Why Cisplatin Fights Cancer but Its Twin Transplatin Does Not
Identify and distinguish structural isomerism — ionisation, linkage, coordination and hydrate — and stereoisomerism, including geometrical and optical isomerism, in coordination compounds, and learn to count isomers.
How can two complexes with the same formula behave completely differently?
Cisplatin, , is a widely used anticancer drug, yet transplatin, with exactly the same atoms, has little effect against tumours. The difference lies purely in how the ligands are arranged around platinum.
Coordination compounds show two broad families of isomerism: structural isomers differ in which atoms or ions are bonded to the metal, while stereoisomers have identical bonds arranged differently in space.
This lesson covers structural isomerism — ionisation, linkage, coordination and hydrate isomerism — and stereoisomerism, including geometrical and optical isomerism.
Coordination compounds show two broad families of isomerism: structural isomers differ in which atoms or ions are bonded to the metal, while stereoisomers have identical bonds arranged differently in space.
This lesson covers structural isomerism — ionisation, linkage, coordination and hydrate isomerism — and stereoisomerism, including geometrical and optical isomerism.
What are ionisation, linkage, coordination and hydrate isomers of complexes?
Structural isomers of coordination compounds have the same formula but differ in which atoms or ions are bonded to the metal, and the four main types are ionisation, linkage, coordination and hydrate isomerism.
Types of structural isomerism:
- Ionisation isomerism — a ligand and a counter ion swap places, giving different ions in solution: gives a precipitate with silver nitrate, while gives one with barium chloride
- Linkage isomerism — an ambidentate ligand binds through different atoms: , bonded through nitrogen, is yellow, while , bonded through oxygen, is red
- Coordination isomerism — ligands swap between a complex cation and a complex anion, as in and
- Hydrate isomerism — water is either a ligand or water of crystallisation, as in violet and grey-green
Worked example — naming the isomerism. and are coordination isomers, because the same ligands are shared differently between the cation and the anion.
An everyday example. The dark green crystals of chromium(III) chloride hexahydrate on a laboratory shelf are a hydrate isomer of the violet form, differing only in how many chloride ions sit inside the coordination sphere.
The substance. Structural isomers can be told apart with simple tests — ionisation isomers give different precipitates, and hydrate isomers release different numbers of free chloride ions in solution.
Types of structural isomerism:
- Ionisation isomerism — a ligand and a counter ion swap places, giving different ions in solution: gives a precipitate with silver nitrate, while gives one with barium chloride
- Linkage isomerism — an ambidentate ligand binds through different atoms: , bonded through nitrogen, is yellow, while , bonded through oxygen, is red
- Coordination isomerism — ligands swap between a complex cation and a complex anion, as in and
- Hydrate isomerism — water is either a ligand or water of crystallisation, as in violet and grey-green
Worked example — naming the isomerism. and are coordination isomers, because the same ligands are shared differently between the cation and the anion.
An everyday example. The dark green crystals of chromium(III) chloride hexahydrate on a laboratory shelf are a hydrate isomer of the violet form, differing only in how many chloride ions sit inside the coordination sphere.
The substance. Structural isomers can be told apart with simple tests — ionisation isomers give different precipitates, and hydrate isomers release different numbers of free chloride ions in solution.
What are geometrical and optical isomers of coordination compounds?
Stereoisomers of coordination compounds have the same bonds but different arrangements in space: geometrical isomers differ in whether identical ligands sit next to or opposite each other, and optical isomers are non-superimposable mirror images.
Geometrical (cis-trans) isomerism:
- **Square planar — two isomers, cis and trans, such as cisplatin and transplatin
- Octahedral ** — cis and trans forms, as in
- **Octahedral — facial (fac), with the three A ligands on one triangular face, and meridional (mer), with them around a meridian
- Tetrahedral complexes show no geometrical isomerism, because every position is adjacent to every other
Optical isomerism:
- The two mirror-image forms, called enantiomers**, rotate plane-polarised light in opposite directions and are labelled d and l
- It is common in octahedral complexes with bidentate ligands, such as
- The cis form of is optically active, but the trans form has a plane of symmetry and is not
**Worked example — counting stereoisomers of :**
- Geometrical: cis and trans
- The cis form exists as two enantiomers; the trans form as one
- Total stereoisomers:
An everyday example. Cisplatin, given as chemotherapy in Indian cancer hospitals, works because its two chloride ligands sit side by side and can bind to neighbouring sites on DNA, while transplatin's chlorides point in opposite directions and cannot.
The substance. Geometrical isomers are different compounds with different properties, but enantiomers are identical in almost every way — they differ only in how they rotate polarised light and interact with other chiral molecules.
Geometrical (cis-trans) isomerism:
- **Square planar — two isomers, cis and trans, such as cisplatin and transplatin
- Octahedral ** — cis and trans forms, as in
- **Octahedral — facial (fac), with the three A ligands on one triangular face, and meridional (mer), with them around a meridian
- Tetrahedral complexes show no geometrical isomerism, because every position is adjacent to every other
Optical isomerism:
- The two mirror-image forms, called enantiomers**, rotate plane-polarised light in opposite directions and are labelled d and l
- It is common in octahedral complexes with bidentate ligands, such as
- The cis form of is optically active, but the trans form has a plane of symmetry and is not
**Worked example — counting stereoisomers of :**
- Geometrical: cis and trans
- The cis form exists as two enantiomers; the trans form as one
- Total stereoisomers:
An everyday example. Cisplatin, given as chemotherapy in Indian cancer hospitals, works because its two chloride ligands sit side by side and can bind to neighbouring sites on DNA, while transplatin's chlorides point in opposite directions and cannot.
The substance. Geometrical isomers are different compounds with different properties, but enantiomers are identical in almost every way — they differ only in how they rotate polarised light and interact with other chiral molecules.
Exam tip
What earns full marks on isomerism in coordination compounds?
Draw every possible isomer in its correct shape, and look for a plane of symmetry before calling any complex optically active.
- Ionisation: a ligand and a counter ion swap; linkage: an ambidentate ligand such as changes its donor atom
- Coordination: ligands exchange between complex cation and anion; hydrate: water inside or outside the coordination sphere
- Square planar and octahedral : cis and trans; octahedral : fac and mer
- Tetrahedral complexes have no geometrical isomers
The trap. Calling trans- optically active. The trans isomer has a plane of symmetry, so only the cis isomer shows optical isomerism.
- Ionisation: a ligand and a counter ion swap; linkage: an ambidentate ligand such as changes its donor atom
- Coordination: ligands exchange between complex cation and anion; hydrate: water inside or outside the coordination sphere
- Square planar and octahedral : cis and trans; octahedral : fac and mer
- Tetrahedral complexes have no geometrical isomers
The trap. Calling trans- optically active. The trans isomer has a plane of symmetry, so only the cis isomer shows optical isomerism.
Did you know
How do your hands help explain optical isomers?
Hold your hands palm to palm: they are mirror images, yet no amount of turning lets one fit exactly over the other, as anyone who has tried a left glove on a right hand knows.
Chiral complexes such as behave the same way. The three ethane-1,2-diamine rings wind around cobalt like the blades of a propeller, twisting either clockwise or anticlockwise, and those two twists are mirror images that cannot be superimposed.
That is why chemists call such molecules chiral, a word built from the Greek for hand.
Chiral complexes such as behave the same way. The three ethane-1,2-diamine rings wind around cobalt like the blades of a propeller, twisting either clockwise or anticlockwise, and those two twists are mirror images that cannot be superimposed.
That is why chemists call such molecules chiral, a word built from the Greek for hand.
Exam relevance
How do JEE Main and NEET test isomerism in coordination compounds?
Coordination Compounds is a recurring chapter in both JEE Main and NEET, and isomerism is a drawing-based topic within it, rewarding a clear picture of each shape.
What gets asked. The type of structural isomerism between two formulas, the number of geometrical and optical isomers of complexes such as and , which complexes are optically active, and linkage isomers of ambidentate ligands.
Question types. Single-correct questions, plus numerical-value questions in JEE Main asking for the total number of stereoisomers.
Why it matters later. Chirality and planes of symmetry return in Haloalkanes and Haloarenes for optically active organic compounds, and the bonding theories in the next part of this chapter explain why some geometries form at all.
The trap that costs marks. Forgetting the enantiomers when counting stereoisomers — cis- counts as two, bringing the total to three.
What gets asked. The type of structural isomerism between two formulas, the number of geometrical and optical isomers of complexes such as and , which complexes are optically active, and linkage isomers of ambidentate ligands.
Question types. Single-correct questions, plus numerical-value questions in JEE Main asking for the total number of stereoisomers.
Why it matters later. Chirality and planes of symmetry return in Haloalkanes and Haloarenes for optically active organic compounds, and the bonding theories in the next part of this chapter explain why some geometries form at all.
The trap that costs marks. Forgetting the enantiomers when counting stereoisomers — cis- counts as two, bringing the total to three.
Key takeaways
What must you be able to do from this lesson?
- Structural isomerism: ionisation, linkage, coordination and hydrate isomers differ in which atoms or ions are bonded to the metal
- Geometrical isomerism: cis and trans in square planar and octahedral complexes, fac and mer for , and none in tetrahedral complexes
- Optical isomerism: non-superimposable mirror images, common with bidentate ligands and ruled out by a plane of symmetry
How many stereoisomers does have — and are any of them geometrical isomers?
- Geometrical isomerism: cis and trans in square planar and octahedral complexes, fac and mer for , and none in tetrahedral complexes
- Optical isomerism: non-superimposable mirror images, common with bidentate ligands and ruled out by a plane of symmetry
How many stereoisomers does have — and are any of them geometrical isomers?