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
The secondary valency and the number of hydrogen bonded water molecule(s) in CuSO₄·5H₂O, respectively, are:
- a
6 and 4
- b✓
4 and 1
- c
6 and 5
- d
5 and 1
4 and 1
🧠 The Werner Picture of Blue Vitriol
is structurally :
- 4 waters sit in the equatorial plane of (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 splits as .
⚡ Why Square-Planar?
is . 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.
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