Trans isomer: Two identical atoms or groups on both sides of a double bond are trans isomers.
Cis-trans nomenclature has a defect: if two carbon atoms on the double bond are connected with four completely different atoms or groups, it cannot be named, so the standard Z-E nomenclature is stipulated internationally. However, Z-E isomerism nomenclature is not suitable for naming cis-trans isomerism caused by alicyclic ring, because any carbon atom in alicyclic ring has been connected with two identical atoms (carbon atoms), and there is no possibility of connecting four completely different groups.
Cis-trans isomers whose two carbon atoms are not all connected to four identical atoms or groups can be named Z-E or cis-trans isomers. However, it should be noted that the naming of cis-trans isomers is not exactly the same as that of Z-E configuration. These are two different nomenclature, so cis is not necessarily Z-shaped, and trans is not necessarily E-shaped.
Isomerization of complex
The isomerization of complexes is mostly caused by different three-dimensional structures or different internal boundary compositions and ligand connection methods. The isomerism of ligands around the central atom due to different arrangements is called stereoisomerism. Cis means that the same ligand is in adjacent position, which is generally expressed by "cis" or "cis-"; Trans means that the same ligand is in a diagonal position, which is generally expressed by "trans" or "trans-".
For coordination compounds with coordination numbers of 2, 3 and 4 (tetrahedron), this isomerization does not exist because the coordination positions in these spatial structures are adjacent and there is no inversion phenomenon. However, cis-trans isomerization is very common for planar square and octahedral coordination compounds.