Isomers are compounds that have the same formula but different arrangement of their constituent atoms. They can be classified into two types. The first one is constitutional isomers. In this type of isomer, the connections between atoms differ. The second type is stereoisomers, and for this, the connections between atoms are the same but there is a different spatial arrangement of the constituent atoms.

     There are two kinds of constitutional isomers. One is coordination isomers, which is also called ionization isomers, and it can produce different complex ions in solution. The other one is called linkage isomers, they differ in the metal-to-ligand linkage.

     By coincidence, there are also two types of stereoisomers: The first type is diastereomers (or geometric isomers), which are non-mirror-image isomers, and enantiomers, which are mirror-image isomers. Diastereomers have the same connections between atoms but differ in the spatial arrangement of the atoms. They include cis and trans. Cis isomers have both substituents on the same side, while trans isomers have subsituents on opposite sides. The second type is enantiomers, which are mirror images of each other and cannot be superimposed even if rotated in space.