Problem 47 - Chemistry and Chemical Reactivity

Question

Give the formula of the complex formed from one \(\mathrm{Co}^{3+}\) ion, two ethylenediamine molecules, one water molecule, and one chloride ion. Is the complex neutral or charged? If charged, give the net charge on the ion.

Step-by-Step Solution

Answer

The complex is \([\mathrm{Co(en)_2(H_2O)Cl}]^{2+}\) with a net charge of +2.

Step 1: Identify the Components

First, list all components involved in the complex formation: one \(\mathrm{Co}^{3+}\) ion, two ethylenediamine (\(\mathrm{en}\)) molecules, one water (\(\mathrm{H_2O}\)) molecule, and one chloride (\(\mathrm{Cl^-}\)) ion.

Step 2: Determine the Ligands

Ethylenediamine (\(\mathrm{en}\)) is a neutral bidentate ligand, meaning it does not contribute to the charge. Water (\(\mathrm{H_2O}\)) is also a neutral ligand. The chloride ion (\(\mathrm{Cl^-}\)) carries a \(-1\) charge.

Step 3: Formulate the Complex

The formula of the complex is assembled as \([\mathrm{Co(en)_2(H_2O)Cl}]\). Here, all ligands are enclosed in square brackets with the central metal ion.

Step 4: Calculate the Charge

Add up the charges: \(\mathrm{Co^{3+}}\) gives a \(+3\) charge, two neutral \(\mathrm{en}\) molecules and \(\mathrm{H_2O}\) give no charge, and \(\mathrm{Cl^-}\) gives a \(-1\) charge. Therefore, the net charge is \(+3 - 1 = +2\).

Key Concepts

Ligand Chemistry

Ligands are the donor molecules or ions that coordinate with a central metal atom or ion to form complex compounds. In the exercise, we come across ethylenediamine (en) and water as ligands. But what exactly does this mean?

• Bidentate Ligand: Ethylenediamine (en) is an example of a bidentate ligand. Bidentate means that a single en molecule can form two bonds with the central metal ion, thanks to its two nitrogen atoms. This property allows the molecule to "bite" the metal from two sides.
• Neutral Ligand: Both ethylenediamine and water (\(H_2O\)) are neutral, which means they do not add any charge to the coordination complex.
• Charged Ligand: On the other hand, chloride (\(Cl^-\)) is a charged ligand, introducing a \(-1\) charge to the overall complex.

Understanding the role and charge of each ligand is crucial for determining the composition and properties of the coordination complex. This is evidenced in their contribution to both the chemical formula and the charge of the complex.

Coordination Number

The coordination number refers to the number of ligand donor atoms bonded to the central metal ion in a coordination complex. In our example, the central metal ion is \(Co^{3+}\).

• Coordination Number Count: As explained, ethylenediamine is a bidentate ligand, meaning each molecule coordinates through its two nitrogen atoms. With two en molecules, we have 4 donor sites.
• Other Ligands: Additionally, there is a water molecule and a chloride ion, which both coordinate through one site each, contributing one more donor atom each.

Therefore, the total coordination number of the cobalt (III) metal in this complex is 4 (from en) + 1 (from water) + 1 (from chloride) = 6. The coordination number reflects how many such donor atoms are connected to the metal ion, impacting the complex’s geometry and stability.

Complex Charge Calculation

Calculating the overall charge of a coordination complex is an important step in determining its properties and behavior in different environments. Let's break down this process using our example.

• Metal Ion Charge: The cobalt ion, \(Co^{3+}\), carries a \(+3\) charge.
• Ligand Charges: Each ethylenediamine and water is neutral, contributing nothing to the charge. However, the chloride ion, \(Cl^-\), brings in a \(-1\) charge.

To calculate the net charge of the complex \([\text{Co(en)}_2 ext{(H}_2 ext{O)Cl}]\), we add these values: \(+3 - 1 = +2\). Thus, the overall charge of the complex is \(+2\). Recognizing these contributions is essential for predicting how the complex will interact with other chemical species and its potential applications in fields like catalysis, biochemistry, and material science.