Problem 116 - Chemistry The Molecular Science

Question

Early coordination chemists relied on close experimental observation to determine the formulas of coordination compounds. They found, for example, that aqueous \(\mathrm{BaCl}_{2}\) did not cause precipitation when added to a solution of a \(\mathrm{Co}^{3+}\) -containing coordination compound, but precipitation occurred when aqueous silver nitrate was added to a solution of the coordination compound. The coordination compound was known to contain one \(\mathrm{Co}^{3+}\) ion, one sulfate ion, one chloride ion, and four ammonia molecules. Write the structural formula of the coordination compound that is consistent with the experimental results.

Step-by-Step Solution

Answer

The structural formula is \[ [\text{Co}(\text{NH}_3)_4\text{Cl}]^{2+}\text{SO}_4^{2-}\] indicating coordination of Cl⁻ in the complex with SO₄²⁻ as outside ion.

Step 1: Understanding the Precipitation Observation

The key observation is that adding BaCl₂ to the solution doesn't cause precipitation, but AgNO₃ does. This suggests that the chloride ion is not free in solution and is actually part of the coordination sphere, while the anion causing precipitation with AgNO₃ (likely chloride) is outside the coordination sphere.

Step 2: Analyzing the Components

The coordination compound contains one Co³⁺, one sulfate ion, one chloride ion, and four ammonia molecules. Given the coordination number is typically 6 for Co³⁺, all available ligands (NH₃ and possibly Cl⁻) should coordinate directly with the cobalt center.

Step 3: Determining Ligand Coordination

The ammonia molecules are neutral ligands and are likely part of the coordination sphere with Co³⁺ due to their good ligand properties. The chloride ion, as suggested by step 1, must also be a part of the coordination complex. If only Cl⁻ was inside, it would not cause any precipitation reactions with AgNO₃ as chloride is usually outside.

Step 4: Assigning Coordination Roles

All four NH₃ molecules coordinate to the Co³⁺ ion. The remaining site should be occupied by Cl⁻ or SO₄²⁻; however, if Cl⁻ is inside and SO₄²⁻ triggers the Ag⁺ precipitation this indicates SO₄²⁻ being outside the coordination sphere.

Step 5: Constructing the Structural Formula

With NH₃ and Cl⁻ as ligands, the coordination sphere involves Co³⁺, 4 NH₃, and 1 Cl⁻, giving \[[\text{Co(NH}_4)_3\text{Cl}]^{2+}\text{SO}_4^{2-}\]Where [Co(NH₃)₄Cl]²⁺ represents the coordination complex and SO₄²⁻ is the counter ion.

Key Concepts

Coordination Sphere

In the study of coordination compounds, the concept of the coordination sphere is critical. It refers to the central atom, typically a metal, and the ligands directly attached to it. In essence, it is the 'core' of the coordination compound. Here, the ext{Co³⁺} ion serves as the central metal atom. Ligands, like ext{NH}_3 (ammonia) and ext{Cl}^-, bind directly to this central metal ion to form the coordination sphere.

In our example, since the coordination number for ext{Co³⁺} is usually six, four ext{NH}_3 molecules and one ext{Cl}^- ion are included within this sphere. This completes the coordination number of the cobalt ion, ensuring that it meets the typical geometric and electronic configuration needs. The coordination sphere is a distinct grouping within a coordination compound that can greatly influence the compound's chemical properties and reactivity.

• The coordination sphere may affect solubility and stability of compounds.
• Coordination number and types of ligands define the geometry of the complex.
• Ions and molecules outside of this sphere, like ext{SO}_4^{2-} in this case, do not directly coordinate with the central metal ion.

Precipitation Reactions

Precipitation reactions play a pivotal role in identifying the structure of coordination compounds, as seen with the given exercise. They occur when two soluble solutions react to form an insoluble solid, called a precipitate. These reactions help in determining which parts of a complex ion are inside or outside the coordination sphere.

In our specific compound, the reaction did not produce a precipitate when ext{BaCl}_2 was added. This implies the ext{Cl}^- ion is not freely available in the solution, indicating its presence within the coordination sphere. Conversely, when ext{AgNO}_3 is introduced, it results in a precipitate. This suggests that the ext{SO}_4^{2-} ion is in the solution, external to the coordination sphere, allowing it to react and form a solid.

• Precipitation reactions can help identify unbound ions in complexes.
• This method is particularly useful for verifying the positions of ions, whether within or outside the coordination sphere.
• Observations of these reactions can inform the structural formulation of coordination compounds.

Structural Formula

A structural formula provides a visual representation of the composition and connectivity of atoms within a compound. For coordination compounds, the structural formula delineates the coordination sphere and how ligands bind to the central metal.

In this specific scenario of the coordination compound, the structural formula displays the configuration as e [[ ext{Co(NH}_3)_4 ext{Cl}]^{2+} ext{SO}_4^{2-}], which tells us:

• The central ext{Co³⁺} ion is directly coordinated to four ext{NH}_3 molecules and one ext{Cl}^- ligand inside the brackets, indicating their inclusion within the coordination sphere.
• The ext{SO}_4^{2-} ion is outside the brackets, serving as a counter ion to balance the charge.
• This configuration ensures the entire compound is electrically neutral.

This formula helps chemists understand not only the makeup of the compound but also potential reaction pathways and stability concerns.

Thus, structural formulas are fundamental in providing clear and informative insights into the nuanced nature of coordination chemistry.