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

A complex in chemistry usually is used to describe molecules or ensembles formed by the combination of ligands and metal ions. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Originally, a complex implied a reversible association of molecules, atoms, or ions through weak chemical bonds. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic 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.

Contents

History

Coordination complexes were known - although not understood in any sense - since the beginning of chemistry, e. g. Prussian blue and copper vitriol. Prussian blue is a very dark blue colorfast non-toxic Pigment – one of the first synthetic Dyes – which was discovered accidentally in Berlin in 1704 Copper(II sulfate is the Chemical compound with the formula Cu[[Sulfur S]] O 4 The key breakthrough occurred when Alfred Werner proposed, inter alia, that Co(III) bears six ligands in an octahedral geometry. Alfred Werner ( December 12, 1866 - November 15, 1919) was a Swiss chemist who was a professor at the University The theory allows one to understand the difference between coordinated and ionic chloride in the cobalt ammine chlorides and to explain many of the previously inexplicable isomers. He resolved the first coordination complex into optical isomers, overthrowing the theory that chirality was necessarily associated with carbon compounds.

Metal complexes

Metal complexes, also known as coordination compounds, include all metal compounds, aside from metal vapors, plasmas, and alloys. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound An alloy is a Solid solution or Homogeneous mixture of two or more elements, at least one of which is a Metal, which itself has The study of "coordination chemistry" is the study of "inorganic chemistry" of all alkali and alkaline earth metals, transition metals, lanthanides, actinides, and metalloids. Trends The alkali metals show a number of trends when moving down the group - for instance decreasing electronegativity increasing reactivity and decreasing melting and boiling Biological occurrences Beryllium's low aqueous solubility means it is rarely available to biological systems it has no known role in living organisms and when encountered In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in Terminology The Trivial name " Rare earths " is sometimes used to describe all the lanthanoids together with Scandium and Yttrium History of the actinoid series From the earlier known chemical properties of actinium (89 up to uranium (92 indicating a relation to the Transition metals it was generally Metalloid is a term used in Chemistry when classifying the Chemical elements On the basis of their general physical and chemical properties nearly every element 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. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally

The ions or molecules surrounding the metal are called ligands. Ligands are generally bound to a metal ion by a coordinate covalent bond (donating electrons from a lone electron pair into an empty metal orbital), and are thus said to be coordinated to the ion. A coordinate covalent bond (formerly also known as dative bond) is a description of Covalent bonding between two atoms in which both electrons shared in the bond come The areas of coordination chemistry can be classified according to the nature of the ligands, broadly speaking:

Examples: [Co(EDTA)]−, [Co(NH3)6]Cl3, [Fe(C2O4)3]K3
Example: (C5H5)Fe(CO)2CH3
Example: hemoglobin
Many natural ligands are "classical" especially including water. Hemoglobin ( also spelled haemoglobin and abbreviated Hb or Hgb) is the Iron -containing Oxygen -transport Metalloprotein
Example Ru3(CO)12
Example: [Fe4S4(Scysteinyl)4]2−, in which a cluster is embedded in a biologically active species. Iron-sulfur proteins are Proteins characterized by the presence of iron-sulfur clusters containing Sulfide -linked di- tri- and tetrairon centers in variable

Mineralogy, 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. Mineralogy is an Earth Science focused around the Chemistry, Crystal structure, and physical (including optical) properties of Minerals Materials Science or Materials Engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of Science and Solid-state chemistry is the study of the synthesis structure and physical properties of solid materials 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 mineralogy, materials science, and solid state chemistry differs from the usual focus of 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, coordination chemistry focuses on reactivity and properties of complexes containing individual metal atoms or small ensembles of metal atoms.

Structure of coordination compounds

Geometry

Structure in complexation chemistry is first described by its "coordination number", the number of ligands attached to the metal (more specifically, the number of σ-type bonds between ligand(s) and the central atom). 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 of the metal ion and the ligands. In Atomic physics and Quantum chemistry, electron configuration is the arrangement of Electrons in an Atom, Molecule, or other Metal ions may have more than one coordination number.

Typically the chemistry of complexes is dominated by interactions between s and p molecular orbitals of the ligands and the d orbitals of the metal ions. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. The s, p, and d orbitals of the metal can accommodate 18 electrons (see 18-Electron rule; for f-block elements, this extends to 32 electrons). The 18-electron rule is a Rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes 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. Terminology The Trivial name " Rare earths " is sometimes used to describe all the lanthanoids together with Scandium and Yttrium History of the actinoid series From the earlier known chemical properties of actinium (89 up to uranium (92 indicating a relation to the Transition metals it was generally

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. What is a polyhedron? We can at least say that a polyhedron is built up from different kinds of element or entity each associated with a different number of dimensions The most observed geometries are listed below, but there are many cases which 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 effects (see e. g. Jahn-Teller distortion):

Some exceptions and provisions should be noted:

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. This article is about the chemical concept For "isomerism" of atomic nuclei see Nuclear isomer.

There exist many kinds of isomerism in coordination complexes, just as in many other compounds. This article is about the chemical concept For "isomerism" of atomic nuclei see Nuclear isomer.

Stereoisomerism

Stereoisomerism occurs with the same bonds in different orientations relative to one another. Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution but which differ in the three dimensional orientations Stereoisomerism can be further classified into:

Geometric isomerism

Geometric isomerism occurs in octahedral and square planar complexes (but not tetrahedral). Trans-2-butenesvg|right|thumb|Trans-2-butene]] In Chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of An octahedron (plural octahedra is a Polyhedron with eight faces When two ligands are opposite 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 and the metal ion 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.

cis-[CoCl2(NH3)4]+
trans-[CoCl2(NH3)4]+
fac-[CoCl3(NH3)3]
mer-[CoCl3(NH3)3]

Optical isomerism

Optical isomerism occurs when the mirror image of a compound is not superimposable with the original compound. The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image It is so called because such isomers are optically active, that is, they rotate the plane of polarized light. Optical rotation or optical activity is the rotation of linearly polarized Light as it travels through certain materials Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations The symbol Λ (lambda) is used as a prefix to describe the left-handed propeller twist formed by three bidentate ligands, as shown. Similarly, the symbol Δ (delta) is used as a prefix for the right-handed propeller twist.

Λ-[Fe(ox)3]3−
Δ-[Fe(ox)3]3−
Λ-cis-[CoCl2(en)2]+
Δ-cis-[CoCl2(en)2]+

Structural isomerism

Structural isomerism occurs when the bonds are themselves different. Structural isomerism, or constitutional isomerism, is a form of isomerism in which Molecules with the same Molecular formula have Atoms Linkage isomerism is only one of several types of structural isomerism in coordination complexes (as well as other classes of chemical compounds). Linkage isomerism is the existence of co-ordination compounds that have the same composition differing with the connectivity of the metal to a Ligand. Linkage isomerism occurs with ambidentate ligands which 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 at an O atom.

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:

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 (CFT is a model that describes the Electronic structure of Transition metal compounds all of which can be considered coordination Crystal field theory, introduced by Hans Bethe in 1929, gives a quantum mechanically based attempt at understanding complexes. Hans Albrecht Bethe (/hans ˈalbʀɛçt ˈbeːtə/ ( July 2 1906 &ndash March 6, 2005) was a German - American Physicist Year 1929 ( MCMXXIX) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons 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 (LFT) and Molecular orbital theory (MO). Ligand field theory (LFT describes the bonding in Coordination complexes. In Chemistry, molecular orbital theory ( MO theory) is a method for determining molecular structure in which Electrons are not assigned to individual 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. Year 1935 ( MCMXXXV) was a Common year starting on Tuesday (link will display full calendar of the Gregorian calendar. 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. Group theory is a mathematical discipline the part of Abstract algebra that studies the Algebraic structures known as 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 are caused by electronic transitions by the absorption of light. 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 electron from a metal-based orbital into an empty ligand based orbital (Metal to Ligand Charge Transfer or MLCT). The converse also occurs: excitation an electron in a ligand-based orbital into an empty metal-based orbital (Ligand to Metal Charge Transfer or LMCT). These phenomenon can be observed with the aid of electronic spectroscopy; also known colloguially as "UV-Vis. "[1] For simple compounds with high symmetry, the d-d transitions can be assigned using Tanabe-Sugano diagrams. Tanabe-Sugano diagrams are used in Coordination chemistry to predict absorptions in the UV and visible Electromagnetic spectrum of Coordination compounds Increasingly, these assignments can be confirmed using computational chemistry. Computational chemistry is a branch of Chemistry that uses computers to assist in solving chemical problems

Magnetism

Metal complexes that have unpaired electrons are magnetic. In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. 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. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field 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). Crystal field theory (CFT is a model that describes the Electronic structure of Transition metal compounds all of which can be considered coordination Crystal field theory (CFT is a model that describes the Electronic structure of Transition metal compounds all of which can be considered coordination 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 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). 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 Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it 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:

A common reaction between coordination complexes involving ligands are inner and outer sphere electron transfers. Inner sphere or bonded electron transfer proceeds via a Covalent linkage between the two redox partners the oxidant and the reductant Outer sphere or nonbonded electron transfer refers to an Electron transfer (ET event that occurs between chemical moieties that remain separate species before during They are two different mechanisms of electron transfer redox reactions, largely defined by the late Henry Taube. Electron transfer (ET is the process by which an Electron moves from one Atom or Molecule to another atom or molecule Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state Professor Henry Taube PhD, MSc, BSc, FRSC ( November 30, 1915 &ndash November 16, 2005 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. 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 A bridging Ligand is an Atom or a polyatomic entity that connects two or more Metal centres in a complex. Through this, electrons are transferred from one centre to another.
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 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 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) 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). Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

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. Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry is the Calculation of Quantitative (measurable Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst

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. )
    • 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 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 or aquo; CO becomes carbonyl; NO becomes nitrosyl. In Coordination chemistry an Ammonia Ligand is called an ammine.
  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 (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. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces Aluminium chloride (AlCl3 is a compound of Aluminium and Chlorine.


See also

References

  1. ^ Harris, D. The IUPAC nomenclature of inorganic chemistry is a systematic method of naming Inorganic Chemical compounds as recommended by the International Union , Bertolucci, M. , Symmetry and Spectroscopy. 1989 New York, Dover Publications

External links

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