Two complexes can have the same metal, the same ligands and the same bonds yet be different substances, because the ligands sit in different places in space. One such pair includes cisplatin, an anticancer drug; its trans isomer is not an effective drug.
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Cis-trans isomerism occurs in square planar complexes of the type MA2B2, such as Pt(NH3)2Cl2, and in octahedral complexes of the type MA4B2, such as [Co(NH3)4Cl2]+: in the cis isomer the two identical ligands are at 90°, in the trans isomer at 180°. Optical isomerism occurs when a complex is chiral: its mirror image is non-superimposable. Octahedral complexes with three bidentate ligands, such as [Ni(en)3]2+, or two bidentate ligands in a cis arrangement, exist as a pair of enantiomers. Cisplatin is an anticancer drug: it loses its Cl− ligands and binds to nitrogen atoms on DNA, stopping cancer cells from dividing.
Place the two identical ligands next to each other (cis, 90°) or opposite each other (trans, 180°). Tetrahedral MA2B2 complexes cannot do this: every position is adjacent to every other.
| Complex | Shape | Isomers | Notes |
|---|---|---|---|
| [Pt(NH3)2Cl2] | square planar | cis, trans | cis is cisplatin (polar); trans is non-polar |
| [Co(NH3)4Cl2]+ | octahedral | cis, trans | Cl at 90° (cis) or 180° (trans) |
| [Ni(H2O)4(NH3)2]2+ | octahedral | cis, trans | NH3 at 90° or 180° |
| [NiCl2(NH3)2] if tetrahedral | tetrahedral | none | all positions are equivalent |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which shape of complex MA2B2 can show cis-trans isomerism?
In cisplatin, what is the Cl–Pt–Cl bond angle?
Why does [Ni(en)3]2+ show optical isomerism?
How does cisplatin act as an anticancer drug?
Rotate the mirror-image pair of [Ni(en)3]2+ in the simulator and try to superimpose them, then apply the same test to other octahedral complexes with bidentate ligands.
| Complex | Chiral? | Stereoisomers in total |
|---|---|---|
| [Ni(en)3]2+ | yes, three bidentate ligands form a propeller | 2 (a pair of enantiomers) |
| [Co(en)2Cl2]+ | cis form yes; trans form no (has a plane of symmetry) | 3 (trans + two cis enantiomers) |
| [Cr(C2O4)3]3− | yes | 2 |
| [Cu(H2O)6]2+ | no | 1 |
Cis-trans isomerism needs square planar or octahedral geometry; a tetrahedral MA2B2 complex has no geometrical isomers. Cis means the identical ligands are at 90°, trans at 180°. In [Co(en)2Cl2]+ only the cis isomer is chiral, giving three stereoisomers in total. It is cisplatin that is the drug.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
How many stereoisomers does [Co(en)2Cl2]+ have in total?
Which complex has NO stereoisomers?
Which statement about the isomers of Pt(NH3)2Cl2 is correct?
Why is trans-[Co(en)2Cl2]+ not optically active?
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