Editing Cis–trans isomerism (section)

=== Coordination complexes ===
[[Coordination complexes]] with octahedral or square planar geometries can also exhibit ''cis-trans'' isomerism.

[[Image:Cisplatin and transplatin.svg|class=skin-invert-image|thumb|right|320px|The two isomeric complexes, cisplatin and transplatin]]

For example, there are two isomers of [[square planar]] Pt(NH<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>, as explained by [[Alfred Werner]] in 1893. The ''cis'' isomer, whose full name is ''cis''-diamminedichloroplatinum(II), was shown in 1969 by [[Barnett Rosenberg]] to have antitumor activity, and is now a chemotherapy drug known by the short name [[cisplatin]]. In contrast, the ''trans'' isomer ([[transplatin]]) has no useful anticancer activity. Each isomer can be synthesized using the [[trans effect]] to control which isomer is produced.

{{multiple image
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| image1 = Cis-dichlorotetraamminecobalt(III).png
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| image2 = Trans-dichlorotetraamminecobalt(III).png
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| footer = ''cis''-[Co(NH<sub>3</sub>)<sub>4</sub> Cl<sub>2</sub>]<sup>+</sup> and ''trans''-[Co(NH<sub>3</sub>)<sub>4</sub> Cl<sub>2</sub>]<sup>+</sup>
}}

For [[octahedral complex]]es of formula MX<sub>4</sub>Y<sub>2</sub>, two isomers also exist. (Here M is a metal atom, and X and Y are two different types of [[ligand]]s.) In the ''cis'' isomer, the two Y ligands are adjacent to each other at 90°, as is true for the two chlorine atoms shown in green in ''cis''-[Co(NH<sub>3</sub>)<sub>4</sub>Cl<sub>2</sub>]<sup>+</sup>, at left. In the ''trans'' isomer shown at right, the two Cl atoms are on opposite sides of the central Co atom.

A related type of isomerism in octahedral MX<sub>3</sub>Y<sub>3</sub> complexes is
[[fac–mer isomerism|facial–meridional]] (or ''fac''–''mer'') isomerism, in which different numbers of ligands are ''cis'' or ''trans'' to each other. Metal carbonyl compounds can be characterized as ''fac'' or ''mer'' using [[Metal carbonyl#Infrared spectra|infrared spectroscopy]].