Problem 12 - Chemistry and Chemical Reactivity

Question

Give the formula of a complex constructed from one \(\mathrm{Cr}^{3+}\) ion, two ethylenediamine ligands, and two ammonia molecules. Is the complex neutral or is it charged? If charged, give the charge.

Step-by-Step Solution

Answer

The complex is \([\mathrm{Cr}( ext{en})_2(\mathrm{NH}_3)_2]\) with a charge of +3.

Step 1: Identify the Components

To construct the complex, we need a total of one Chromium ion, two ethylenediamine ligands, and two ammonia molecules. Chromium is in the +3 oxidation state, denoted as \(\mathrm{Cr}^{3+}\). Ethylenediamine is a neutral ligand, often abbreviated as 'en', and ammonia, \(\mathrm{NH}_3\), is also a neutral ligand.

Step 2: Determine the Basic Structure

The basic structure of the complex can be written as \([\mathrm{Cr}(\text{en})_2(\mathrm{NH}_3)_2]\). This notation indicates that the central \(\mathrm{Cr}^{3+}\) ion is coordinated with two ethylenediamine ligands (each represented by \(\text{en}\)) and two ammonia molecules.

Step 3: Calculate the Overall Charge

Now, we calculate the charge of the overall complex. The \(\mathrm{Cr}^{3+}\) ion contributes a charge of +3. Both ethylenediamine and ammonia are neutral ligands, contributing a total charge of 0. Therefore, the overall charge of the complex is \(+3 + 0 + 0 = +3\).

Step 4: Determine the Charge of the Complex

Since we calculated that the overall charge of the complex is +3, the complex is charged, specifically with a charge of +3.

Key Concepts

Coordinate Bonds

In the world of chemistry, complex ions are an intriguing domain where atoms bond in unique ways. One key feature of complex ions is the formation of **coordinate bonds**. These bonds occur when a pair of electrons is shared between two atoms. However, what's special here is that both electrons come from the same atom, usually a ligand.

In our exercise, the complex comprises the ion \[\mathrm{Cr}^{3+}\], which is at the center, and it forms coordinate bonds with the ligands, ethylenediamine and ammonia. These ligands provide the lone pairs of electrons that donate to the chromium ion, creating a stable complex. This fascinating bond mechanism is crucial in forming complex ions and stabilizing their structure.

Oxidation State

The oxidation state gives us an idea of how many electrons an atom has lost or gained during a reaction. In this case, the central ion is chromium in a \[\mathrm{Cr}^{3+}\] state. It means that chromium has lost three electrons.

Understanding the oxidation state is essential because it affects how the metal ion will bond with ligands. The positive charge of the \[\mathrm{Cr}^{3+}\] ion attracts the neutral ligands, facilitating the formation of the complex. This concept helps predict the chemical behavior and properties of the complex ion.

Neutral Ligands

Ligands are molecules that can form bonds with metal ions to create complex structures. In our example, both **ethylenediamine** (\(\text{en}\)) and **ammonia** (\(\mathrm{NH}_3\)) act as neutral ligands.

Neutral ligands are unique because they do not possess a charge. Consequently, when they form coordinate bonds with a metal ion, like \[\mathrm{Cr}^{3+}\], they do not alter the overall charge of the ion. This means that despite their involvement in the complex, the total contribution of these ligands to the complex's charge remains 0. This concept is essential in determining the overall behavior and charge of the complex ion.

Charge of Complex Ions

The overall charge of a complex ion is a sum of the charges contributed by the central metal ion and the ligands. In our exercise, we have the central ion \[\mathrm{Cr}^{3+}\], contributing a charge of +3. The ligands—ethylenediamine and ammonia—are neutral and therefore contribute 0 to the charge.

Thus, the total charge on the complex ion is entirely derived from the chromium ion, resulting in a total charge of +3. Recognizing the **charge of complex ions** is crucial in predicting how they will interact with other ions and molecules. It also affects their solubility, reactivity, and roles in various chemical processes.