Geometric and optical isomerism
Stereoisomers have the same bonds but a different arrangement in space. Geometric isomers are cis or trans; optical isomers are non-superimposable mirror images.
Two families of isomers
Isomers are compounds with the same chemical formula but a different arrangement of atoms. They differ in physical or chemical properties such as colour, solubility or the ions they give. Stereoisomers have the same atoms bonded to the same atoms, but arranged differently in space: these are geometric and optical isomers. Structural isomers have different bonds; they are the next topic.
Geometric (cis-trans) isomerism
| Type of complex | Example | Isomers |
|---|---|---|
| Square planar [MX₂L₂] | [PtCl₂(NH₃)₂] | 2: cis (Cl-Pt-Cl at 90°) and trans (180°) |
| Square planar [MABXL] | [PtBrCl(NH₃)(py)], py = pyridine | 3: two cis forms and one trans form |
| Octahedral [MX₂L₄] | [CoCl₂(NH₃)₄]⁺ | 2: cis (violet) and trans (green) |
| Octahedral [MX₂(AA)₂], AA = didentate ligand | [CoCl₂(en)₂]⁺ | 2: cis and trans |
| Octahedral [MA₃B₃] | [Co(NH₃)₃(NO₂)₃] | 2: fac (three like groups on one face) and mer (three like groups round a meridian) |
| Tetrahedral [MA₂B₂] | [ZnCl₂(NH₃)₂] | None: every corner is next to every other corner |
How to draw cis and trans isomers
- Square planar: draw a square with the metal at the centre and one ligand at each corner.
- cis-[PtCl₂(NH₃)₂]: put the two Cl on neighbouring corners and the two NH₃ on the other two. Label it cis.
- trans-[PtCl₂(NH₃)₂]: put the two Cl on opposite corners. Label it trans.
- Octahedral: put the metal at the centre. Draw one bond straight up, one straight down, and a parallelogram round the middle for the four positions in the plane.
- For cis, put the two like ligands on adjacent corners (90°). For trans, put one at the top and one at the bottom (180°).
- Draw en as a curved line joining two adjacent corners. It can never span two opposite (trans) corners.
Optical isomerism
A molecule that cannot be superimposed on its mirror image is chiral. Its two mirror-image forms are enantiomers, or optical isomers. They rotate the plane of polarised light by equal amounts in opposite directions: dextro (d) to the right and laevo (l) to the left. In complexes, optical isomerism is most common in octahedral complexes with didentate ligands. [Co(en)₃]³⁺ exists as d and l forms. cis-[PtCl₂(en)₂]²⁺ is optically active, but the trans form is not, because it has a plane of symmetry. Draw the d and l forms side by side with a dotted mirror line between them.
Counting stereoisomers
| Complex | Geometric isomers | Optically active forms | Total stereoisomers |
|---|---|---|---|
| [PtCl₂(NH₃)₂] | 2 (cis, trans) | None: the complex is planar | 2 |
| [CoCl₂(NH₃)₄]⁺ | 2 (cis, trans) | None | 2 |
| [CoCl₃(NH₃)₃] | 2 (fac, mer) | None | 2 |
| [CoCl₂(en)₂]⁺ | 2 (cis, trans) | cis exists as d and l | 3 |
| [Co(en)₃]³⁺ | 1 | d and l | 2 |
| [CrCl₂(C₂O₄)₂]³⁻ | 2 (cis, trans) | cis exists as d and l | 3 |
Geometric vs optical isomers
A form with a plane of symmetry, such as trans-[CoCl₂(en)₂]⁺, is never optically active.
Cisplatin, cis-[PtCl₂(NH₃)₂], is a medicine used in cancer hospitals against several cancers. The trans isomer has the same formula but is not useful as a drug: the arrangement in space alone changes how it acts in the body.
2 marks: “Draw the geometric isomers of [PtCl₂(NH₃)₂]. Which one is used as an anticancer drug?” Square planar cis (Cl on neighbouring corners) and trans (Cl on opposite corners). The cis isomer, cisplatin.
3 marks: “Draw the optical isomers of [Co(en)₃]³⁺. Why is trans-[CoCl₂(en)₂]⁺ optically inactive?” Two octahedra, each with three en rings, drawn as mirror images with a mirror line (d and l). The trans form has a plane of symmetry, so it is superimposable on its mirror image.
1 marks: “Why do tetrahedral complexes not show geometric isomerism?” All four positions in a tetrahedron are adjacent to each other, so cis and trans arrangements are not possible.
Cis = side by side, trans = across. Mirror twins need rings: en in an octahedron makes d and l forms.