Which of the following postulates of Werner theory is incorrect?

The correct answer is Option (3) → The secondary valence is equal to the oxidation number of metal and is fixed for a metal.

Werner explained the structure of coordination compounds with two kinds of valence. The primary valence is ionisable; it equals the oxidation number of the metal and is satisfied by negative ions. The secondary valence is non-ionisable; it equals the coordination number and is satisfied by neutral molecules or negative ions bonded to the metal.

For example, in \( \ce{[Co(NH3)6]Cl3} \) the primary valence of cobalt is 3 (its oxidation state, satisfied by the three \( \ce{Cl^-} \) ions), while the secondary valence is 6 (the six \( \ce{NH3} \) ligands).

• Option (1) In coordination compounds metals show two types of linkages (valences)-primary and secondary. — A postulate of Werner theory. This is the first postulate.

• Option (2) The primary valences are normally ionisable and are satisfied by negative ions. — A postulate of Werner theory. These are the ions outside the coordination sphere, such as the \( \ce{Cl^-} \) ions precipitated by silver nitrate.

• Option (3) The secondary valence is equal to the oxidation number of metal and is fixed for a metal. — Not a postulate of Werner theory as written. The secondary valence equals the coordination number; it is the primary valence that corresponds to the oxidation number.

• Option (4) The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. — A postulate of Werner theory. These arrangements are the coordination polyhedra, such as octahedral for coordination number 6.

NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – …