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Step-by-Step Explanation
Step 1: Understand the Nature of the Complexes
Each compound mentioned is a cobalt(III) complex with different ammonia (NH3) to chloride (Cl−) ratios. A cobalt(III) complex is octahedral, meaning the coordination number (number of sigma bonds to the cobalt center) is 6.
Step 2: Determine the Binding Mode of Chloride Ions
In an octahedral complex of cobalt(III) with ammonia, the ligands around cobalt can be either ammonia molecules or chloride ions. If all chloride ions are inside the coordination sphere (i.e., directly bonded to the cobalt), they will not be free in solution and thus will not precipitate with AgNO3. Conversely, any chloride ion outside the coordination sphere exists as a free ion that can be tested with AgNO3 to form AgCl (a white precipitate).
Step 3: Analyze Each Given Formula
CoCl3 · 6NH3:
Typically written as [Co(NH3)6]Cl3. Here, none of the chlorides is within the coordination sphere; all three chlorides are outside as counter-ions. These chlorides are free to react with AgNO3, giving a chloride test.
CoCl3 · 5NH3:
Can be formulated as [Co(NH3)5Cl]Cl2 or a similar structure, depending on the exact complex. Still, there would be free chloride ions in solution to react with AgNO3.
CoCl3 · 4NH3:
Likely formulated as [Co(NH3)4Cl2]Cl, indicating at least one chloride remains outside the coordination sphere, so the Cl− would still be available to react with AgNO3.
CoCl3 · 3NH3:
When three chloride ions and three ammonia molecules all bind to cobalt, the formula can be represented as [Co(NH3)3Cl3]. In this case, all three chlorides are within the coordination sphere, leaving no free chloride ions in solution to give a test with AgNO3.
Step 4: Conclusion
The complex CoCl3 · 3NH3 (or [Co(NH3)3Cl3]) will not give the chloride ion test with silver nitrate because all the chloride ions are coordinated to cobalt inside the coordination sphere, and thus they are not free to precipitate as silver chloride.
