Cobalt (III) chloride forms several octahedral complexes with ammonia. Which of the following will not give test for chloride ions with silver nitrate at 25 ?
- CoCl3 5NH3
- CoCl3 6NH3
- CoCl3 3NH3
- CoCl3 4NH3
Answer
(3) CoCl3 3NH3
Solution
Co(III) has coordination number 6; ligands inside the coordination sphere are not ionisable.
CoCl3 6NH3 = [Co(NH3)6]Cl3 ⇒ 3 ionisable Cl-
CoCl3 5NH3 = [Co(NH3)5Cl]Cl2 ⇒ 2 ionisable Cl-
CoCl3 4NH3 = [Co(NH3)4Cl2]Cl ⇒ 1 ionisable Cl-
CoCl3 3NH3 = [Co(NH3)3Cl3] ⇒ no ionisable Cl-
∴ CoCl3 3NH3 gives no precipitate of AgCl with AgNO3.
Answer: (3)