ChemistryCBSE Class 12 · Board exam 2025, set 56/1/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2025: question 32(B)

Set in the CBSE Class 12 board exam of 2025, paper 56/1/1, section E, question 32(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(B) · 5 marks · Long answerCoordination Compounds

(a) What is meant by 'Chelate effect' ? Give an example. (2 + 2 + 1 = 5)
(b) Write the hybridization and magnetic behaviour of $[Fe (CN)_{6}]^{4 -}$ . (Atomic number : Fe = 26)
(c) If $PtC l_{2} \cdot 2N H_{3}$ does not react with $AgN O_{3}$ , what will be its formula ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

For (a), think about ligands like 'en' that grip the metal at two points and form a ring.

Hint 2

For (b), find the oxidation state of Fe, write its d-configuration, and remember that CN⁻ is a strong field ligand that forces pairing.

Hint 3

For (c), AgNO₃ precipitates only chloride ions that lie outside the coordination sphere.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Chelate effect: A complex formed by a chelating (polydentate) ligand, which binds the metal through two or more donor atoms to form a ring, is more stable than a similar complex formed by comparable monodentate (non-chelating) ligands. This extra stability is called the chelate effect.
1 mark
Example: $[Co (en)_{3}]^{3 +}$ is more stable than $[Co (N H_{3})_{6}]^{3 +}$ , because each ethane-1,2-diamine (en) forms a five-membered chelate ring with Co. (Another example: $[Ni (en)_{3}]^{2 +}$ compared with $[Ni (N H_{3})_{6}]^{2 +}$ .)
1 mark
(b) In $[Fe (CN)_{6}]^{4 -}$ : $x + 6 (- 1) = - 4$ , so $x = + 2$ . Fe ( $Z = 26$ ) is $[Ar] 3 d^{6} 4 s^{2}$ , so $F e^{2 +}$ is $[Ar] 3 d^{6}$ . $C N^{-}$ is a strong field ligand, so the six 3d electrons pair up in three 3d orbitals, leaving two 3d orbitals empty. Two 3d, one 4s and three 4p orbitals mix to accept six electron pairs from $C N^{-}$ . Hybridisation: $d^{2} s p^{3}$ (inner orbital, octahedral).
1 mark
All electrons are paired ( $t_{2 g}^{6} e_{g}^{0}$ ), with no unpaired electron, so the complex is diamagnetic.
1 mark
(c) No precipitate of AgCl forms with $AgN O_{3}$ . So no $C l^{-}$ is present outside the coordination sphere, and both Cl atoms are bonded to Pt along with the two $N H_{3}$ . Formula: $[Pt (N H_{3})_{2} C l_{2}]$ .
1 mark

Answer: (a) Extra stability of complexes with chelating ligands over those with non-chelating ligands, e.g. [Co(en)3]3+; (b) d2sp3, diamagnetic; (c) [Pt(NH3)2Cl2]

Where the marks are

Correct definition of chelate effect (1 mark)
Correct example of a chelate complex (1 mark)
Hybridisation of [Fe(CN)6]4- is d2sp3 (1 mark)
States the complex is diamagnetic (1 mark)
Formula [Pt(NH3)2Cl2] (1 mark)

Revise the chapter

Chapter: Coordination Compounds (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Coordination Compounds, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 56/1/1: every question, by section.

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CBSE Class 12 Chemistry 2025: question 32(B)

Hints

Step-by-step solution

Revise the chapter

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