Stereoisomerism in transition element complexes

Chemistry A2 · Chemistry of transition elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Stereoisomers have the same formula and the same bonds, but the ligands are arranged differently in space around the metal ion.

Geometrical (cis/trans) isomerism: in the cis isomer two identical ligands are next to each other (90° apart). In the trans isomer they are opposite each other (180° apart). It occurs in square planar complexes such as [Pt(NH₃)₂Cl₂] and in octahedral complexes such as [Co(NH₃)₄(H₂O)₂]²⁺.

Optical isomerism: the complex and its mirror image cannot be superimposed. It occurs in octahedral complexes with two or three bidentate ligands, such as 1,2-diaminoethane, H₂NCH₂CH₂NH₂ (en). A complex is polar when its bond dipoles do not cancel.

Rules to remember

Square planar [MA₂B₂]: cis and trans isomers. Tetrahedral [MA₂B₂]: one form only.
Octahedral [MA₄B₂]: two isomers, cis (B ligands at 90°) and trans (B ligands at 180°).
[Ni(en)₃]²⁺: two optical isomers, no cis/trans forms.
[Ni(en)₂(H₂O)₂]²⁺: three stereoisomers. The trans form has a plane of symmetry; the cis form exists as two optical isomers.
Trans isomers: the dipoles are opposite and cancel, so the complex is non-polar. Cis isomers: the dipoles do not cancel, so the complex is polar.
A bidentate ligand can only join two positions that are next to each other (90° apart).

Common mistake

Looking for a chiral carbon atom. In a complex the whole ion is chiral: check whether it has a plane of symmetry. If it has one, it is not optically active.

Worked example

The octahedral complex [Fe(C₂O₄)₃]³⁻ contains three bidentate ethanedioate ligands. Which type of stereoisomerism does it show, and how many stereoisomers are there?

All three ligands are the same, so there is no pair of ligands that could be next to or opposite each other in two different ways: no cis/trans isomers.
The three ligands are arranged around the metal like the blades of a propeller, which can turn to the left or to the right.
The two forms are mirror images that cannot be superimposed, the same pattern as [Ni(en)₃]²⁺.

Answer: Optical isomerism only: two stereoisomers (a pair of optical isomers).

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