Question

A coordination compound of cobalt (III) contains four ammonia molecules, one sulfate ion, and one chloride ion. Addition of aqueous \(\mathrm{BaCl}_{2}\) solution to an aqueous solution of the compound gives no precipitate. Addition of aqueous \(\mathrm{AgNO}_{3}\) to an aqueous solution of the compound produces a white precipitate. Propose a structure for this coordination compound.

Short answer

The structure of the coordination compound is [Co(NH3)4(SO4)]Cl2. The coordination sphere contains the cobalt (III) ion, four ammonia ligands, and a sulfate ion. The chloride ion acts as a counterion outside the coordination sphere. The chemical tests indicate that the sulfate ion is within the coordination sphere, while the chloride ion is acting as a counterion.

Step-by-step solution

List the given information

We know that the coordination compound contains: - Cobalt (III) as the central metal ion - Four ammonia molecules as ligands - One sulfate ion - One chloride ion We also know that when the compound is mixed with: - Aqueous \(\mathrm{BaCl}_{2}\), no precipitate forms - Aqueous \(\mathrm{AgNO}_{3}\), a white precipitate forms

Identify the coordination sphere

The coordination sphere contains the central metal ion and its ligands. In this case, the coordination sphere will contain the cobalt (III) ion and four ammonia molecules. The coordination sphere is usually written as [Co(NH3)4]

Analyze the results of the tests

When the compound is mixed with aqueous \(\mathrm{BaCl}_{2}\), no precipitate forms. This indicates that the sulfate ion is not acting as a counterion, because \(\mathrm{BaSO}_{4}\) is insoluble in water and would form a precipitate if the sulfate ion was acting as a counterion. Thus, the sulfate ion must also be within the coordination sphere. When the compound is mixed with aqueous \(\mathrm{AgNO}_{3}\), a white precipitate forms, which indicates the formation of \(\mathrm{AgCl}\), an insoluble compound in water. Thus, the chloride ion must be acting as a counterion outside the coordination sphere.

Propose the structure of the coordination compound

Considering the information obtained from the chemical tests, we conclude that the coordination sphere contains both the ammonia ligands and the sulfate ion. Since the charge of cobalt (III) is +3, the charge of the coordination sphere becomes +2 due to the presence of the sulfate ion with charge -1. The coordination sphere: [Co(NH3)4(SO4)] As the chloride ion acts as a counterion and the coordination sphere has a charge of +2, we will need two chloride ions to balance the overall charge of the complex. The final structure of the coordination compound is: [Co(NH3)4(SO4)]Cl2

Key concepts

Cobalt (III) complex

Cobalt (III) complexes are coordination compounds featuring cobalt in the +3 oxidation state. In coordination chemistry, the central metal atom, in this case, cobalt, connects with molecules or ions called ligands to form a structured complex. These complexes are notable for their ability to coordinate with various ligands to create diverse structures.

One of the intriguing aspects of cobalt (III) complexes is their ability to stabilize unusual coordination environments due to the specific electronic configuration of cobalt. This characteristic is crucial in many biological and industrial processes. Cobalt (III) acts as an excellent center owing to its magnetic properties and ability to engage in low-spin or high-spin states, depending on the ligands associated with it.

In our coordination compound, cobalt (III) is bonded prominently within the coordination sphere, binding with ammonia molecules, enhancing the stability of the complex through covalent bond formation.

Coordination sphere

In coordination chemistry, the coordination sphere includes the central metal ion and its directly bonded ligands, forming a distinct entity within the compound. It's represented in brackets, denoting its uniqueness in terms of chemical and structural characteristics.

For our cobalt (III) complex, the coordination sphere includes the central cobalt ion and four ammonia molecules. This arrangement is crucial as it defines the core structure of the coordination compound, influencing both its reactivity and properties. Notably, in this compound, the sulfate ion is also part of the coordination sphere, meaning it directly interacts with cobalt, which prevents it from acting like a counterion.

• The coordination sphere contains the metal ion with its ligands.
• It determines the geometry and spatial arrangement of the complex.
• In our example: [Co(NH3)4(SO4)]

Understanding the composition of the coordination sphere offers vital insights into how the complex will interact in different environments.

Ligands

Ligands are ions or neutral molecules that form bonds with a central metal atom in a complex compound by donating electron pairs. They are crucial for understanding how coordination complexes behave and stabilize.

In our case, ammonia acts as a ligand, coordinating with the central cobalt ion. Its ability to easily share its lone pair of electrons makes ammonia a typical ligand in the formation of coordination complexes. Ligands dictate the coordination number, which represents the number of ligand attachments to the central metal, and influence the geometry.

Ammonia, being a neutral ligand, doesn't affect the charge directly. However, it creates a stable electrostatic and covalent interaction with cobalt (III), impacting the geometry and overall form of the complex.

Counterion

Counterions play a balancing role in coordination complexes, compensating for any charge discrepancies created by the coordination sphere. They are often not directly bonded to the central metal ion but exist outside the coordination sphere.

In the cobalt (III) complex discussed, the chloride ion serves as the counterion. It remains outside the coordination sphere, balancing the overall charge of the complex. A key aspect is that counterions can be replaced or removed, affecting the solubility and reactivity of the entire compound.

• Counterions maintain neutrality within coordination compounds.
• They are not directly involved in the primary structure of coordination.
• Examples include chloride in [Co(NH3)4(SO4)]Cl2, where Cl balances charge.

Understanding counterions aids in predicting precipitate formation and exploring reaction pathways.

Precipitation reactions

Precipitation reactions are processes where insoluble solids form when two solutions are mixed. In coordination chemistry, such reactions often indicate the presence of particular ions or counterions.

In our scenario, adding \(\mathrm{AgNO}_{3}\) to the cobalt complex solution results in a white precipitate. This formation of \(\mathrm{AgCl}\) reveals that the chloride ion is not within the coordination sphere but acts as a counterion. Conversely, no precipitate forms with \(\mathrm{BaCl}_{2}\), indicating sulfate is within the sphere.

• Precipitation reactions help identify the location of ions within a complex.
• They provide clues about counterion placement and coordination sphere contents.
• Reaction with \(\mathrm{AgNO}_{3}\) confirms chloride's role as a counterion by forming \(\mathrm{AgCl}\).

These reactions are essential in analyzing the composition and structure of coordination compounds.