CBSE 2025, Set 3

Question

Werner's coordination theory in 1893 was the first attempt to explain the bonding in coordination complexes. Werner was able to explain that in complexes, the metal shows two different sorts of valency: primary and secondary. Primary valences are normally ionisable whereas secondary valences are non ionisable.

One mole of CrCl$_3 \cdot$ 4H$_2$O precipitates one mole of AgCl when treated with excess of AgNO$_3$ solution.

Write 
(i) the structural formula of the complex, and 
(ii) the secondary valency of Cr.

Edvaya · Accepted Answer

The correct answer is Subjective. One mole of the compound precipitates **one mole of AgCl** with excess AgNO$_3$. AgCl is precipitated only by **free (ionisable) Cl$^-$** ions outside the coordination sphere. So only **1 Cl$^-$** is free (primary valency / ionisable), and **2 Cl$^-$** are inside the coordination sphere. Total ligands inside coordination sphere = 2 Cl + 4 H$_2$O = **6 ligands** (coordination number 6) **(i) Structural formula:** $$[	ext{Cr(H}_2	ext{O)}_4	ext{Cl}_2]	ext{Cl}$$ The complex ion is [Cr(H₂O)₄Cl₂]⁺ (cation) with Cl⁻ as the outer sphere counter ion. **(ii) Secondary valency of Cr:** Secondary valency = coordination number = **6** (4 H₂O + 2 Cl inside the coordination sphere)

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Chemistry > Inorganic Chemistry > Coordination Compounds

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