Question

When CoCl\(_3\) solution is treated with excess ammonia, a violet coloured complex is formed which conducts current. Also, it gives one mole of AgCl when treated with AgNO\(_3\). What is the chemical formula of the complex

• A. [CoCl\(_2\)(NH\(_3\))\(_4\)]Cl
• B. [CoCl\(_3\)(NH\(_3\))\(_3\)]
• C. [CoCl(NH\(_3\))\(_5\)]Cl\(_2\)
• D. [Co(NH\(_3\))\(_6\)]Cl\(_3\)
• E. [CoCl(NH\(_3\))\(_5\)]Cl\(_2\).H\(_2\)O

Hint:

The number of chloride ions precipitated by AgNO\(_3\) equals the number of chloride ions outside the coordination sphere.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
This question involves Werner's theory of coordination compounds. We need to deduce the formula of a complex based on its reaction with silver nitrate (AgNO₃), which indicates the number of ionizable chloride ions (counter ions) outside the coordination sphere.
Step 2: Detailed Explanation
1. Starting Material: We start with CoCl₃. This means the oxidation state of cobalt is +3. 2. Conducts Current: The fact that the complex conducts current means it is an ionic compound that dissociates into ions in solution. 3. Reaction with AgNO₃: The complex reacts with AgNO₃ to give one mole of AgCl precipitate. This is the crucial piece of information. The reaction for the precipitate is \( \text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) \). This tells us that for every one mole of the complex, there is only one free chloride ion available to precipitate. This means one Cl⁻ ion is outside the coordination sphere as a counter ion. 4. Formula Structure: Based on the above point, the formula of the complex must be of the form \([\dots]\text{Cl}\). 5. Coordination Sphere: - The central metal is Cobalt (Co) in the +3 oxidation state. - The ligands are ammonia (NH₃) and chloride (Cl⁻). - Since one Cl⁻ is outside, the other two Cl⁻ ions must be inside the coordination sphere, acting as ligands. - Cobalt(III) complexes typically have a coordination number of 6. To satisfy this, if we have two Cl⁻ ligands, we must have four NH₃ ligands. - This gives the coordination sphere as \([\text{CoCl}_2(\text{NH}_3)_4]\). 6. Charge of the Coordination Sphere: Let's calculate the charge of this sphere: Charge = (charge of Co) + 2 (charge of Cl⁻) + 4 (charge of NH₃) Charge = (+3) + 2 (-1) + 4 (0) = +3 - 2 + 0 = +1. The coordination sphere has a charge of +1: \([\text{CoCl}_2(\text{NH}_3)_4]^+\). 7. Final Formula: To balance the +1 charge of the coordination sphere, we need one counter ion with a -1 charge, which is the chloride ion we identified in step 3. Therefore, the full formula of the complex is \([\text{CoCl}_2(\text{NH}_3)_4]\text{Cl}\). This formula is consistent with all the information given in the problem. The complex dichloridotetraamminecobalt(III) chloride exists as cis (violet) and trans (green) isomers.
Step 3: Final Answer
The chemical formula of the complex is \([\text{CoCl}_2(\text{NH}_3)_4]\text{Cl}\).