Which of the following statements is incorrect?

The correct answer is Option (2) → In K₃[Fe(CN)₆] the ligand has satisfied only the secondary valency of ferric ion

• In Werner’s theory the primary valency of a metal ion is its oxidation state; it is satisfied by negative ions. The secondary valency is its coordination number; it is satisfied by neutral molecules or negative ions bound directly to the metal.

• So a negative ligand inside the square bracket satisfies both valencies at once, while a neutral ligand satisfies only the secondary valency.

Option (1) In K₃[Fe(CN)₆], the ligand has satisfied both primary and secondary valencies of ferric ion — Correct statement. \( \ce{Fe^3+} \) has primary valency 3 and secondary valency 6. The six \( \ce{CN^-} \) ions occupy the six coordination positions and, being negative ions, also neutralise the +3 charge of the metal.

Option (2) In K₃[Fe(CN)₆] the ligand has satisfied only the secondary valency of ferric ion — Incorrect statement. \( \ce{CN^-} \) is a negative ligand, so it satisfies the primary valency too; the \( \ce{K^+} \) ions are only counter ions balancing the charge of \( \ce{[Fe(CN)6]^3-} \).

Option (3) In K₄[Fe(CN)₆] the ligand has satisfied both primary and secondary valencies of ferrous ion — Correct statement. \( \ce{Fe^2+} \) has primary valency 2 and secondary valency 6; the six negative \( \ce{CN^-} \) ligands satisfy both.

Option (4) In [Cu(NH₃)₄]SO₄, the ligand has satisfied only the secondary valency of copper — Correct statement. \( \ce{NH3} \) is a neutral molecule, so the four \( \ce{NH3} \) satisfy only the secondary valency (4) of \( \ce{Cu^2+} \); its primary valency (2) is satisfied by the ionisable \( \ce{SO4^2-} \) ion outside the bracket.

NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “2. The primary valences are normally ionisable and are satisfied by negative ions.”

“3. 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.”