3 moles of metal complex with formula Co(en)₂Cl₃ gives 3 moles of silver chloride on treatment with excess of silver nitrate. The secondary valency of Co in the complex is ___________.
(Round off to the nearest integer)

Solution

Chemistry ma’am · 4 steps

Silver chloride counts only the chlorides outside the bracket. The ones left inside are bonded to cobalt.

1. Ionisable chloride. Only free Cl⁻ ions in solution precipitate with AgNO₃. Here 3 mol of complex give 3 mol of AgCl, so each formula unit has just 1 ionisable chloride.
2. Write the formula. The other 2 chlorides must be bound to cobalt inside the coordination sphere. So the complex is [Co(en)₂Cl₂]Cl.
3. Count donor atoms. Secondary valency is the coordination number, the number of donor atoms bonded to Co. Ethylenediamine is bidentate: each en binds through two N atoms.

   C.N. = 2 × 2 (en) + 2 × 1 (Cl⁻) = 6

4. So. The secondary valency of Co in [Co(en)₂Cl₂]Cl is 6.

The common slip: 4. Counting each en as one donor atom gives 2 + 2 = 4, not 6. Ethylenediamine binds through both of its N atoms.

The idea to keep. Moles of AgCl per mole of complex tell you the chlorides outside the bracket. Secondary valency counts donor atoms inside it, so a bidentate ligand counts twice.

Answer from the NTA final answer key. Solution by PrepWiser.