AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Shapes of complex ions (A-level only): Cisplatin is the cis isomer.
Cisplatin is cis-Pt(NH₃)₂Cl₂, a square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Cisplatin is cis-Pt(NH₃)₂Cl₂, a square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent. In the body a chloride ligand is replaced by water, and the resulting complex bonds to a nitrogen atom on a guanine base in DNA, then repeats with a second guanine. The DNA strand is locked together, so it cannot unwind and replicate and the cell cannot divide. Geometry is everything here: only in the cis isomer are the two leaving groups close enough to reach two bases on the same strand, so the trans isomer is inactive as a drug. Because cisplatin attacks any rapidly dividing cell it also damages hair follicles and bone marrow, so dose is a balance of benefit against side effects.
What earns the marks
- Drawing cisplatin as square planar with the two chloride ligands adjacent.
- A ligand substitution step in which a chloride is replaced, allowing platinum to bond to a nitrogen atom on guanine in DNA.
- Stating that the DNA cannot replicate so the cell cannot divide.
- Explaining that the trans isomer is inactive because its leaving groups are too far apart.
- A named side effect linked to damage to healthy rapidly dividing cells.
Common mistakes and how to put them right
Drawing cisplatin as tetrahedral, which destroys the whole cis and trans argument.
Saying cisplatin kills the cancer cell directly rather than preventing DNA replication.
Naming the bonding site as the sugar or the phosphate of DNA instead of a nitrogen atom on a base.
Explaining side effects by saying the drug is poisonous rather than by damage to healthy dividing cells.
Exam tips
- Label the ammonia and chloride ligands on your drawing so the cis arrangement is unmistakable.
- Mention DNA replication explicitly, because it stops the cancer does not score.
- If asked to evaluate the use of the drug, give one benefit and one specific side effect.
Try the lesson
The start of the Shapes of complex ions (A-level only): Cisplatin is the cis isomer. lesson, made from checked teaching notes for this specification point.
Have a go first
Cisplatin is a square planar complex with the formula [Pt(NH₃)₂Cl₂]. Which description matches the cis isomer?
Listen
Cisplatin is cis-Pt(NH₃)₂Cl₂, a square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent. In the body a chloride ligand is replaced by water, and the resulting complex bonds to a nitrogen atom on a guanine base in DNA, then repeats with a second guanine. The DNA strand is locked together, so it cannot unwind and replicate and the cell cannot divide. Geometry is everything here: only in the cis isomer are the two leaving groups close enough to reach two bases on the same strand, so the trans isomer is inactive as a drug. Because cisplatin attacks any rapidly dividing cell it also damages hair follicles and bone marrow, so dose is a balance of benefit against side effects. A strong answer earns marks for these points. drawing cisplatin as square planar with the two chloride ligands adjacent. a ligand substitution step in which a chloride is replaced, allowing platinum to bond to a nitrogen atom on guanine in DNA. stating that the DNA cannot replicate so the cell cannot divide. explaining that the trans isomer is inactive because its leaving groups are too far apart. Exam tip: Label the ammonia and chloride ligands on your drawing so the cis arrangement is unmistakable.
Key words
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cisplatin structure
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ligand substitution with DNA
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inactive trans isomer
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risks and benefits
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See every definition
- cisplatin structure
- A square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent.
- ligand substitution with DNA
- A chloride ligand is replaced by water, and the resulting complex bonds to a nitrogen atom on a guanine base in DNA, then repeats with a second guanine.
- inactive trans isomer
- Only in the cis isomer are the two leaving groups close enough to reach two bases on the same strand, so the trans isomer is inactive as a drug.
- risks and benefits
- Because cisplatin attacks any rapidly dividing cell it also damages hair follicles and bone marrow, so dose is a balance of benefit against side effects.
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