Consider the following statements about the cobalt(III) chloride–ammonia complexes studied by Werner.
(A) 1 mol of the yellow complex, CoCl3.6NH3, gives 3 mol of AgCl with an excess of silver nitrate solution in cold.
(B) In the purple complex, CoCl3.5NH3, two chloride ions are bound to cobalt inside the square brackets.
(C) The green and the violet complexes have identical empirical formula, CoCl3.4NH3, but distinct properties.
(D) The solution conductivity of the violet complex corresponds to a 1:2 electrolyte.
(E) In each of the four complexes, the secondary valence of cobalt is six.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (D) only
Werner explained the AgCl and conductivity results by keeping six groups bound to cobalt inside a square bracket. Only the chloride ions outside the bracket are precipitated as AgCl or move as free ions in solution.
(A) 1 mol of the yellow complex, CoCl3.6NH3, gives 3 mol of AgCl with an excess of silver nitrate solution in cold. — Correct statement. The yellow complex is \( \ce{[Co(NH3)6]^3+ 3Cl-} \). All three chloride ions are outside the bracket, so all three are precipitated.
(B) In the purple complex, CoCl3.5NH3, two chloride ions are bound to cobalt inside the square brackets. — Incorrect statement. The purple complex gives 2 mol AgCl, so two chloride ions are outside the bracket. Table 5.1 formulates it as \( \ce{[CoCl(NH3)5]^2+ 2Cl-} \), with only one chloride ion bound to cobalt.
(C) The green and the violet complexes have identical empirical formula, CoCl3.4NH3, but distinct properties. — Correct statement. Both are formulated as \( \ce{[CoCl2(NH3)4]^+ Cl-} \) and differ in colour. Such compounds are isomers.
(D) The solution conductivity of the violet complex corresponds to a 1:2 electrolyte. — Incorrect statement. The violet complex, \( \ce{[CoCl2(NH3)4]^+ Cl-} \), has only one chloride ion outside the bracket. Table 5.1 lists its solution conductivity as that of a 1:1 electrolyte.
(E) In each of the four complexes, the secondary valence of cobalt is six. — Correct statement. In every complex six groups (chloride ions, ammonia molecules or both) are bound directly 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”
“…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.”
“Note that the last two compounds in Table 5.1 have identical empirical formula, CoCl3.4NH3, but distinct properties. Such compounds are termed as isomers.”
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes