28.4 Stereoisomerism in transition element 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.

A complex is overall polar when its bond or ligand dipoles do not cancel as vectors. The metal–ligand bonds may be polar individually, but symmetry and three-dimensional arrangement determine the net dipole.

Draw or inspect the geometry before deciding. Opposite equal dipoles can cancel in a symmetric arrangement; cis/trans changes or different ligands can destroy that cancellation.

A trans square-planar complex with matching opposite ligands can have cancellation that its cis isomer lacks. An octahedral complex with six identical ligands is more symmetric than one containing different ligand types.

Do not infer polarity from the overall ionic charge or from one polar bond. Charge and molecular dipole are different properties, and a charged complex can still have zero net dipole.