Which of the following is a characteristic of the secondary valences of a metal ion according to Werner’s theory?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → They are non-ionisable.
The secondary valence is the number of groups bound directly to the metal ion, that is, the coordination number. These groups stay inside the square bracket as one entity, so they do not ionise in solution. In \( \ce{[Co(NH3)6]Cl3} \), for example, the six \( \ce{NH3} \) molecules stay on the cobalt, while the three \( \ce{Cl^-} \) ions are free and precipitate as AgCl.
Why the other options are incorrect
Option (1) They are normally ionisable. — Incorrect. It is the primary valences that are normally ionisable, satisfied by negative ions such as the free \( \ce{Cl^-} \).
Option (3) They correspond to the oxidation state of the metal. — Incorrect. The primary valence matches the oxidation state: \( \ce{CrCl3} \) has primary valence 3 (Cr is +3). The secondary valence is the coordination number.
Option (4) They have no definite arrangement in space around the metal ion. — Incorrect. Groups held by secondary linkages have characteristic spatial arrangements (octahedral, tetrahedral, square planar), now called coordination polyhedra.
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “3. The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions.”