A coordination complex compound of cobalt has molecular formula conta…

The correct answer is Option (3) → [Co(NH₃)₅(NO₂)]Cl₂

In a coordination compound, the groups written inside the square bracket are bonded directly to the metal and form one entity that does not break up in water. The ions written outside the bracket are counter ions; they are ionisable and go into solution as free ions. Only a chloride ion that is free in solution can be precipitated as AgCl by silver nitrate; a chloride bonded to the metal is not.

The data decide the formula in three steps:

• 2 mol AgCl per mole: two chloride ions are outside the bracket, as counter ions.

• 3 mol of ions per mole: one complex ion plus two \( \ce{Cl^-} \) ions, a 1:2 electrolyte.

• The rest is inside the bracket: five \( \ce{NH3} \) and one \( \ce{NO2^-} \) are bonded to cobalt, which gives cobalt its usual coordination number of 6.

• Oxidation state of Co: from the charges, x + 5(0) + (−1) + 2(−1) = 0, so x = +3.

In water the compound ionises as:

    \( \displaystyle \ce{[Co(NH3)5(NO2)]Cl2 -> [Co(NH3)5(NO2)]^2+ + 2Cl^-} \)

With excess silver nitrate, the two free chloride ions are precipitated:

    \( \displaystyle \ce{[Co(NH3)5(NO2)]Cl2 + 2AgNO3 -> [Co(NH3)5(NO2)](NO3)2 + 2AgCl v} \)

So the formula is [Co(NH₃)₅(NO₂)]Cl₂, pentaamminenitrito-N-cobalt(III) chloride.

Why the other options are incorrect

Option (1) [Co(NH₃)₄(NO₂)Cl][(NH₃)Cl] — Incorrect.

• Ammonia is a neutral molecule and does not form a counter ion with chloride, so “[(NH₃)Cl]” is not a real ion.

• Here one chloride is bonded to cobalt, so at most one chloride could be precipitated, not two.

Option (2) [Co(NH₃)₅Cl][Cl(NO₂)] — Incorrect.

• There is no ion “[Cl(NO₂)]”.

• Even if chloride and nitrite were both counter ions, one chloride would be bonded to cobalt inside the bracket, so only 1 mol of AgCl would form instead of 2.

Option (4) [Co(NH₃)₅][(NO₂)Cl₂] — Incorrect.

• It gives cobalt a coordination number of only 5 and puts the two chlorides inside an anion “[(NO₂)Cl₂]”, which does not exist.

• Chlorides held inside a bracket are not free to give AgCl, so this formula cannot explain 2 mol of precipitate.

NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “… In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution.”

Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes