JEE Main · 2021mediumCORD-068

The secondary valency and the number of hydrogen bonded water molecule(s) in CuSO₄·5H₂O, respectively, are:

Coordination Compounds · Class 12 · JEE Main Previous Year Question

Question

The secondary valency and the number of hydrogen bonded water molecule(s) in CuSO₄·5H₂O, respectively, are:

Options
  1. a

    6 and 4

  2. b

    4 and 1

  3. c

    6 and 5

  4. d

    5 and 1

Correct Answerb

4 and 1

Detailed Solution

🧠 The Werner Picture of Blue Vitriol

CuSO45H2O\mathrm{CuSO_4 \cdot 5H_2O} is structurally [Cu(H2O)4]SO4H2O[\mathrm{Cu(H_2O)_4}]\mathrm{SO_4 \cdot H_2O}:

  • 4 waters sit in the equatorial plane of Cu2+\mathrm{Cu^{2+}} (square-planar coordination).
  • 1 water sits outside the coordination sphere, hydrogen-bonded to two coordinated waters and to the sulfate.
  • (Sulfate provides 2 weak axial Cu–O contacts in the crystal, but these are not counted in Werner's "secondary valency" by NCERT convention.)

So:

  • Secondary valency (Werner's term for primary coordination) = 4.
  • H-bonded waters = 1 (the lattice water).

🗺️ The 4+1 Anchor

Memorise the picture: 4 waters bound to Cu, 1 water in the lattice. The "5" in CuSO45H2O\mathrm{CuSO_4 \cdot 5H_2O} splits as 4+14 + 1.

Why Square-Planar?

Cu2+\mathrm{Cu^{2+}} is d9\mathrm{d^9}. Strong Jahn–Teller distortion elongates two trans bonds so much that the equatorial plane dominates → effectively square-planar with weak axial contacts. The four waters in the plane give CN = 4 by Werner's tally.

⚠️ Don't Add Up to 5

The secondary valency is not 5. The 5th water is hydrogen-bonded only — it's not a primary Cu–O bond. Similarly, secondary valency is not 6 even though the actual CN with axial sulfate contacts is sometimes given as 6 in advanced texts; for JEE, use the NCERT picture: 4.

Answer: (2) 4 and 1\boxed{\text{Answer: (2) 4 and 1}}

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