A complex has a composition corresponding to the formula `CoBr_(2)Cl.4NH_(3)`. What is the structural formula if conductance measurements show two ions per formula unit ? Silver nitrate solution given an immediate precipitate of AgCl but AgBr :-

Options

A. `[CoBrCl(NH_(3))_(4)]Br`
B. `[CoCl(NH_(3))_(4)]Br_(2)`
C. `[CoBr_(2)Cl(NH_(3))_(4)]`
D. `[CoBr_(2)(NH_(3))_(4)]Cl`

Answer

To solve the problem, we need to analyze the given coordination compound and the information provided regarding its behavior in solution. Here’s a step-by-step breakdown of the solution:

### Step 1: Analyze the Given Formula
The coordination compound is given as `CoBr2Cl·4NH3`. This indicates that cobalt (Co) is the central metal ion, surrounded by two bromide ions (Br⁻), one chloride ion (Cl⁻), and four ammonia ligands (NH₃).

### Step 2: Understand Conductance Measurements
The problem states that conductance measurements show two ions per formula unit. This suggests that when the compound dissociates in solution, it produces two charged entities.

### Step 3: Identify the Ionization and Coordination Spheres
In coordination chemistry:
- The **coordination sphere** refers to the central metal ion and the ligands directly bonded to it (inside the square brackets).
- The **ionization sphere** refers to the ions that are not directly bonded to the metal and can dissociate in solution.

From the conductance measurement indicating two ions, we can infer:
- The chloride ion (Cl⁻) is likely in the ionization sphere because it is the only ion that precipitates with AgNO₃, indicating it is free in solution.
- The bromide ions (Br⁻) must be part of the coordination sphere since they do not precipitate with AgNO₃.

### Step 4: Formulate the Structural Formula
Based on the above analysis:
- The structural formula should have the ammonia ligands and the bromide ions in the coordination sphere, while the chloride ion is outside in the ionization sphere.
- Thus, the structural formula can be represented as:
\[ [Co(NH_3)_4Br_2]Cl \]

### Step 5: Confirm with Silver Nitrate Reaction
When this compound reacts with silver nitrate (AgNO₃), the chloride ion will precipitate as AgCl:
\[ [Co(NH_3)_4Br_2]Cl + AgNO_3 \rightarrow [Co(NH_3)_4Br_2]^+ + NO_3^- + AgCl \downarrow \]
This confirms that the chloride ion is indeed in the ionization sphere.

### Final Structural Formula
Thus, the final structural formula is:
\[ [Co(NH_3)_4Br_2]Cl \]