When Ethane-1,2-diamine is added progressively to an aqueous solution of Nickel (II) chloride, the sequence of colour change observed will be:

Green arrow Pale Blue arrow Blue arrow Violet

Pale Blue arrow Blue arrow Green arrow Violet

Pale Blue arrow Blue arrow Violet arrow Green

Violet arrow Blue arrow Pale Blue arrow Green

Coordination Compounds PYQ — JEE Main 2025 · 24 Jan · Shift 2 · Q18 with Solution

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Correct answer: C

Initial species in aqueous solution of $\text{NiCl}_2$

In water, $\text{Ni}^{2+}$ exists mainly as the hexaaqua complex:

[\text{Ni}(\text{H}_2\text{O})_6]^{2+}

This complex is typically green in aqueous nickel(II) chloride solution.

So the starting colour is:

\text{Green}

On progressive addition of ethane-1,2-diamine (en)

Ethane-1,2-diamine ( $\text{en}$ ) is a bidentate ligand and forms increasingly stable chelate complexes with $\text{Ni}^{2+}$ by replacing water molecules stepwise.

The sequence of complex formation is:

[\text{Ni}(\text{H}_2\text{O})_6]^{2+} \rightarrow [\text{Ni}(\text{en})(\text{H}_2\text{O})_4]^{2+} \rightarrow [\text{Ni}(\text{en})_2(\text{H}_2\text{O})_2]^{2+} \rightarrow [\text{Ni}(\text{en})_3]^{2+}

Their observed colours are known to change as follows:

$[\text{Ni}(\text{H}_2\text{O})_6]^{2+}$ : Green
intermediate aquo/ammine-like substitution stage : Pale blue
further substitution : Blue
$[\text{Ni}(\text{en})_3]^{2+}$ : Violet

Hence the colour sequence is:

\text{Green} \rightarrow \text{Pale Blue} \rightarrow \text{Blue} \rightarrow \text{Violet}

Compare with options

A: Pale Blue $\rightarrow$ Blue $\rightarrow$ Green $\rightarrow$ Violet ❌
B: Pale Blue $\rightarrow$ Blue $\rightarrow$ Violet $\rightarrow$ Green ❌
C: Green $\rightarrow$ Pale Blue $\rightarrow$ Blue $\rightarrow$ Violet ✅
D: Violet $\rightarrow$ Blue $\rightarrow$ Pale Blue $\rightarrow$ Green ❌

Final answer

The correct option is:

\boxed{\text{C}}

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View written solutionFree

Correct answer: C

Initial species in aqueous solution of $\text{NiCl}_2$

In water, $\text{Ni}^{2+}$ exists mainly as the hexaaqua complex:

[\text{Ni}(\text{H}_2\text{O})_6]^{2+}

This complex is typically green in aqueous nickel(II) chloride solution.

So the starting colour is:

\text{Green}

On progressive addition of ethane-1,2-diamine (en)

Ethane-1,2-diamine ( $\text{en}$ ) is a bidentate ligand and forms increasingly stable chelate complexes with $\text{Ni}^{2+}$ by replacing water molecules stepwise.

The sequence of complex formation is:

[\text{Ni}(\text{H}_2\text{O})_6]^{2+} \rightarrow [\text{Ni}(\text{en})(\text{H}_2\text{O})_4]^{2+} \rightarrow [\text{Ni}(\text{en})_2(\text{H}_2\text{O})_2]^{2+} \rightarrow [\text{Ni}(\text{en})_3]^{2+}

Their observed colours are known to change as follows:

$[\text{Ni}(\text{H}_2\text{O})_6]^{2+}$ : Green
intermediate aquo/ammine-like substitution stage : Pale blue
further substitution : Blue
$[\text{Ni}(\text{en})_3]^{2+}$ : Violet

Hence the colour sequence is:

\text{Green} \rightarrow \text{Pale Blue} \rightarrow \text{Blue} \rightarrow \text{Violet}

Compare with options

A: Pale Blue $\rightarrow$ Blue $\rightarrow$ Green $\rightarrow$ Violet ❌
B: Pale Blue $\rightarrow$ Blue $\rightarrow$ Violet $\rightarrow$ Green ❌
C: Green $\rightarrow$ Pale Blue $\rightarrow$ Blue $\rightarrow$ Violet ✅
D: Violet $\rightarrow$ Blue $\rightarrow$ Pale Blue $\rightarrow$ Green ❌

Final answer

The correct option is:

\boxed{\text{C}}

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Coordination Compounds question

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