Problem 25 - Chemistry and Chemical Reactivity

Question

Give the formula of a complex constructed from one \(\mathrm{Ni}^{2+}\) ion, one ethylenediamine ligand, three ammonia molecules, and one w$$\left[\mathrm{Ni}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{2+}$$ter molecule. Is the complex neutral or is it charged? If charged, give the charge.

Step-by-Step Solution

Answer

The complex is charged and has a 2+ charge.

Step 1: Identify the Components

The complex consists of one \( \mathrm{Ni}^{2+} \) ion, one ethylenediamine (en) ligand, three ammonia (\( \mathrm{NH}_3 \)) molecules, and one water (\( \mathrm{H}_2 \mathrm{O} \)) molecule.

Step 2: Write the Complex Formula

Combine the components into the chemical formula: \[ \left[ \mathrm{Ni} (\mathrm{en}) \left( \mathrm{NH}_3 \right)_3 \left( \mathrm{H}_2 \mathrm{O} \right) \right]^{2+} \]. Here, the brackets denote the coordination sphere.

Step 3: Determine the Charge

The central metal ion \( \mathrm{Ni}^{2+} \) carries a +2 charge. The ligands (ethylenediamine, ammonia, and water) are neutral, thus they do not contribute to the charge. Therefore, the complex itself is charged at \( 2+ \).

Key Concepts

Coordination Sphere

The coordination sphere plays a crucial role in forming coordination complexes. It is made up of a central metal ion surrounded by molecules or ions known as ligands. In the case of the complex \[ \left[ \mathrm{Ni} (\mathrm{en}) \left( \mathrm{NH}_3 \right)_3 \left( \mathrm{H}_2 \mathrm{O} \right) \right]^{2+} \], the metal ion is \( \mathrm{Ni}^{2+} \). This forms the central core of the coordination sphere.
The coordination sphere is enclosed by square brackets, which differentiate the entities involved within the complex from any other components it might be associated with. Everything inside these brackets contributes to the properties and behavior of the compound as a whole.
Recognizing the components inside the coordination sphere enables understanding of the complex’s stability and reactivity.

Ligands

Ligands are the entities surrounding the central metal ion within the coordination sphere. They attach themselves to the metal through coordinate covalent bonds, where one atom donates both electrons used in the bond. These can be ions or neutral molecules.
For instance, in the complex \( \left[ \mathrm{Ni} (\mathrm{en}) \left( \mathrm{NH}_3 \right)_3 \left( \mathrm{H}_2 \mathrm{O} \right) \right]^{2+} \), **ethylenediamine (en)**, a bidentate ligand, binds with two donor atoms. Meanwhile, **ammonia (\( \mathrm{NH}_3 \))** and **water (\( \mathrm{H}_2 \mathrm{O} \))** are monodentate ligands, each attaching through a single atom.
Here is a breakdown of ligands in the given complex:

• **Ethylenediamine (en):** Binds with two metal sites providing greater stability.
• **Ammonia (NH₃):** Provides additional metal-ligand interactions.
• **Water (H₂O):** Complements the coordination often enhancing solubility.

In helping determine properties such as geometry and reactivity of the complex, ligands play an essential role.

Charge Determination

A coordination complex can be neutral, cationic, or anionic based on the charges associated with its constituents. To determine the overall charge, you consider the charge on the central metal and the charges on the ligands.
In the given complex, the nickel ion \( \mathrm{Ni}^{2+} \) inherently possesses a +2 charge. However, the ligands present, namely ethylenediamine, ammonia, and water, are neutral. They do not add any extra charge to the overall complex. As a result, the charge of the entire coordination complex is the same as the central metal ion, which in this case is **2+**.
Here are the key steps to simplify charge calculation:

• Determine the charge of the central metal ion.
• Assess the charges of all ligands. Neutral ligands contribute nothing extra.
• Add these values together for the total charge.

Once the charge is determined, it helps in forecasting the complex's interactions with other chemical or biological entities.