Editing Coordination complex (section)

===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 distinct orientations. Stereoisomerism can be further classified into:<ref>von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. {{ISBN|0-471-95599-X}}.</ref>

=====Cis–trans isomerism and facial–meridional isomerism=====
[[Cis–trans isomerism]] occurs in octahedral and [[square planar]] complexes (but not tetrahedral). When two ligands are 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>
Image:Cis-dichlorotetraamminecobalt(III).png|{{chem2|''cis''\-[CoCl2(NH3)4]+}}
Image:Trans-dichlorotetraamminecobalt(III).png|{{chem2|''trans''\-[CoCl2(NH3)4]+}}
Image:Fac-trichlorotriamminecobalt(III).png|{{chem2|''fac''\-[CoCl3(NH3)3]}}
Image:Mer-trichlorotriamminecobalt(III).png|{{chem2|''mer''\-[CoCl3(NH3)3]}}
</gallery>
</div>

=====Optical isomerism=====
[[Optical isomerism]] occurs when a complex is not superimposable with its mirror image.  It is so called because the two isomers are each [[optically active]], that is, they rotate the plane of [[polarized light]] in opposite directions. In the first molecule shown, the symbol Λ (''[[lambda]]'') is used as a prefix to describe the left-handed propeller twist formed by three bidentate ligands.  The second molecule is the mirror image of the first, with the symbol Δ (''[[delta (letter)|delta]]'') as a prefix for the right-handed propeller twist. The third and fourth molecules are a similar pair of Λ and Δ isomers, in this case with two bidentate ligands and two identical monodentate ligands.<ref>{{Cite book|isbn=978-0-13-841891-5 |last=Miessler|first=Gary L.|author2=Donald Arthur Tarr|title=Inorganic Chemistry|year=1999|pages=315, 316|chapter=9|publisher=Prentice Hall }}</ref>

<div align="center">
<gallery>
Image:Delta-tris(oxalato)ferrate(III)-3D-balls.png|[[Potassium ferrioxalate|{{chem2|Λ\-[Fe(ox)3](3-)}}]]
Image:Lambda-tris(oxalato)ferrate(III)-3D-balls.png|{{chem2|Δ\-[Fe(ox)3](3-)}}
Image:Delta-cis-dichlorobis(ethylenediamine)cobalt(III).png|[[cis-Dichlorobis(ethylenediamine)cobalt(III) chloride|{{chem2|Λ\-''cis''\-[CoCl2(en)2]+}}]]
Image:Lambda-cis-dichlorobis(ethylenediamine)cobalt(III).png|{{chem2|Δ\-''cis''\-[CoCl2(en)2]+}}
</gallery>
</div>

====Structural isomerism====
[[Structural isomerism]] occurs when the bonds are themselves different. Four types of structural isomerism are recognized: ionisation isomerism, solvate or hydrate isomerism, linkage isomerism and coordination isomerism.
# '''Ionisation isomerism''' – the isomers give different ions in solution although they have the same composition. This type of isomerism occurs when the counter ion of the complex is also a potential ligand. For example, pentaamminebromocobalt(III) sulphate {{chem2|[Co(NH3)5Br]SO4}} is red violet and in solution gives a precipitate with barium chloride, confirming the presence of sulphate ion, while pentaamminesulphatecobalt(III) bromide {{chem2|[Co(NH3)5SO4]Br}} is red and tests negative for sulphate ion in solution, but instead gives a precipitate of AgBr with silver nitrate.<ref name=Huheey>Huheey, James E., ''Inorganic Chemistry'' (3rd ed., Harper & Row 1983), p.524–5 {{ISBN|0-06-042987-9}}</ref>
# [[Hydration isomerism|Solvate or hydrate isomerism]] – the isomers have the same composition but differ with respect to the number of molecules of solvent that serve as ligand vs simply occupying sites in the crystal. Examples: {{chem2|[Cr(H2O)6]Cl3}} is violet colored, {{chem2|[CrCl(H2O)5]Cl2·H2O}} is blue-green, and {{chem2|[CrCl2(H2O)4]Cl*2H2O}} is dark green. See [[water of crystallization]].<ref name=Huheey/>
# '''[[Linkage isomerism]]''' occurs with ligands with more than one possible donor atom, known as [[ambidentate ligand]]s.<ref name=Jolly>{{cite book |last1=Jolly |first1=William L. |title=Modern Inorganic Chemistry |date=1984 |publisher=McGraw-Hill |isbn=0-07-032760-2 |pages=357–9}}</ref> For example, [[nitrite]] can coordinate through O or N.<ref>Huheey, James E., ''Inorganic Chemistry'' (3rd ed., Harper & Row 1983), p.513–24 {{ISBN|0-06-042987-9}}</ref> One pair of nitrite linkage isomers have structures {{chem2|(NH3)5CoNO2(2+)}} (nitro isomer) and {{chem2|(NH3)5CoONO(2+)}} (nitrito isomer).<ref name=Jolly/>
# '''[[Coordination isomerism]]''' occurs when both positive and negative ions of a salt are complex ions and the two isomers differ in the distribution of ligands between the cation and the anion. For example, {{chem2|[Co(NH3)6][Cr(CN)6]}} and {{chem2|[Cr(NH3)6][Co(CN)6]}}.<ref name=Huheey/>