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 (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) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}_{2} .2 \mathrm{NH}_{3}\)

Short answer

The correct structure is (b) \([\mathrm{Co}(\mathrm{NH}_{3})_{5} \mathrm{Cl}] \cdot \mathrm{Cl}_{2}\).

Step-by-step solution

Understanding the Ion Formation

Given: The complex compound \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3} \) gives 3 moles of ions when dissolved in water. This indicates that two ions must be from chlorine dissociating as separate ions, given the chloride ion typically forms \( \mathrm{Cl}^- \). With this, we infer the structure must have at least two chlorine ions dissociating from the main coordination sphere to produce three total ions: \(\mathrm{cation\;complex}^+\), \(\mathrm{Cl}^-\), and \(\mathrm{Cl}^-\).

Reaction with Silver Nitrate

The reaction of one mole of the complex with two moles of \( \mathrm{AgNO}_3 \) forming two moles of \( \mathrm{AgCl(s)} \) suggests that two chlorine atoms are free or labile enough in the solution to react with silver. This supports having two chloride ions outside the coordination sphere, which would react with \( \mathrm{Ag}^+ \) ions to precipitate as \( \mathrm{AgCl} \).

Determining Coordination Sphere

Knowing that two \( \mathrm{Cl}^- \) are crucial for ionization, these chlorines must reside outside the coordination sphere. The coordination number of cobalt, considering ammonia and chloride ligands, is typically 6. This coordination number can be satisfied with 5 ammine groups and one chloride ligand within the coordination sphere. The structure of the coordination complex aligns with this if written as \( [\mathrm{Co}(\mathrm{NH}_3)_5 \mathrm{Cl}] \cdot \mathrm{Cl}_2 \), where the dot signifies two outer-sphere (distinguishable) chloride ions.

Verify with Options

Review the provided options:(a) \([\mathrm{Co}(\mathrm{NH}_{3})_{3} \mathrm{Cl}_{3}].2 \mathrm{NH}_{3}\) - Incorrect as it implies no free chloride ion.(b) \([\mathrm{Co}(\mathrm{NH}_{3})_{5} \mathrm{Cl}] \cdot \mathrm{Cl}_{2}\) - Correct as explained by three ions and reaction with 2 moles \(\mathrm{AgNO}_3\) forming 2 moles \( \mathrm{AgCl} \).(c) & (d) also imply coordination numbers or structures inconsistent with the number of required free chloride ions.

Confirm Answer

Hence, option (b) \([\mathrm{Co}(\mathrm{NH}_{3})_{5} \mathrm{Cl}] \cdot \mathrm{Cl}_{2}\) matches all details: accounting for 3 ions formed and reaction with 2 moles \( \mathrm{AgNO}_{3} \).

Key concepts

Coordination Compounds

Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ligands. These ligands donate electrons to form coordinate bonds with the metal. For instance, in the complex compound \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3} \), cobalt is the central metal, and the ammonia (\( \mathrm{NH}_3 \)) and chloride ions act as ligands.

Coordination compounds are crucial in chemistry due to their diverse applications, from catalysis to biological systems. Understanding their structures helps predict their reactivity and properties. In coordination chemistry, the metal along with its immediate ligands forms the 'coordination sphere', while other species outside this sphere, such as counter ions, help balance the charge but are not directly bonded to the metal.

For example, in \( [\mathrm{Co}(\mathrm{NH}_3)_5 \mathrm{Cl}] \cdot \mathrm{Cl}_2 \), the part inside the brackets constitutes the coordination sphere, whereas the chloride ions after the dot are outside, acting as counter ions to balance the complex's overall charge.

Coordination Number

The term 'coordination number' refers to the number of ligand donor atoms bonded to the central metal ion in a coordination compound. It is a key descriptor in coordination chemistry.

In our example of the cobalt complex \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3} \), the coordination number is 6, implying cobalt is bonded to five ammonia molecules and one chloride ion. These ligands form the essential part of the complex that decides its geometry based on their spatial arrangement.

Understanding the coordination number helps predict geometrical shapes and potential reactions of coordination compounds. Typically, a coordination number of 6 leads to an octahedral geometry, which is common in complexes involving transition metals like cobalt.

Ions in Solution

When coordination compounds dissolve in water, they often dissociate into smaller ions based on their structure. This process greatly influences the properties of the solution. The number of ions produced upon dissociation can reveal aspects about the compound's structure.

In the case of \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3} \), the solution yields three ions: one complex cation and two chloride anions. This occurs as two of the chloride ligands remain outside the coordination sphere, thus dissociating freely in the solution.

By analyzing the resulting ions, scientists can deduce the structural organization of the coordination compound, like which ligands are bound within the coordination sphere and which are free.Getting insights into ions in solution is pivotal for applications like drug formulations and chemical analysis.

Complex Ion Reactions

Reactions involving complex ions can provide valuable insight into the stability and structure of coordination compounds. Understanding these reactions helps chemists predict how a compound will interact with other chemicals.

Consider the reaction of \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}_{3} \) with silver nitrate \( (\mathrm{AgNO}_3) \). In this scenario, two chloride ions from outside the coordination sphere react with silver ions from the silver nitrate to form solid silver chloride \( (\mathrm{AgCl}) \). This solid formation indicates those chloride ions are not tightly bound and remain accessible for reaction.

Analyzing such complex ion reactions allows us to confirm which ligands are part of the coordination sphere and which are not. These reactions also play a profound role in practical and industrial applications, such as refining, purification processes, and testing for the presence of particular ions.