Read the passage carefully and answer the Questions.
Werner in 1898, propounded his theory of coordination compounds. In coordination compounds metals show two types of linkages (valences)-primary and secondary. The primary valences are normally ionisable and are satisfied by negative ions. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. The ions or group bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to the different coordination numbers. Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, Mohr's salt, potash alum etc. dissociate into simple ions completely when dissolved in water. But complex ions do not dissociate completely into simple ions.
Which of the following statement is correct as per Werner theory?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the number of groups bound directly to the metal ion corresponding to the coordination number are Secondary valences
Werner’s secondary valence is non-ionisable and is equal to the coordination number; the primary valence is ionisable and corresponds to the oxidation state.
In \( \ce{[Co(NH3)6]Cl3} \), for example, the primary valence of cobalt is 3 (three ionisable \( \ce{Cl^-} \) ions, all precipitated by silver nitrate), and the secondary valence is 6 (six \( \ce{NH3} \) molecules bound to cobalt). So the groups bound directly to the metal, equal in number to the coordination number, make up the secondary valences.
Why the other options are incorrect
Option (1) Secondary valences are ionisable. — Incorrect. Secondary valences are non-ionisable; the groups in the coordination sphere stay bound to the metal in solution.
Option (3) primary valences are non-ionisable — Incorrect. Primary valences are normally ionisable; the counter ions outside the bracket are free in solution.
Option (4) primary valences correspond to the coordination number of the metal — Incorrect. The coordination number is the secondary valence; the primary valence corresponds to the oxidation state of the metal.
NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – …
