Cis/Trans isomerism is a type of stereoisomerism in which the relative positions of 2 functional groups differ between the isomers. An isomer is cis if the 2 functional groups of interest are closer to each other, and trans if they are farther from each other. You may find these definitions to be non-rigorous based on the subjectivity of “closer” and “farther”, but cis/trans isomers have only 2 possible relative positions for these functional groups, so “closer” and “farther” are actually obvious to identify. It’s easier to illustrate this with some examples.
Let’s start with an organic molecule.
Image courtesy of Roland1952 on Wikimedia.
The molecule on the left is trans-1,2-dibromoethylene, and the molecule on the right is cis-1,2-dibromoethylene. The 2 functional groups of interest are the 2 bromides, and the isomerism arises from the 2 different ways that these bromides can be positioned relative to each other. (Notice that the 2 bromides are bonded to different carbon atoms, thus the “1,2-” designation in its name.) Relative to the other bromide, one bromide can either be on the same of the double bond (“closer”) or on the opposite side of the double bond (“farther”). To view the isomerism from another perspective, the double bond serves as the plane of separation, and the bromides can be on different sides of that plane (trans) or the same sides of the plane (cis). Cis/Trans isomerism often arises in organic chemistry because of a bond with restricted rotation, and such restriction is often due to a double bond or a ring structure. Such a bond often serves as the plane of separation on which the relative positions of the 2 functional groups can be established.
Let’s now consider a coordination complex in inorganic chemistry.
Image courtesy of Anypodetos on Wikimedia.
Cisplatin and transplatin are both 4-coordinated complexes with a square planar geometry. Their ligands are 2 chlorides and 2 ammonias. When looking at the pictures above, it’s obvious that there are only 2 relative positions for one chloride to take compared to the other chloride – they can be either closer to each other (cis) or farther apart (trans).
Cis/Trans isomerism can also arise in 6-coordinated octahedral complexes in inorganic chemistry.