Problem 16 - Chemistry: An Atoms-Focused Approach

Question

When \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\) dissolves in water, which molecules or ions occupy the inner coordination sphere around the \(\mathrm{Ni}^{2+}\) ions?

Step-by-Step Solution

Answer

Answer: In the inner coordination sphere around the central Ni2+ ions, there are 6 molecules of NH3 (ammonia).

Step 1: Write down the formula for the coordination compound

The formula of the given coordination compound is \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\). Here, \(\mathrm{Ni}\) is the central metal ion, and \(\mathrm{NH}_{3}\) is the ligand. The coordination number is 6, which means 6 ammonia molecules (ligands) are present around the central metal ion in the complex.

Step 2: Dissolve the coordination compound in water

When the coordination compound \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\) is dissolved in water, the following reaction takes place: $$\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2} \xrightarrow{H_{2}O} [\mathrm{Ni}(\mathrm{NH}_{3})_{6}]^{2+} + 2\mathrm{Cl}^{-}$$

Step 3: Identify the inner coordination sphere

The inner coordination sphere consists of the central metal ion and the surrounding ligands. From the dissolution reaction, we can see that the central metal ion is \(\mathrm{Ni}^{2+}\), and it is surrounded by 6 ammonia molecules, \(\mathrm{NH}_{3}\), as ligands. Therefore, the inner coordination sphere around the \(\mathrm{Ni}^{2+}\) ion consists of 6 molecules of \(\mathrm{NH}_{3}\).

Step 4: Provide the answer

When \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\) dissolves in water, the inner coordination sphere around the \(\mathrm{Ni}^{2+}\) ions consists of 6 molecules of \(\mathrm{NH}_{3}\) (ammonia).

Key Concepts

Inner Coordination Sphere

The inner coordination sphere is a key concept in coordination chemistry. It involves the central metal ion and the surrounding ligands directly attached to it. In the context of the coordination compound \[\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2},\]the inner coordination sphere is defined by what surrounds the nickel ion (\(\mathrm{Ni}^{2+}\)).

The inner coordination sphere is tightly bound. This means that the ligands here are covalently bonded or very closely associated with the central metal ion. These are distinct from any outer sphere components which are more loosely associated or free like water molecules or counter ions.

• The central metal ion is \(\mathrm{Ni}^{2+}\).
• It is surrounded by 6 \(\mathrm{NH}_{3}\) molecules acting as ligands.

These characteristics mean that the inner coordination sphere retains its identity in solution. The specific arrangement of ligands allows the complex ion to exhibit unique properties, including color and reactivity characteristics.

Ligands

Ligands are molecules or ions that donate at least one pair of electrons to a central metal ion to form a coordination complex. In the case of \[\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2},\]the ligand is the ammonia molecule (\(\mathrm{NH}_{3}\)).

Ammonia acts as a neutral ligand in this complex, meaning it does not possess any overall charge. It binds to the metal ion through a lone pair of electrons on the nitrogen atom. This creates a coordinate covalent bond—a bond in which both electrons are shared between the ligand and the metal.

• Ammonia (\(\mathrm{NH}_{3}\)) is a monodentate ligand. This term means each ligand uses a single atom to bind to the metal.
• In this complex, there are 6 ammonia ligands, each providing one electron pair, perfectly matching the coordination number of 6 for nickel.

The nature of ligands plays a pivotal role in defining the properties of the coordination compound, influencing its geometry, stability, and color.

Complex Ions

A complex ion is a charged species consisting of a central metal ion surrounded by ligands. In our example of \[\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2},\]the complex ion is \[[\mathrm{Ni}(\mathrm{NH}_{3})_{6}]^{2+}.\]

Here, the nickel ion (\(\mathrm{Ni}^{2+}\)) is coordinated by the ammonia ligands, forming a stable, octahedral arrangement as consistent with its coordination number.

• The nickel ion provides a positive charge, balanced partially by the surrounding ligands.
• The complex ion itself carries a 2+ charge, with the chloride ions (\(\mathrm{Cl}^{-}\)) acting as counter ions outside the inner sphere.

These complex ions play a crucial role in various chemical processes and applications such as catalysis, material synthesis, and biomedical applications. Understanding their nature and how they interact helps in predicting the behaviors and outcomes in both practical and theoretical scenarios.