What are hydrate isomers ? Explain with an example.

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Q. What are hydrate isomers ? Explain with an example.

Tamil Nadu DgeTamil Nadu HSC (DGE) Board 2025Subjective· 3mImportance★★★★★

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Hydrate isomers are coordination compounds with the same overall formula that differ only in how many water molecules are bound directly to the metal (inside the coordination sphere) versus present as free water of crystallisation (outside it) — a classic case being chromium(III) chloride hexahydrate, which exists in three distinctly coloured forms.

Hydrate isomerism is a type of structural isomerism shown by coordination compounds in which water molecules can be distributed differently between being direct ligands (coordinated to the metal, inside the coordination sphere, shown inside square brackets) and being simple water of crystallisation (outside the coordination sphere, not bonded to the metal). Since a coordinated water molecule and a water of crystallisation behave chemically quite differently, these isomers have distinguishably different properties (colour, conductivity, and the number of ions/molecules of water they release on treatment with a dehydrating agent like conc. $H_2SO_4$ ).

Example: chromium(III) chloride hexahydrate, overall formula $CrCl_3\cdot6H_2O$ , exists as three hydrate isomers:

$[Cr(H_2O)_6]Cl_3$ — violet, all 6 water molecules coordinated, all 3 $Cl^-$ ionisable (ionic, gives 4 ions in solution)
$[Cr(H_2O)_5Cl]Cl_2\cdot H_2O$ — grey-green, 5 waters coordinated + 1 free water, 2 $Cl^-$ ionisable …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 16 on this concept.

CBSE 2026Set V11 markMCQ
$[\text{Co(NH}_3)_5(\text{NO}_2)]\text{Cl}_2$
(a) Orange
(b) Blue
(c) White
(d) Yellow
›Reveal solutionSolution

The nitrite ligand bound through N ( $-\text{NO}_2$ , nitro) gives the yellow linkage isomer, while binding through O ( $-\text{ONO}$ , nitrito) gives the red form — so the answer is (d) Yellow.

The $\text{NO}_2^-$ ion is an ambidentate ligand: it can coordinate to the metal either through nitrogen (nitro, $-\text{NO}_2$ ) or through oxygen (nitrito, $-\text{ONO}$ ). These two modes give a pair of linkage isomers.

For the pentaamminecobalt(III) complex: …
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CBSE 2026Set A1 markMCQ
$Q. [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are related to each other as$
(a) Ionisation isomers
(b) Linkage isomers
(c) Co-ordination isomers
(d) Geometric isomers
›Reveal solutionSolution

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 mark
$Q. Give one example of coordination isomerism.$
›Reveal solutionSolution

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 $[Co(NH_3)_6][Cr(CN)_6]$ , all six ammine ligands are bound to cobalt and all six cyanide ligands are bound to chromium. In its coordination isomer, $[Cr(NH_3)_6][Co(CN)_6]$ , the same six $NH_3$ and six $CN^-$ ligands are distributed the opposite way — now $NH_3$ is on chromium and $CN^-$ is on cobalt. Both compounds have an identical overall composition ( $Co$ , $Cr$ , $6\,NH_3$ , $6\,CN^-$ ) but differ in which ligands coo …

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CBSE 2025Set X11 markMCQ
$Q. Which of the following is not a subdivision of structural isomerism?$
(a) Coordination isomerism
(b) Linkage isomerism
(c) Ionisation isomerism
(d) Geometrical isomerism
›Reveal solutionSolution
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. …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. [Mn(CO)5SCN] and/or [Mn(CO)5NCS] are/is -$
(a) Linkage isomers
(b) Coordination isomers
(c) Ionisation isomers
(d) All of these
›Reveal solutionSolution

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 …

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CBSE 2025Set D1 markMCQ
$Q. Which of the following cannot show linkage isomerism?$
(a) NO2-
(b) SCN-
(c) CN-
(d) NH3
›Reveal solutionSolution
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) …
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CBSE 2025Set ANNUAL1 mark
$Q. What type of isomerism is shown by the complex [Co(NH3)6][Cr(CN)6] ?$
›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.

$[\text{Co(NH}_3)_6][\text{Cr(CN)}_6]$ is built from a complex cation $[\text{Co(NH}_3)_6]^{3+}$ and a complex anion $[\text{Cr(CN)}_6]^{3-}$ .

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CBSE 2025Set ANNUAL1 markMCQ
$Q. What type of isomers [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl are?$
(a) Linkage
(b) Coordination
(c) Ionisation
(d) Solvate
›Reveal solutionSolution

[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 markMCQ
$Q. The co-ordination compounds [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] are examples of -$
(i) Ionisation Isomerism
(ii) Linkage Isomerism
(iii) Coordination Isomerism
(iv) Geometrical Isomerism
›Reveal solutionSolution

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: …

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CBSE 2024Set B1 markMCQ
$Q. Isomerism exhibited by the complex compound [Co(NH3)5(Cl)]SO4 and [Co(NH3)5(SO4)]Cl is$
(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). …
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CBSE 2024Set ANNUAL1 mark
$Q. What type of isomerism is shown by the complex [Cr(H2O)5(NCS)]^+2 ?$
›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) …
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CBSE 2023Set ANNUAL1 markMCQ
$Q. The type of Isomerism exhibited by the isomers [Co(NO₂)(NH₃)₅]Cl₂ and [Co(ONO)(NH₃)₅]Cl₂ is -$
(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.

$[Co(NO_2)(NH_3)_5]Cl_2$ has the nitrite ligand bonded to Co through nitrogen (nitrito-N / nitro form), while $[Co(ONO)(NH_3)_5]Cl_2$ 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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Q.What are hydrate isomers ? Explain with an example.

Concept understanding — Structural Isomerism in Coordination Compounds

Q.What are hydrate isomers ? Explain with an example.

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