Coordination complexes with octahedral or square planar geometries can also exhibit cis-trans isomerism.
For example, there are two isomers of square planar Pt(NH)Cl, 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.
For octahedral complexes of formula MXY, two isomers also exist. (Here M is a metal atom, and X and Y are two different types of ligands.) 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)Cl], 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 MXY complexes is
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 infrared spectroscopy.