Q.Define linkage isomerism. Give an example.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Structural Isomerism in Coordination Compounds
Structural (constitutional) isomers share one molecular formula but differ in how the atoms are actually connected -- four types are recognised. Linkage isomers arise from an ambidentate ligand (like NO₂⁻) bonding through a DIFFERENT donor atom in each isomer: [Co(NH₃)₅(NO₂)]²⁺ bound through nitrogen (nitro) versus through oxygen (nitrito/ONO) is the textbook pair.
Coordination isomers arise only when BOTH the cation and the anion of a salt are themselves complex ions, and ligands are interchanged between them: [Co(NH₃)₆][Cr(CN)₆] versus [Cr(NH₃)₆][Co(CN)₆].
Ionisation isomers arise when an ionisable counter ion can itself act as a ligand, and is swapped with a ligand already inside the coordination sphere -- [Co(NH₃)₅Cl]SO₄ (sulfate free, gives a precipitate with BaCl₂ but not AgNO₃) versus [Co(NH₃)₅SO₄]Cl (chloride free, the reverse reactivity); the two are told apart experimentally by exactly which precipitate test fires. …
Some ligands, called ambidentate ligands, have two different atoms capable of donating to the metal, and which atom actually bonds in a given complex is what defines this type of isomerism. …
Linkage isomerism occurs when a ligand that has two different donor atoms (an ambidentate ligand) coordinates to the central metal through one atom in one isomer and through the other atom in a different isomer.
Some ligands possess more than one potential donor atom but can bind through only one of them at a time in a given complex - these are called ambidentate ligands (e.g. -NO2/-ONO, -SCN/-NCS, -CN/-NC). When two complexes differ only in which atom of such a ligand is bonded to the metal, they are called linkage isomers.
Example: The nitrite ion (NO2-) can bond either through nitrogen or through oxygen:
- [Co(NH3)5(NO2)]Cl2 - nitro isomer, nitrogen-bonded (Co-NO2)
- [Co(NH3)5(ONO)]Cl2 - nitrito isomer, oxygen-bonded (Co-ONO) …
Showing the 12 most recent of 16 on this concept.
- CBSE 2026Set V11 markMCQQ.The complex [Co(NH3)5(NO2)]Cl2 is obtained in the red form when nitrite ligand is bound to cobalt through oxygen (−ONO) and its Linkage isomer is obtained as a __________ form when nitrite ligand is bound to "Co" through nitrogen (−NO2).(a) Orange(b) Blue(c) White(d) Yellow
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The nitrite ligand bound through N (−NO2, nitro) gives the yellow linkage isomer, while binding through O (−ONO, nitrito) gives the red form — so the answer is (d) Yellow.
The NO2− ion is an ambidentate ligand: it can coordinate to the metal either through nitrogen (nitro, −NO2) or through oxygen (nitrito, −ONO). These two modes give a pair of linkage isomers.
For the pentaamminecobalt(III) complex: …
- CBSE 2026Set A1 markMCQQ.[Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are related to each other as(a) Ionisation isomers(b) Linkage isomers(c) Co-ordination isomers(d) Geometric isomers
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Interchanging a ligand and the counter ion (Br- vs SO4^2-) gives ionisation isomers that yield different ions in solution.
Ionisation isomers have the same molecular formula but exchange an anionic ligand inside the coordination sphere with the counter ion outside it, so they give different ions when dissolved:
[Co(NH3)5Br]SO4 -> [Co(NH3)5Br]2+ + SO4^2- (gives a sulphate test)
[Co(NH3)5SO4]Br -> [Co(NH3)5SO4]+ + Br- (gives a bromide test)
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- CBSE 2026Set ANNUAL1 markQ.Give one example of coordination isomerism.
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Coordination isomerism occurs only in compounds containing both a complex cation and a complex anion; swapping which ligands sit on which metal gives isomeric compounds of identical overall composition.
In [Co(NH3)6][Cr(CN)6], all six ammine ligands are bound to cobalt and all six cyanide ligands are bound to chromium. In its coordination isomer, [Cr(NH3)6][Co(CN)6], the same six NH3 and six CN− ligands are distributed the opposite way — now NH3 is on chromium and CN− is on cobalt. Both compounds have an identical overall composition (Co, Cr, 6NH3, 6CN−) but differ in which ligands coo …
- CBSE 2025Set X11 markMCQQ.Which of the following is not a subdivision of structural isomerism?(a) Coordination isomerism(b) Linkage isomerism(c) Ionisation isomerism(d) Geometrical isomerism
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Coordination, linkage and ionisation isomerism are all structural (constitutional) isomerisms; geometrical isomerism is a STEREOisomerism, so (d) is the odd one out.
Isomerism in coordination compounds is broadly divided into two classes:
- Structural (constitutional) isomerism — same formula but different bonds/connectivity. Its subdivisions include: linkage isomerism, coordination isomerism, ionisation isomerism, solvate (hydrate) isomerism. …
- CBSE 2025Set ANNUAL1 markMCQQ.[Mn(CO)5SCN] and/or [Mn(CO)5NCS] are/is -(a) Linkage isomers(b) Coordination isomers(c) Ionisation isomers(d) All of these
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SCN- is an ambidentate ligand that can bind through S or N, giving linkage isomers.
The thiocyanate ligand SCN− is ambidentate — it can coordinate to the metal through the sulfur atom (−SCN) or through the nitrogen atom (−NCS). [Mn(CO)5SCN] (S-bonded) and [Mn(CO)5NCS] (N-bonded) differ only in the p …
- CBSE 2025Set D1 markMCQQ.Which of the following cannot show linkage isomerism?(a) NO2-(b) SCN-(c) CN-(d) NH3
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Linkage isomerism needs an ambidentate ligand; NH3 is not ambidentate.
Linkage isomerism arises when a ligand can coordinate to the metal through two different donor atoms (an ambidentate ligand):
- NO2- : binds through N (nitro) or O (nitrito)
- SCN- : binds through S (thiocyanato) or N (isothiocyanato)
- CN- : binds through C (cyano) or N (isocyano) …
- CBSE 2025Set ANNUAL1 markQ.What type of isomerism is shown by the complex [Co(NH3)6][Cr(CN)6] ?
›Reveal solutionSolution
When a compound is made up of two complex ions (one cationic, one anionic), swapping ligands between the two metal centres gives a different valid compound - this is coordination isomerism.
[Co(NH3)6][Cr(CN)6] is built from a complex cation [Co(NH3)6]3+ and a complex anion [Cr(CN)6]3−.
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- CBSE 2025Set ANNUAL1 markMCQQ.What type of isomers [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl are?(a) Linkage(b) Coordination(c) Ionisation(d) Solvate
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[Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl have identical overall composition but swap which group (Cl- or SO4^2-) sits INSIDE the coordination sphere versus OUTSIDE as the free counter-ion — this exchange is called ionisation isomerism.
In [Co(NH3)5Cl]SO4, Cl- is a ligand bonded to Co, and SO4^2- is the free (ionisable) counter-ion — this solution would give a white precipitate with BaCl2 (free SO4^2- present) but not with AgNO3 (Cl- is locked in the complex).
In [Co(NH3)5(SO4)]Cl, SO4^2- is bonded to Co as a ligand, and Cl- is the free counter-ion — this solution gives a white AgCl precipitate with AgNO3 but not a BaSO4 precipitate with BaCl2.
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- CBSE 2025Set ANNUAL1 markMCQQ.The co-ordination compounds [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] are examples of –(i) Ionisation Isomerism(ii) Linkage Isomerism(iii) Coordination Isomerism(iv) Geometrical Isomerism
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When the metal ions (and their ligands) in the cationic and anionic complex parts of a salt are interchanged, the two compounds are coordination isomers.
In [Co(NH₃)₆][Cr(CN)₆], Co³⁺ is the central metal of the cationic complex (with NH₃ ligands) and Cr³⁺ is the central metal of the anionic complex (with CN⁻ ligands).
In [Cr(NH₃)₆][Co(CN)₆], the roles are exactly swapped: Cr³⁺ is now bound to NH₃ (cationic part) and Co³⁺ is bound to CN⁻ (anionic part).
This interchange of metal ions/ligand-distribution between the cationic and anionic complex ions of a compound (while the overall composition stays the same) is the definition of coordination isomerism. It is distinct from: …
- CBSE 2024Set B1 markMCQQ.Isomerism exhibited by the complex compound [Co(NH3)5(Cl)]SO4 and [Co(NH3)5(SO4)]Cl is(a) Bond isomerism(b) Hydrate isomerism(c) Co-ordination isomerism(d) Ionisation isomerism
›Reveal solutionSolution
[Co(NH3)5Cl]SO4 and [Co(NH3)5SO4]Cl have the identical molecular formula but different ions inside vs outside the coordination sphere — this is ionisation isomerism.
Ionisation isomers are coordination compounds that have an identical molecular formula but yield different ions in aqueous solution, because a ligand and the counter (ionisable) ion exchange positions.
- [Co(NH3)5Cl]SO4: Cl- is inside the coordination sphere (bonded to Co), SO4(2-) is the counter ion outside. In solution it gives [Co(NH3)5Cl]2+ and free SO4(2-) — a white precipitate forms with BaCl2 (test for free sulfate).
- [Co(NH3)5SO4]Cl: SO4(2-) is inside the coordination sphere, Cl- is the counter ion. In solution it gives [Co(NH3)5SO4]+ and free Cl- — a white precipitate forms with AgNO3 (test for free chloride). …
- CBSE 2024Set ANNUAL1 markQ.What type of isomerism is shown by the complex [Cr(H2O)5(NCS)]^+2 ?
›Reveal solutionSolution
Linkage isomerism occurs when an ambidentate ligand (one with two different possible donor atoms) can coordinate to the metal through either donor atom, giving two distinct complexes with the same formula.
The ligand NCS- (thiocyanate) is ambidentate: it can bond to the metal through its nitrogen atom (as -NCS, "isothiocyanato") or through its sulfur atom (as -SCN, "thiocyanato").
So [Cr(H2O)5(NCS)]2+ can exist as two distinct linkage isomers:
- [Cr(H2O)5(NCS)]2+ - bonded through N (isothiocyanato) …
- CBSE 2023Set ANNUAL1 markMCQQ.The type of Isomerism exhibited by the isomers [Co(NO₂)(NH₃)₅]Cl₂ and [Co(ONO)(NH₃)₅]Cl₂ is -(i) Linkage isomerism(ii) Hydrate isomerism(iii) Coordination isomerism(iv) Ionisation isomerism
›Reveal solutionSolution
The two complexes differ only in which atom of the ambidentate NO₂⁻ ligand is bonded to the metal — that is the definition of linkage isomerism.
[Co(NO2)(NH3)5]Cl2 has the nitrite ligand bonded to Co through nitrogen (nitrito-N / nitro form), while [Co(ONO)(NH3)5]Cl2 has the same ligand bonded through oxygen (nitrito-O). NO₂⁻ is an ambidentate ligand — it can coordinate through either the N atom or an O atom. When two coordination compounds differ only in the donor atom used by such an ambidentate ligand, they are called linkage isomers.
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