Stereoisomerism in transition element complexes

· Stereoisomers = complexes with the same formula and same bonding, but a different 3D arrangement of ligands around the metal ion.
· CIE focus: geometrical isomerism, optical isomerism, and overall polarity.
· Common shapes tested: square planar and octahedral complexes.
· Bidentate ligands are especially important because they can create optical isomers in octahedral complexes.
· Key bidentate ligand: 1,2-diaminoethane, en, H₂NCH₂CH₂NH₂.

Geometrical isomerism: cis/trans

· Geometrical isomerism occurs when ligands can occupy different positions around the central metal ion.
· Cis = two identical or specified ligands are adjacent / 90° apart.
· Trans = two identical or specified ligands are opposite / 180° apart.
· Square planar complexes can show cis/trans isomerism, e.g. [Pt(NH₃)₂Cl₂].
· Octahedral complexes can show cis/trans isomerism, e.g. [Co(NH₃)₄(H₂O)₂]²⁺.
· In exam diagrams, check whether the two repeated ligands are next to each other or opposite each other.

Square planar complexes: [Pt(NH₃)₂Cl₂]

· [Pt(NH₃)₂Cl₂] has 2 NH₃ ligands and 2 Cl⁻ ligands arranged around Pt in a square planar shape.
· Cis-[Pt(NH₃)₂Cl₂]: the two NH₃ ligands are adjacent and the two Cl⁻ ligands are adjacent.
· Trans-[Pt(NH₃)₂Cl₂]: the two NH₃ ligands are opposite and the two Cl⁻ ligands are opposite.
· Exam tip: square planar MA₂B₂ complexes can show cis/trans isomerism.
· Do not confuse square planar with tetrahedral: simple tetrahedral MA₂B₂ complexes do not show cis/trans isomerism because positions are equivalent by rotation.

This image shows the general MA₂B₂ square planar case. It helps students recognise that cis means adjacent ligands and trans means opposite ligands. Source

Octahedral complexes: cis/trans

· Octahedral complexes have 6 ligand positions around the central metal ion.
· [Co(NH₃)₄(H₂O)₂]²⁺ can form cis/trans isomers because the two H₂O ligands can be adjacent or opposite.
· Cis-[Co(NH₃)₄(H₂O)₂]²⁺: the two H₂O ligands are 90° apart.
· Trans-[Co(NH₃)₄(H₂O)₂]²⁺: the two H₂O ligands are 180° apart.
· [Ni(en)₂(H₂O)₂]²⁺ also shows geometrical isomerism, with two en ligands and two H₂O ligands in an octahedral complex.
· A bidentate ligand such as en usually binds to adjacent positions, forming a chelate ring.

These diagrams show cis and trans octahedral arrangements. They are useful for visualising whether two ligands are 90° apart or 180° apart in a six-coordinate complex. Source

Optical isomerism in complexes

· Optical isomerism occurs when a complex has non-superimposable mirror images.
· The two mirror-image forms are called optical isomers or enantiomers.
· Optical isomerism in transition metal complexes often occurs with octahedral complexes containing bidentate ligands.
· [Ni(en)₃]²⁺ shows optical isomerism because three en ligands wrap around the metal ion to give two non-superimposable mirror images.
· [Ni(en)₂(H₂O)₂]²⁺ can also show optical isomerism, especially in the cis form.
· Exam tip: look for bidentate ligands and ask whether the structure has a plane of symmetry; if it has a plane of symmetry, it is usually not optically active.

These images help visualise ethylenediamine (en) as a bidentate ligand. Each en ligand forms two coordinate bonds, which can create chiral octahedral arrangements. Source

Optical isomerism: exam examples

· [Ni(en)₃]²⁺:
· Contains three bidentate en ligands.
· Has two optical isomers.
· The isomers are mirror images that cannot be superimposed.

· [Ni(en)₂(H₂O)₂]²⁺:
· Contains two bidentate en ligands and two H₂O ligands.
· Can show cis/trans geometrical isomerism.
· The cis form can show optical isomerism.
· The trans form is usually not optically active because it has greater symmetry.

Overall polarity of complexes

· To deduce overall polarity, consider whether the bond dipoles cancel due to symmetry.
· A complex is more likely non-polar if it is symmetrical and opposite dipoles cancel.
· A complex is more likely polar if ligands are arranged unsymmetrically, so dipoles do not cancel.
· Trans-[Pt(NH₃)₂Cl₂] is usually treated as non-polar because opposite ligand dipoles cancel.
· Cis-[Pt(NH₃)₂Cl₂] is polar because the dipoles do not cancel.
· Trans-[Co(NH₃)₄(H₂O)₂]²⁺ is more symmetrical, so dipoles are more likely to cancel.
· Cis-[Co(NH₃)₄(H₂O)₂]²⁺ is less symmetrical, so it is more likely to be polar.
· Exam tip: always link polarity to symmetry and cancellation of dipoles, not just the presence of polar bonds.

Common exam traps

· Do not say stereoisomers have different formulas: they have the same formula.
· Do not call cis/trans isomerism structural isomerism; it is stereoisomerism.
· Do not assume every octahedral complex is optically active; check for non-superimposable mirror images.
· Do not assume bidentate ligands automatically mean optical isomerism; check for symmetry.
· Do not draw bidentate ligand donor atoms opposite each other in an octahedral complex; they usually occupy adjacent positions.
· For polarity questions, do not just state polar/non-polar; explain using dipole cancellation.