Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

A mole of complex compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3}\) gives 3 mole of ions, when dissolved in water. One mole of the same complex reacts with two mole of \(\mathrm{AgNO}_{3}\) solution to form two mole of \(\mathrm{AgCl}(\mathrm{s})\). The structure of the complex is \([2003]\) (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right] .2 \mathrm{NH}_{3}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right] \cdot \mathrm{Cl}_{2}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl} .2 \mathrm{NH}_{3}\) (d) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}_{2} .2 \mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The structure of the complex is (b) \([\text{Co(NH}_3\text{)}_5\text{Cl}]\cdot \text{Cl}_2\).

Step 1: Analyze the Ionization of the Complex

The complex compound \(\text{Co(NH}_3\text{)}_5\text{Cl}_3\) ionizes in solution to give 3 moles of ions. This implies the compound can dissociate into 3 distinct ions in solution. Structural options must result in the dissociation into 3 overall ions.

Step 2: Determine the Reaction with AgNO3

When one mole of the complex reacts with two moles of \(\text{AgNO}_3\) to form two moles of \(\text{AgCl(s)}\), this indicates that there are two chloride ions in the outer sphere (uncoordinated). Thus, 2 chloride ions are free and precipitate as \(\text{AgCl}\) when reacted with silver nitrate.

Step 3: Identify the Coordination and Hydration Sphere

Given the complex's interaction and extent of reaction with \(\text{AgNO}_3\), options must fit the form where 2 chlorides are not coordinated but free in the solution. The complex should be configured to have \(\text{Co(NH}_3\text{)}_5\text{Cl}\), and the extra \(\text{Cl}_2\) should be outside coordination.

Step 4: Deduce Correct Structural Option

Analyzing each option:- Option (a): \(\text{[Co(NH}_3\text{)}_3\text{Cl}_3]\cdot 2\text{NH}_3\) - 3 coordinated chlorides cannot release 2 free \(\text{Cl}^-\).- Option (b): \(\text{[Co(NH}_3\text{)}_5\text{Cl]}\cdot \text{Cl}_2\) - Exactly fits the disassociation and precipitation criteria.- Option (c) and (d) also involve too many or poorly coordinated ions to explain the given reactions.

Key Concepts

Ionization of Complexes

When a complex compound dissolves in water, ionization occurs, breaking the complex into individual ions. Specifically for the compound \(\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}_3\), ionization results in the formation of 3 moles of ions from one mole of the compound.
This dissociation often involves separating the metal center from its ligands, and any free ions present.In coordination chemistry, the ability of a compound to ionize depends on the bonds between the central atom and the ligands.
Stronger bonds mean less ionization, whereas weaker bonds mean more ions are produced.

• For \(\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}_3\), the ionization process suggests one cobalt and two chloride ions are part of the coordination sphere, remaining intact, while other chloride ions are free to dissociate.

This aligns with the structural possibility where cobalt forms a stable inner coordination sphere, leaving some chloride ions free. Thus, allowing for three ions in solution, consistent with the structure \(\left[\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}\right] \cdot \mathrm{Cl}_2\).

Chloride Precipitation

The interaction of chloride ions within a coordination compound can be determined by their reaction with agents like silver nitrate (\(\mathrm{AgNO}_3\)).
In our case, two moles of \(\mathrm{AgNO}_3\), when added to one mole of the complex \(\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}_3\), result in the formation of two moles of \(\mathrm{AgCl(s)}\).This precipitation reaction indicates the presence of free chloride ions outside the coordination sphere.
These are readily available to react with silver ions to form silver chloride.

• Only chloride ions located outside the primary coordination shell would precipitate out when \(\mathrm{AgNO}_3\) is added.
• In this structure, this implies that two chloride ions are typically found in the "outer sphere," leading to their precipitation.

This supports the formula \(\left[\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}\right]\cdot \mathrm{Cl}_2\), which has two moles of chloride ions free and available for reaction.

Coordination Chemistry

Coordination chemistry revolves around the behavior and bonding of metal complexes.
Central to this is the coordination sphere, consisting of the metal ion and its directly bonded ligands.In \(\mathrm{Co(NH}_3\mathrm{)}_5\mathrm{Cl}_3\), the cobalt is the central metal ion, forming bonds with ammonia ligands and a chloride within the inner coordination sphere.
The remaining chloride ions are located outside the coordination sphere.

• Ligands like ammonia (\(\mathrm{NH}_3\)) are neutral, contributing no charge to the ionization process. But they stabilize the metal center through coordination bonds.
• What falls outside this coordination sphere, such as the two chloride ions, plays a critical role in reactions like precipitation with \(\mathrm{AgNO}_3\).

Understanding these interactions within the coordination sphere helps in predicting not just ionization but also how complexes interact with other chemical entities, leading to practical applications in synthesis and industrial chemistry.