Question

Cobalt (III) chloride forms several octahedral complexes with ammonia. Which of the following will not give test for chloride ions with silver nitrate at $25^{\circ } C$?

• A. ${CoCl3.5NH3 }$
• B. ${ CoCl3.6NH3 }$
• C. ${ CoCl3.3NH3}$
• D. ${ CoCl3.4NH_3}$

Answer 1

The Correct Option is C

To solve this question, we need to understand the coordination chemistry of cobalt (III) complexes and the behavior of chloride ions within these complexes.

1. The concern here is which complex does not give a chloride ion test with silver nitrate (AgNO_3) at 25^{\circ} C. This would mean that chloride ions are not available in the solution in their free form.
2. Cobalt (III) chloride forms various coordination complexes with ammonia. The general formula is CoCl_3\cdot xNH_3, where the ammonia molecules act as ligands, and some or all chloride ions may become part of the coordination sphere.
3. When a complex is formed where chloride ions are inside the coordination sphere, they do not dissociate into the solution, and thus do not react with silver ions from AgNO_3 to form a precipitate of AgCl.
4. Analyzing the given options:

• CoCl_3\cdot 5NH_3: This complex is [CoCl(NH_3)_5]Cl₂. In this form, two chloride ions are outside the coordination sphere and will give a test with AgNO_3.
• CoCl_3\cdot 6NH_3: This complex is [Co(NH_3)_6]Cl₃. Here, all three chloride ions are outside the coordination sphere and will give a test with AgNO_3.
• CoCl_3\cdot 3NH_3: This complex is [CoCl_3(NH_3)_3]. This "ammine chloride" complex does not dissociate any chloride ions into the solution, so it will not give a test with AgNO_3.
• CoCl_3\cdot 4NH_3: This complex is [CoCl_2(NH_3)_4]Cl. One chloride ion is outside the coordination sphere and will give a test with AgNO_3.

Based on the analysis, the complex which will not give a test for chloride ions with silver nitrate is CoCl_3\cdot 3NH_3.