28.4.1—Types of stereoisomerism shown by complexes

Stereoisomers have the same connectivity but different three-dimensional arrangements. Complexes may show geometrical cis/trans isomerism, or optical isomerism when a pair of arrangements are non-superimposable mirror images.

For cis/trans isomers, identical ligands are adjacent (cis) or opposite (trans). Bidentate ligands can create chirality by wrapping around the metal, so the mirror image cannot be rotated onto the original.

Square-planar [Pt(NH₃)₂Cl₂] has cis and trans forms. Octahedral [Ni(en)₃]²⁺ is a standard optical-isomer example; [Co(NH₃)₄(H₂O)₂]²⁺ illustrates octahedral cis/trans arrangements.

Different colours or conformations are not automatically stereoisomers. First check whether connectivity is unchanged and whether the spatial arrangements are genuinely non-superimposable.