Question

Chromium(III) forms many complexes, among them those with the following ligands. Give the formula and charge of each chromium complex ion described below. (a) two oxalate ions \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right)\) and two water molecules (b) five ammonia molecules and one sulfate ion (c) one ethylenediamine molecule, two ammonia molecules, and two iodide ions

Short answer

Question: Determine the formula and charge of the following chromium(III) complex ions: a) The complex with two oxalate ions and two water molecules. b) The complex with five ammonia molecules and one sulfate ion. c) The complex with one ethylenediamine molecule, two ammonia molecules, and two iodide ions. Answer: a) The formula and charge of the complex with two oxalate ions and two water molecules are [Cr(C_2O_4)_2(H_2O)_2]^{-1}. b) The formula and charge of the complex with five ammonia molecules and one sulfate ion are [Cr(NH_3)_5(SO_4)]^{+1}. c) The formula and charge of the complex with one ethylenediamine molecule, two ammonia molecules, and two iodide ions are [Cr(en)(NH_3)_2I_2]^{+1}.

Step-by-step solution

(a) Formula and charge of the complex with two oxalate ions and two water molecules

1. Write the formula of chromium(III) ion as Cr^{3+} 2. Write the formula of oxalate ion as C_2O_4^{2-} 3. Write the formula of a water molecule as H_2O (neutral, no charge) 4. Combine Cr^{3+} with two oxalate ions and two water molecules to form the complex: [Cr(C_2O_4)_2(H_2O)_2]^{n+} 5. Calculate the net charge of the complex by adding the chromium(III) ion charge and the charges of the two oxalate ions (water molecules do not contribute to the charge): n = 3 + (-2) * 2 = -1 6. Write the final formula and charge of the complex [Cr(C_2O_4)_2(H_2O)_2]^{-1}

(b) Formula and charge of the complex with five ammonia molecules and one sulfate ion

1. Write the formula of chromium(III) ion as Cr^{3+} 2. Write the formula of ammonia molecule as NH_3 (neutral, no charge) 3. Write the formula of sulfate ion as SO_4^{2-} 4. Combine Cr^{3+} with five ammonia molecules and one sulfate ion to form the complex: [Cr(NH_3)_5(SO_4)]^{n+} 5. Calculate the net charge of the complex by adding the chromium(III) ion charge and the charge of the sulfate ion (ammonia molecules do not contribute to the charge): n = 3 + (-2) = 1 6. Write the final formula and charge of the complex [Cr(NH_3)_5(SO_4)]^{+1}

(c) Formula and charge of the complex with one ethylenediamine molecule, two ammonia molecules, and two iodide ions

1. Write the formula of chromium(III) ion as Cr^{3+} 2. Write the formula of ethylenediamine molecule as en (neutral, no charge) 3. Write the formula of ammonia molecule as NH_3 (neutral, no charge) 4. Write the formula of iodide ion as I^{-} 5. Combine Cr^{3+} with one ethylenediamine molecule, two ammonia molecules, and two iodide ions to form the complex: [Cr(en)(NH_3)_2I_2]^{n+} 6. Calculate the net charge of the complex by adding the chromium(III) ion charge and the charges of the two iodide ions (ethylenediamine and ammonia molecules do not contribute to the charge): n = 3 + (-1) * 2 = 1 7. Write the final formula and charge of the complex [Cr(en)(NH_3)_2I_2]^{+1}

Key concepts

Coordination Chemistry

Coordination chemistry is a fascinating field that deals with the interaction between metal ions and molecules called ligands. In these interactions, metal ions like chromium(III) serve as central atoms that attract ligands to form complex structures. These complexes are pivotal in various chemical reactions and industrial processes.

At the heart of this chemistry is the concept of coordination compounds. These compounds consist of a central metal ion bonded to a certain number of ligands. The number of ligands attached to the central ion is known as the coordination number. For example, in the exercise provided, chromium(III) forms three different complexes with different coordination environments.

Such understanding helps in predicting the properties of these compounds, which is vital for applications like metallurgy, medicine, and catalysis.

Complex Ion Formation

Complex ion formation occurs when ligands, which are ions or molecules with electron pairs, bond to a central metal ion. This bonding can occur through a coordinate covalent bond, where both electrons in the bond come from the ligand.

For example, in one of the exercises provided, chromium(III) bonds with oxalate ions and water to form the complex \[ [Cr(C_2O_4)_2(H_2O)_2]^{-1} \]This shows how ligands like oxalate, which has a -2 charge, influence the overall charge and stability of the complex.

Understanding these formation principles allows chemists to design complexes with desired properties, such as specific reactivity or color, which are important in fields ranging from material science to biochemistry.

Ligands

Ligands are key players in coordination chemistry. They are the molecules or ions that connect to the central metal ion. Ligands can vary from simple ions like chloride to more complex molecules like oxalate and ethylenediamine.

In the exercise, ligands such as ammonia, sulfate, and iodide are involved in forming different chromium complexes. These ligands do not contribute to the charge but influence the shape and reactivity of the complex.

Ligands are classified based on the number of bonding sites they have:

• Monodentate: Ligands that bond through one site, like water or ammonia.
• Bidentate: Ligands that can attach at two places, such as oxalate or ethylenediamine.

This classification is crucial for understanding the geometry and stability of complexes.

Oxidation States

Oxidation states describe the degree of oxidation of an atom within a compound. In coordination chemistry, it helps to determine the charge of a metal ion and predict its reactivity.

For chromium, which often exists in a +3 oxidation state (Cr³⁺), understanding oxidation states is essential for calculating the net charge of a complex.

For example, the complex \[ [Cr(NH_3)_5(SO_4)]^{+1} \]shows the chromium ion and its interaction with neutral ammonia and a sulfate ion. These calculations highlight how the metal's oxidation state influences the overall charge and stability of the complex.

Oxidation states guide chemists in balancing chemical reactions and designing compounds with specific electronic properties, enhancing our ability to manipulate chemical systems effectively.