Question

One mole of the octahedral complex compound \( \text{Co(NH}_3)_5\text{Cl}_3 \) gives 3 moles of ions on dissolution in water. One mole of the same complex reacts with excess of \( \text{AgNO}_3 \) solution to yield two moles of \( \text{AgCl}(s) \). The structure of the complex is:

• A. \( [{Co(NH}_3{)}_5{Cl}]{Cl}_2 \)
• B. \( [{Co(NH}_3{)}_4{Cl}]\cdot {Cl}_2{NH}_3 \)
• C. \( [{Co(NH}_3{)}_4{Cl}_2]{Cl}\cdot {NH}_3 \)
• D. \( [{Co(NH}_3{)}_3{Cl}]\cdot {Cl}_3\cdot 2{NH}_3 \)

Hint:

The number of free ions in a complex can be determined by how they interact with reagents like \( {AgNO}_3 \). In this case, the number of chloride ions reacting with \( {AgNO}_3 \) determines which chloride ions are free.

Answer 1

The Correct Option is A

To ascertain the correct configuration of the complex compound \( \text{Co(NH}_3)_5\text{Cl}_3 \), we must examine its behavior in aqueous solution and its reactivity with an excess of \( \text{AgNO}_3 \):

1. Dissolving one mole of the complex in water yields three moles of ions.
2. Reaction of one mole of the complex with excess \( \text{AgNO}_3 \) produces two moles of \( \text{AgCl} \).

We will assess each potential structural arrangement against these observations:

1. Option 1: \([ \text{Co(NH}_3)_5\text{Cl}] \text{Cl}_2\)

In water, this complex dissociates as follows:

\([ \text{Co(NH}_3)_5\text{Cl}]^{2+} + 2\text{Cl}^- \rightarrow 3 \text{ ions}\)

Upon reaction with excess \( \text{AgNO}_3 \), the two chloride ions precipitate as \( \text{AgCl} \):

\(2\text{Cl}^- + 2\text{Ag}^+ \rightarrow 2\text{AgCl} \)

Consequently, Option 1 satisfies both conditions: it generates three ions upon dissolution and yields two moles of \( \text{AgCl} \) when treated with \( \text{AgNO}_3 \).

1. Option 2: \([ \text{Co(NH}_3)_4\text{Cl} ] \cdot \text{Cl}_2\text{NH}_3\)

This structure is inconsistent with the specified conditions due to the implication of additional coordinated water/ammonia molecules, which would alter the dissociation into more than the stated 3 ions.

1. Option 3: \([ \text{Co(NH}_3)_4\text{Cl}_2 ]\text{Cl} \cdot \text{NH}_3\)

This arrangement implies the presence of two coordinated chlorides, which would result in different reactivity with \( \text{AgNO}_3 \), making the formation of two moles of \( \text{AgCl} \) improbable.

1. Option 4: \([ \text{Co(NH}_3)_3\text{Cl} ] \cdot \text{Cl}_3 \cdot 2\text{NH}_3\)

This option suggests an incorrect stoichiometry, leading to a number of ions that do not align with the provided data.

Therefore, based on the provided data, the definitive structure of the complex is:

\([Co(NH_3)_5Cl]Cl_2\).