The shape of the lesson
By the end of the lesson, learners can
- Cis and trans: Place the two identical ligands next to each other (cis, 90°) or opposite each other (trans, 180°).
- Mirror-image complexes: 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.
- Cis-trans: square planar MA2B2 or octahedral MA4B2.
- Cis 90°, trans 180°.
- Optical: three bidentate ligands, or cis-[M(en)2X2].
- Cisplatin: binds DNA, stops cell division.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Same ligands, different arrangement.. 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. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Cis and trans | Place the two identical ligands next to each other (cis, 90°) or opposite each other (trans, 180°). Project tab 5 of the lab (Transition elements lab, isomerism) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Mirror-image complexes | 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. Project tab 5 of the lab (Transition elements lab, isomerism) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Mirror-image complexes, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Cis-trans isomerism needs square planar or octahedral geometry; a tetrahedral MA 2 B 2 complex has no geometrical isomers. | Cis means the identical ligands are at 90°, trans at 180°. In [Co(en) 2 Cl 2 ] + only the cis isomer is chiral, giving three stereoisomers in total. It is cis platin that is the drug. |
Stereoisomerism in complexes: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Draw the two isomers of Pt(NH3)2Cl2, name the type of isomerism and identify cisplatin.
Describe how cisplatin acts as an anticancer drug.
Explain why a tetrahedral complex [ZnCl2(NH3)2] does not show cis-trans isomerism, but square planar [Pt(NH3)2Cl2] does.
Draw the stereoisomers of [Co(en)2Cl2]+ and state the total number.
Explain why [Ni(en)3]2+ shows optical isomerism.
Octahedral [Co(NH3)4Cl2]+ exists as two isomers, one violet and one green. Name the type of isomerism and explain why there are only two.
Answer key and marking notes
- Square planar Pt with the two Cl adjacent (Cl–Pt–Cl 90°): cisplatin.
- Square planar Pt with the two Cl opposite (180°): the trans isomer.
- Geometrical (cis-trans) isomerism.
- In the cell a chloride ligand is replaced by water (ligand exchange), making the platinum reactive.
- Pt forms dative bonds with nitrogen atoms on two adjacent guanine bases in DNA.
- This distorts the DNA so it cannot replicate and the cancer cell cannot divide.
- In a tetrahedron every ligand position is adjacent to the other three (all angles 109.5°), so only one arrangement exists.
- In a square plane two identical ligands can be adjacent (90°) or opposite (180°), giving two isomers.
- Trans isomer: the two Cl opposite each other, both en ligands in one plane; has a plane of symmetry so not chiral.
- Cis isomer: the two Cl at 90°; it is chiral and exists as two non-superimposable mirror images.
- Total = 3 stereoisomers.
- The three bidentate en ligands wrap around Ni2+ like a propeller, in a left- or right-handed sense.
- The two forms are non-superimposable mirror images (no plane of symmetry), so they are enantiomers.
- Geometrical (cis-trans) isomerism.
- The two Cl ligands can only be adjacent (90°, cis) or opposite (180°, trans); all other arrangements are the same as one of these after rotation.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows the two isomers of [Pt(NH3)2Cl2]. Why does this complex have two geometrical isomers?
- Ait is square planar, so the two Cl ligands can be adjacent (90°, cis) or opposite (180°, trans)
- Bit is tetrahedral, so the Cl ligands can be on the same or opposite sides
- Cthe platinum atom is a chiral centre with four different groups
- Dthe Pt-Cl bonds cannot rotate because they are double bonds
Cisplatin is used as an anticancer drug. Which statement about it is correct?
- Ait is the cis isomer; it binds to DNA in cancer cells (Cl ligands are replaced by N atoms of the bases), stopping cell division
- Bit is the trans isomer, because only the trans form can bond to DNA
- Cit kills cancer cells by releasing chlorine gas inside them
- Dboth cis and trans isomers are equally effective, because they have the same formula
How many geometrical (cis/trans) isomers does the octahedral ion [Co(NH3)4Cl2]+ have?
- Atwo: cis (Cl at 90°) and trans (Cl at 180°)
- Bthree: cis, trans and a mirror image of cis
- Cone, because octahedral complexes do not show isomerism
- Dfour, because each Cl can occupy any of four positions
Which statement about [Ni(en)3]2+ (en = 1,2-diaminoethane) is correct?
- Ait exists as two optical isomers: non-superimposable mirror images
- Bit exists as cis and trans isomers
- Cit has no stereoisomers, because all three ligands are identical
- Dit is optically active because nickel is bonded to four different groups
Which complex ion can show optical isomerism?
- Acis-[Co(en)2Cl2]+
- Btrans-[Co(en)2Cl2]+
- Ccis-[Co(NH3)4Cl2]+
- Dcis-[Pt(NH3)2Cl2]
How many stereoisomers in total are there of [Co(en)2Cl2]+?
- A3
- B2
- C4
- D6
Why is trans-[Co(en)2Cl2]+ NOT optically active, while the cis isomer is?
- Athe trans isomer has a plane of symmetry (through the Co and the two N-N rings), so it is superimposable on its mirror image
- Bthe trans isomer has no bidentate ligands
- Cthe trans isomer is square planar, so it is flat
- Dthe trans isomer contains a chiral carbon atom in each en ligand
Which statement compares the two isomers of [Pt(NH3)2Cl2] correctly?
- Athe cis isomer is polar, but in the trans isomer the bond dipoles cancel, so it is non-polar
- Bboth isomers are non-polar because the complex has no overall charge
- Cboth isomers are polar because Pt-Cl bonds are polar
- Dthe trans isomer is polar, but the cis isomer is non-polar
Quiz key
In a square planar complex, two identical ligands can be next to each other (cis, isomer A) or across from each other (trans, isomer B). These are different compounds. In a tetrahedron every position is adjacent to every other, so a tetrahedral MA2B2 complex has only one form. Pt carries two pairs of identical ligands, not four different groups, and the square planar molecule has a mirror plane, so it is not chiral. The Pt-ligand bonds are single dative bonds; restricted rotation about C=C is the organic origin of cis/trans, not the reason here.
Cisplatin is cis-[Pt(NH3)2Cl2]. Inside the cell the two Cl ligands are replaced, and the platinum bonds to two nitrogen atoms on neighbouring bases of the same DNA strand, which needs the two sites to be cis. The distorted DNA cannot be copied, so the cell cannot divide. The trans isomer cannot bind in the same way and is not the drug. No chlorine gas is released: the chloride leaves as Cl− in ligand exchange. Isomers have the same formula but different shapes, and here the shape decides the biological effect.
Any two positions in an octahedron are either 90° apart (cis) or 180° apart (trans), so [Co(NH3)4Cl2]+ has two geometrical isomers. The cis form has a mirror plane, so it has no optical isomer: with only monodentate ligands, there is no third form. Octahedral complexes certainly show isomerism, as this example shows. Counting four positions double counts arrangements that are identical after rotating the ion.
The three en ligands can twist round the nickel in a left-handed or a right-handed sense. The two forms are mirror images that cannot be superimposed, so [Ni(en)3]2+ shows optical isomerism. With three identical bidentate ligands there is only one way to place them relative to each other, so there are no cis/trans isomers. Identical ligands do not rule out chirality: the arrangement in space is what matters. The nickel is bonded to six N atoms, not four different groups; the 'four different groups' rule applies to a chiral carbon, not to octahedral complexes.
In cis-[Co(en)2Cl2]+ the two en rings and the two cis Cl ligands make a twisted shape with no mirror plane, so it exists as a pair of enantiomers. trans-[Co(en)2Cl2]+ has a mirror plane (the plane of the four N atoms, with one Cl above it and one below), so it is identical to its mirror image. cis-[Co(NH3)4Cl2]+ has only monodentate ligands and contains a mirror plane. Square planar cis-[Pt(NH3)2Cl2] is flat, and the molecular plane is a mirror plane.
There is one trans isomer, which is not chiral, and a cis isomer that exists as two non-superimposable mirror images. Total = 1 + 2 = 3. 2 counts only cis and trans and misses the optical isomers. 4 gives the trans form a mirror image too, but it has a plane of symmetry. 6 counts every way of placing the ligands, many of which are the same ion rotated.
In the trans isomer, the two en ligands lie in one plane around the cobalt with the two Cl ligands above and below it; that plane is a mirror plane, so the ion is identical to its mirror image and does not rotate plane-polarised light. In the cis isomer there is no such plane. Both isomers contain the same two bidentate en ligands. The complex is octahedral, not square planar. The en ligand (H2NCH2CH2NH2) has no chiral carbon, and even if it did, that would not explain the difference between cis and trans.
In the trans isomer, the two Pt-Cl dipoles point in opposite directions and cancel, and so do the two Pt-N dipoles, so it has no overall dipole. In the cis isomer the two Pt-Cl dipoles point to the same side, so the molecule is polar. A neutral complex can still be polar; charge and polarity are different things. Polar bonds do not guarantee a polar molecule when the shape is symmetrical. The last option swaps the two isomers.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.