Stereoisomerism in transition element complexes — common mistakes

When drawing 3D isomers, clarity is essential. Use solid lines for bonds in the plane of the paper, solid wedges for bonds coming out towards you, and dashed lines for bonds going away from you. Always label your isomers clearly (e.g., 'cis', 'trans', 'enantiomer 1', 'enantiomer 2'). For bidentate ligands, a simple arc connecting the two coordination sites is usually sufficient and much clearer than drawing the full structure.

In a tetrahedral geometry, all four ligand positions are equivalent relative to each other. The angle between any two ligands is 109.5°. There are no 'opposite' positions (which would be 180° apart), so cis-trans isomerism is not possible.

Structural isomers have the same molecular formula but a different pattern of atomic connectivity (i.e., the atoms are bonded together in a different order). Stereoisomers have the same molecular formula and the same connectivity, but the atoms are arranged differently in three-dimensional space.

Visualise an imaginary plane cutting through the molecule. If this plane divides the molecule into two halves that are perfect mirror reflections of each other, it has a plane of symmetry and is achiral. For example, in trans-[Co(en)₂(Cl)₂]⁺, a plane can be drawn through the Co ion and the two Cl⁻ ions, which reflects one half of the molecule onto the other. This is not possible for the cis-[Co(en)₂(Cl)₂]⁺ isomer.