hans Bethe in 1945. ... 1929 (MCMXXIX) was a common year starting on Tuesday (link will take you to calendar). ... For a non-technical introduction to the topic, please see Introduction to Quantum mechanics. ...

More sophisticated models embrace covalency, and this approach is described by Ligand field theory (LFT) and Molecular orbital theory (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 can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry based solutions to the formal equations. Ligand field theory was developed during the thirties and fourties of the twentieth century as an expansion of the electrostatic crystal field theory, which offered a good description of the electronic structure of metal ions in coordination complexes but was not able to provide a proper explanation for their bonding. ... In quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. ... 1935 (MCMXXXV) was a common year starting on Tuesday (link will take you to calendar). ... Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ... Group theory is that branch of mathematics concerned with the study of groups. ...

Chemists tend to employ the simplest model required to predict the propeties 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. These colors arises from the absorption of light which excites electrons within the metal's orbitals or, in the case of charge-transfer, excites electrons from metal-based orbitals to ligand-based orbitals (or the reverse). An aid to explain and interpret electronic excitations within the collection of metal-based orbitals are the Tanabe-Sugano diagrams (for simple compounds with a good defined geometry) or the use of computational chemistry. Computational chemistry is a branch of chemistry that uses the results of theoretical chemistry incorporated into efficient computer programs to calculate the structures and properties of molecules and solids, applying these programs to real chemical problems. ...

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, 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 physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. ... Paramagnetism is the tendency of the atomic magnetic dipoles to align with an external magnetic field. ... Ligand field theory was developed during the thirties and fourties of the twentieth century as an expansion of the electrostatic crystal field theory, which offered a good description of the electronic structure of metal ions in coordination complexes but was not able to provide a proper explanation for their bonding. ... Ligand field theory was developed during the thirties and fourties of the twentieth century as an expansion of the electrostatic crystal field theory, which offered a good description of the electronic structure of metal ions in coordination complexes but was not able to provide a proper explanation for their bonding. ...

In bi- and polymetallic complexes, in which the individual centers have an odd number of electrons or which 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, resulting in a diamagnetic compound), or they may enhance each other (ferromagnetic coupling). When there is no interaction, the two (or more) individual metal centers behave as if in two separate molecules. In materials that exhibit antiferromagnetism, the spins of magnetic electrons align in a regular pattern with neighboring spins pointing in opposite directions. ... Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ...

Reactivity

A common reaction between coordination complexes involving ligands are inner and outer sphere electron transfers. They are two different mechanisms of electron transfer redox reactions, largely defined by the late Henry Taube. In an inner sphere reaction, a ligand with two lone electron pairs acts as a bridging ligand, a ligand to which both coordination centres can bond. Through this, electrons are transferred from one centre to another. Inner sphere electron transfer is a specific type of electron transfer reaction (ET) in which a common ligand bridges the two metal redox centers during the electron transfer event. ... Electron transfer (ET) is the process by which an electron moves from one atom or molecule to another atom or molecule. ... Redox reactions include all chemical processes in which atoms have their oxidation number (oxidation state) changed. ... Professor Henry Taube, Ph. ... A lone pair is an electron pair without bonding or sharing with other atoms. ...

The reactivity of metal complexes can be predicted from the electronic and atomic structure. 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 thermodyanically. Typical labile metal complexes have either 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 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 only exist 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.

Complexes that do not have a closed shell electronic configuration often show the capability to react with substrates. If the ligands around the metal are carefully chosen, the metal can aid in transformations of molecules, be used as a sensor, or in catalysis. Generic graph showing the effect of a catalyst in an hypotetical exothermic chemical reaction. ...

Naming complexes

The basic procedure for naming a complex:

1. When naming a complex ion, the ligands are named before the metal ion.
2. Write the names of the ligands in alphabetical order. (Numerical prefixes do not affect the order.)
   • Multiply 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 o. This replaces the final 'e' when the anion ends with '-ate', e.g. sulfate becomes sulfato. It replaces 'ide': cyanide becomes cyano.
   • Neutral ligands are given their usual name, with some exceptions: NH₃ becomes ammine; H₂O becomes aqua; CO becomes carbonyl; NO becomes nitrosyl.
3. 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).
4. 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.
5. Name cation then anion as separate words (if applicable, as in last example)

Examples:

[NiCl₄]^2- → tetrachloronickelate(II) ion

[CuNH₃Cl₅]^3- → amminepentachlorocuprate(II) ion

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

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

The coordination number of ligands attached to more than one metal is indicated by a subscript to the Greek symbol μ. Thus the dimer of aluminium trichloride is described by Al₂Cl₄(μ₂-Cl)₂. Sucrose, or common table sugar, is composed of glucose and fructose. ...

Receptor-ligand complexes

Receptors are proteins that bind small ligands. A typical example of a receptor-ligand complex is a neurotransmitter bound to a neurotransmitter receptor in the cell membrane of the synapse. The dissociation constant K_d is used as an indicator of the electron affinity of the ligand to the receptor. In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... Transmembrane receptors are integral membrane proteins, which reside and operate typically within a cells plasma membrane, but also in the membranes of some subcellular compartments and organelles. ... Drawing of a cell membrane A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... Illustration of the major elements in a prototypical synapse. ... In chemistry, electron affinity is the amount of energy absorbed when an electron is added to a neutral isolated gaseous atom to form a gaseous ion with a 1- charge. ...

See also

• Inclusion compounds
• Organometallic chemistry deals with a special class of coordination compounds where organic fragments are bonded to a metal.

In host-guest chemistry an inclusion compound is a complex in which one chemical compound the host forms a cavity which molecules of a second compound the guest are located. ... Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. ...

References

• De Vito, D.; Weber, J. ; Merbach, A. E. “Calculated Volume and Energy Profiles for Water Exchange on t_2g ⁶ Rhodium(III) and Iridium(III) Hexaaquaions: Conclusive Evidence for an I_a Mechanism” Inorganic Chemistry, 2005, Volume 43, pages 858-863.
• Zumdahl, Steven S. Chemical Principles, Fifth Edition. New York: Houghton Mifflin, 2005. 943-946, 957.

http://www.chemistry.wustl.edu/~edudev/LabTutorials/naming_coord_comp.html

External links

• Naming Coordination Compounds