Match List-I with List-II for the cobalt(III) chloride–ammonia complexes studied by Werner.
| List-I (Quantity) | List-II (Value) |
|---|---|
| (A) Moles of AgCl precipitated from 1 mol of the yellow complex by an excess of silver nitrate solution in cold | (I) 1 |
| (B) Number of chloride ions bound directly to cobalt, inside the square brackets, in the purple complex | (II) 3 |
| (C) Secondary valence of cobalt in the green complex | (III) 4 |
| (D) Number of ammonia molecules bound directly to cobalt in the violet complex | (IV) 6 |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A-II, B-I, C-IV, D-III
In Werner’s formulation, six groups (chloride ions, ammonia molecules or both) are bound to cobalt inside the square brackets. Only the chloride ions outside the brackets are precipitated as AgCl.
(A) Moles of AgCl from 1 mol of the yellow complex — This matches (II) 3. The yellow complex is \( \ce{[Co(NH3)6]^3+ 3Cl-} \). All three chloride ions are outside the bracket, so 3 mol of AgCl is precipitated.
(B) Chloride ions bound directly to cobalt in the purple complex — This matches (I) 1. The purple complex is \( \ce{[CoCl(NH3)5]^2+ 2Cl-} \). One chloride ion is inside the bracket; the other two are counter ions.
(C) Secondary valence of cobalt in the green complex — This matches (IV) 6. The green complex is \( \ce{[CoCl2(NH3)4]^+ Cl-} \). The secondary valence counts all groups bound directly to cobalt: 2 Cl + 4 NH3 = 6.
(D) Ammonia molecules bound directly to cobalt in the violet complex — This matches (III) 4. The violet complex has the same formula as the green one, \( \ce{[CoCl2(NH3)4]^+ Cl-} \). Four NH3 molecules are bound to cobalt.
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (Violet) gave 1 mol AgCl”
“These observations, together with the results of conductivity measurements in solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 5.1, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions. Werner proposed the term secondary valence for the number of groups bound directly to the metal ion; in each of these examples the secondary valences are six.”
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes