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[[File:Cisplatin-3D-balls.png|thumb|200px|<center>[[Cisplatin]], PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub><br />A platinum atom with four [[ligand]]s</center>]]

In [[chemistry]], a '''coordination complex''' or '''metal complex''', is a structure consisting of a central atom or ion (usually metallic), [[chemical bond|bonded]] to a surrounding array of molecules or anions ([[ligand]]s, complexing agents).
<ref>[http://goldbook.iupac.org/C01203.html IUPAC definition of complex]</ref>
<ref>[http://goldbook.iupac.org/C01330.html IUPAC definition of coordination entity]</ref>
The atom within a ligand that is directly bonded to the central atom or ion is called the '''donor atom'''. [[Ligand#Polydentate_and_polyhapto_ligand_motifs_and_nomenclature|Polydentate]] (multiple bonded) ligands can form a [[chelate complex]]. A ligand donates at least one [[lone pair|pair of electrons]] to the central atom/ion.

Compounds that contain a coordination complex are called '''coordination compounds'''. The
central atom or ion, together with all ligands form the [[coordination sphere]].
<ref>[http://www.chemistry-dictionary.com/definition/coordination+sphere.php chemistry-dictionary.com - Definition of coordination sphere]</ref>
<ref>[http://www.chem.purdue.edu/gchelp/cchem/whatis.html ''What Is A Coordination Compound?'']</ref>

'''Coordination''' refers to the "coordinate covalent bonds" ([[dipolar bond]]s) between the ligands and the central atom. Originally, a complex implied a reversible association of [[molecule]]s, [[atom]]s, or [[ion]]s through such weak [[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.<ref>{{Cite book|isbn=0471199575, 9780471199571|pages=1355|last=Cotton|first=Frank Albert|coauthors=Geoffrey Wilkinson, Carlos A. Murillo|title=Advanced Inorganic Chemistry|year=1999}}</ref><ref>{{Cite book|isbn=0138418918, 9780138418915|pages=642|last=Miessler|first=Gary L.|coauthors=Donald Arthur Tarr|title=Inorganic Chemistry|year=1999}}</ref>
[[Image:Copper complex.jpg|frame|right|Synthesis of copper(II)-tetraphenylporphyrin, a metal complex, from tetraphenylporphyrin and copper(II) acetate monohydrate.]]

==History==
{{main|Alfred Werner}}
[[File:Hexol1.gif|thumb|Structure of hexol]]
Coordination complexes were known - although not understood in any sense - since the beginning of chemistry, e.g. [[Prussian blue]] and [[Copper(II) sulfate|copper vitriol]].  The key breakthrough occurred when [[Alfred Werner]] 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]] chlorides and to explain many of the previously inexplicable isomers.

In 1914, Werner resolved the first coordination complex, called [[hexol]], into optical isomers, overthrowing the theory that [[Chirality (chemistry)|chirality]] was necessarily associated with carbon compounds.

==Structure of coordination complexes==
The ions or molecules surrounding the central atom are called ligands. Ligands are generally bound to the central atom by a [[coordinate covalent bond]] (donating electrons from a [[lone pair|lone electron pair]] into an empty metal orbital), and are said to be '''coordinated''' to the atom. There are also organic ligands such as [[alkene]]s whose [[pi bonds]] can coordinate to empty metal orbitals. An example is [[ethene]] in the complex known as [[Zeise's salt]], K<sup>+</sup>[PtCl<sub>3</sub>(C<sub>2</sub>H<sub>4</sub>)]<sup>-</sup>.

===Geometry===
In coordination chemistry, a structure 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.  Metal ions may have more than one coordination number.

Typically the chemistry of complexes is dominated by interactions between s and p [[molecular orbital]]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]]; 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)<sub>8</sub>]<sup>4-</sup>.  Small metals with large ligands lead to low coordination numbers, e.g. Pt[P(CMe<sub>3</sub>)]<sub>2</sub>.  Due to their large size, [[lanthanide]]s, [[actinide]]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]] 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 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 effect]]s (see e.g. [[Jahn-Teller distortion]]):

*[[Linear molecular geometry|Linear]] for two-coordination, 
*[[Trigonal planar]] for three-coordination, 
*[[Tetrahedral molecular geometry|Tetrahedral]] or [[square planar]] for four-coordination
*[[Trigonal bipyramid molecular geometry|Trigonal bipyramidal]] or [[square pyramid]]al for five-coordination, 
*[[Octahedral molecular geometry|Octahedral]] (orthogonal) or [[prism (geometry)|trigonal prismatic]] for six-coordination,
*[[Pentagonal bipyramid]]al for seven-coordination, 
*[[Square antiprism]]atic for eight-coordination, and 
*[[Triaugmented triangular prism|Tri-capped trigonal prismatic]] (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 distortion|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]].

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

====Stereoisomerism====
[[Stereoisomerism]] 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 [[plane (mathematics)|square planar]] 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.

<div align="center">
<gallery widths="170">
Image:Cis-dichlorotetraamminecobalt(III).png|''cis''-[CoCl<sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>]<sup>+</sup>
Image:Trans-dichlorotetraamminecobalt(III).png|''trans''-[CoCl<sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>]<sup>+</sup>
Image:Fac-trichlorotriamminecobalt(III).png|''fac''-[CoCl<sub>3</sub>(NH<sub>3</sub>)<sub>3</sub>]
Image:Mer-trichlorotriamminecobalt(III).png|''mer''-[CoCl<sub>3</sub>(NH<sub>3</sub>)<sub>3</sub>]
</gallery>
</div>

=====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.  Similarly, the symbol Δ (''delta'') is used as a prefix for the right-handed propeller twist.<ref>{{Cite book|isbn=0138418918, 9780138418915|pages=642|last=Miessler|first=Gary L.|coauthors=Donald Arthur Tarr|title=Inorganic Chemistry|year=1999|pages=315, 316|chapter=9}}</ref>

<div align="center">
<gallery widths="170">
Image:Delta-tris(oxalato)ferrate(III)-3D-balls.png|Λ-[Fe(ox)<sub>3</sub>]<sup>3−</sup>
Image:Lambda-tris(oxalato)ferrate(III)-3D-balls.png|Δ-[Fe(ox)<sub>3</sub>]<sup>3−</sup>
Image:Delta-cis-dichlorobis(ethylenediamine)cobalt(III).png|[[cis-Dichlorobis(ethylenediamine)cobalt(III) chloride|Λ-''cis''-[CoCl<sub>2</sub>(en)<sub>2</sub>]<sup>+</sup>]]
Image:Lambda-cis-dichlorobis(ethylenediamine)cobalt(III).png|Δ-''cis''-[CoCl<sub>2</sub>(en)<sub>2</sub>]<sup>+</sup>
</gallery>
</div>

====Structural isomerism====
[[Structural isomerism]] occurs when the bonds are themselves different. [[Linkage isomer]]ism 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 which can bind in more than one place.  For example, NO<sub>2</sub> is an ambidentate ligand: it can bind to a metal at either the N atom or at an O atom.

==Electronic properties of metal complexes==
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]] (CFT).  Crystal field theory, introduced by [[Hans Bethe]] in 1929, gives a [[quantum mechanics|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.

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.

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:

===Colour===
Metal complexes often have spectacular colors 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 an electron from a metal-based orbital into an empty ligand-based orbital ([[Charge transfer complex|Metal-to-Ligand Charge Transfer]] or MLCT).  The converse also occurs: excitation of an electron in a ligand-based orbital into an empty metal-based orbital ([[Charge transfer complex|Ligand to Metal Charge Transfer]] or LMCT).  These phenomena can be observed with the aid of electronic spectroscopy; also known as [[UV-Vis]].<ref>Harris, D., Bertolucci, M., ''Symmetry and Spectroscopy''. 1989 New York, Dover Publications</ref> For simple compounds with high symmetry, the d-d transitions can be assigned using [[Tanabe-Sugano diagram]]s.  Increasingly, these assignments can be supported with [[computational chemistry]].

[[File:Complex ion colours.jpg]]

===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 [[paramagnetism|(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 TiX<sub>2</sub>[(CH<sub>3</sub>)<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>P(CH<sub>3</sub>)<sub>2</sub>]<sub>2</sub>: when X = Cl, the complex is paramagnetic ([[high spin]] configuration) whereas when X=CH<sub>3</sub>, 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 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 ([[Antiferromagnetism|antiferromagnetic coupling]], resulting in a diamagnetic compound), or they may enhance each other ([[Ferromagnetism|ferromagnetic coupling]]).  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 sphere electron transfer|inner]] and [[outer sphere electron transfer]]s. 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 pair]]s acts as a ''[[bridging ligand]]'', 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(H<sub>2</sub>O)<sub>6</sub>]<sup>n+</sup> 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<sup>+</sup>), electrons in d-orbitals that are antibonding with respect to the ligands (Zn<sup>2+</sup>), or lack covalency (Ln<sup>3+</sup>, 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.
* 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]]) 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 (physics)|plasma]]s, and [[alloy]]s.  The study of "coordination chemistry" is the study of "inorganic chemistry" of all [[alkali metal|alkali]] and [[alkaline earth metal]]s, [[transition metal]]s, [[lanthanide]]s, [[actinides]], and [[metalloid]]s.  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.

The areas of coordination chemistry can be classified according to the nature of the ligands, broadly speaking:
*Classical (or "[[Alfred Werner|Werner]] Complexes"): Ligands in classical coordination chemistry bind to metals, almost exclusively, via their "[[lone pair]]s" of electrons residing on the main group atoms of the ligand. Typical ligands are H<sub>2</sub>O, NH<sub>3</sub>, [[chloride|Cl<sup>−</sup>]], [[Cyanide|CN<sup>−</sup>]], [[ethylenediamine|en<sup>−</sup>]]
:Examples: [Co([[EDTA]])]<sup>−</sup>, [[Cobalt(III) hexammine chloride|[Co(NH<sub>3</sub>)<sub>6</sub>]Cl<sub>3</sub>]], [[Potassium ferrioxalate|[Fe(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]K<sub>3</sub>]]
*Organometallic Chemistry:  Ligands are organic (alkenes, alkynes, alkyls) as well as "organic-like" ligands such as phosphines, hydride, and CO.
:Example: [[Cyclopentadienyliron dicarbonyl dimer|(C<sub>5</sub>H<sub>5</sub>)Fe(CO)<sub>2</sub>CH<sub>3</sub>]]
*Bioinorganic Chemistry: Ligands are those provided by nature, especially including the side chains of amino acids, and many [[cofactor (biochemistry)|cofactor]]s such as [[porphyrin]]s.
:Example: [[hemoglobin]]
:Many natural ligands are "classical"  especially including water.
*Cluster Chemistry: Ligands are all of the above  also include other metals as ligands.
: Example Ru<sub>3</sub>(CO)<sub>12</sub>

*In some cases there are combinations of different fields:
:Example: [[Iron-sulfur protein|[Fe<sub>4</sub>S<sub>4</sub>(Scysteinyl)<sub>4</sub>]<sup>2−</sup>]], in which a cluster is embedded in a biologically active species.

[[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.  In many cases these ligands are oxides or sulfides, but the metals are coordinated nonetheless, and the principles and guidelines discussed below apply. In [[water of crystallization|hydrate]]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 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.

===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]] 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]].

==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 ''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: NH<sub>3</sub> becomes ''[[ammine]]''; H<sub>2</sub>O 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:
: [NiCl<sub>4</sub>]<sup>2−</sup> → tetrachloronickelate(II) ion
: [CuNH<sub>3</sub>Cl<sub>5</sub>]<sup>3−</sup> → amminepentachlorocuprate(II) ion
: [Cd(en)<sub>2</sub>(CN)<sub>2</sub>] → dicyanobis(ethylenediamine)cadmium(II) 
: [Co(NH<sub>3</sub>)<sub>5</sub>Cl]SO<sub>4</sub> → 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 (chemistry)|dimer]] of [[aluminium trichloride]] is described by Al<sub>2</sub>Cl<sub>4</sub>(μ<sub>2</sub>-Cl)<sub>2</sub>.

==Application of coordination compounds==
(1) They are used in photography, i.e., AgBr forms soluble complex with [[sodium thiosulfate]] in photography.
 
(2) K[Ag(CN)<sub>2</sub>] is used for [[electroplating]] of silver, and K[Au(CN)<sub>2</sub>]is used for [[gold plating]].

(3) Some ligands [[oxidation|oxidise]] Co<sup>2+</sup> to Co<sup>3+</sup> ion.
 
(4) [[EDTA]] is used for estimation of Ca<sup>2+</sup> and Mg<sup>2+</sup>in [[hard water]].
 
(5) Silver and gold are extracted by treating zinc with their cyanide complexes.

==See also==
{{commonscat|Coordination compounds}}
*[[Coordination geometry]]
*[[Inclusion compound]]s
*[[Organometallic chemistry]] deals with a special class of coordination compounds where organic fragments are bonded to a metal.
*[[IUPAC nomenclature of inorganic chemistry]]

==External links==
{{Wikibooks|A-level Chemistry/OCR (Salters)|Complexes}}
*[http://www.chemistry.wustl.edu/~edudev/LabTutorials/naming_coord_comp.html Naming Coordination Compounds]
*[http://www.wou.edu/las/physci/ch462/tmcolors.htm Transition Metal Complex Colors]

==References==
<references />
*De Vito, D.; Weber, J. ; Merbach, A. E. “Calculated Volume and Energy Profiles for Water Exchange on t<sub>2g</sub> <sup>6</sup> Rhodium(III) and Iridium(III) Hexaaquaions: Conclusive Evidence for an I<sub>a</sub> Mechanism” Inorganic Chemistry, 2005, Volume 43, pages 858-863. 
*Zumdahl, Steven S.  Chemical Principles, Fifth Edition. New York: Houghton Mifflin, 2005. 943-946, 957.
*Harris, D., Bertolucci, M., ''Symmetry and Spectroscopy''. 1989 New York, Dover Publications

{{BranchesofChemistry}}

[[Category:Coordination compounds|Coordination compounds]]
[[Category:Inorganic chemistry]]
[[Category:Transition metals]]
[[Category:Coordination chemistry]]

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