Problem 177 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among the following will be named as dibromidobis (ethylene diamine) chromium (III) bromide? (a) \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right] \mathrm{Br}\) (b) \(\left[\mathrm{Cr}(\mathrm{en}) \mathrm{Br}_{2}\right] \mathrm{Br}\) (c) \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right] \mathrm{Br}_{3}\) (d) \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right] \mathrm{Br}\)

Step-by-Step Solution

Answer

Option (d) is named dibromidobis (ethylene diamine) chromium (III) bromide.

Step 1: Identify ligand and metal oxidation state

Determine the ligands used and calculate the oxidation state of the central ion in each compound. Ethylene diamine (en) is neutral, while bromide (Br) is -1. To find the oxidation state of chromium (Cr), set the net charge of the coordination complex equal to the charge of the ion.

Step 2: Analyze Option (a)

For \([\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}] \mathrm{Br}\), the complex is coordinated with two ethylene diamines and two bromides. Set up the equation to find the oxidation state of Cr:\[\text{Charge of complex} = 0 (neutral complex)\Rightarrow x + 2(0) + 2(-1) = 0\Rightarrow x - 2 = 0\Rightarrow x = +2\]Thus, chromium here is not in the +3 oxidation state as needed.

Step 3: Analyze Option (b)

In \([\mathrm{Cr}(\mathrm{en}) \mathrm{Br}_{2}] \mathrm{Br}\), the complex has one ethylene diamine and two bromides. Calculate the oxidation state of Cr:\[\text{Charge of complex} = -1\Rightarrow x + 0 + 2(-1) = -1\Rightarrow x - 2 = -1\Rightarrow x = +1\]Cr does not have the correct oxidation state.

Step 4: Analyze Option (c)

The coordination complex\([\mathrm{Cr}(\mathrm{en})_{3}] \mathrm{Br}_{3}\) consists of three ethylene diamines (neutral). Determine the oxidation state of Cr:\[\text{Charge of complex} = -3\Rightarrow x + 3(0) = -3\Rightarrow x = +3\]Despite the correct oxidation state, there are no bromide ligands inside the complex to match with 'dibromido'.

Step 5: Analyze Option (d)

In \([\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}] \mathrm{Br}\), two ethylene diamine (neutral) and two bromide ligands (\(-1\)). Calculate Cr's oxidation state:\[\text{Charge of complex} = -1\Rightarrow x + 2(0) + 2(-1) = -1\Rightarrow x - 2 = -1\Rightarrow x = +3\]This matches 'dibromido' with two bromide atoms within the complex and the overall charge. Chromium is in the +3 state.

Key Concepts

Ligands

Ligands are molecules or ions that bond to a central metal atom to form a coordination compound. They play a crucial role in defining the properties and stability of these compounds. In coordination chemistry, ligands can be:

• Neutral - These do not contribute to the overall charge of the complex. A common example is ethylene diamine (en), which is a bidentate ligand that can attach to the metal atom at two positions.
• Anionic - These contribute a negative charge. Halides like bromide (Br) are classic examples; they provide a -1 charge for each bromide ion.
• Cationic - Although less common, these ligands would contribute a positive charge if present.

Studying ligands helps us understand how they influence the geometry and reactivity of coordination compounds.

Oxidation State

The oxidation state of a metal in a coordination compound is a key factor that influences the compound's chemical behavior. To determine the oxidation state of the central metal, consider the total charge of the coordination complex and the known charges of the ligands. For example, in the given exercise, we calculate the oxidation state of chromium (Cr) by:

• Adding the charges of the ligands and equating it to the net charge of the entire coordination complex.
• Solving the equation to find the unknown, which represents the oxidation state of the metal.

Calculations show that chromium in the correct option has an oxidation state of +3. This is essential because the configuration of the oxidation state often influences both the physical and chemical properties of the compound, like color, coordination number, and reactivity.

Complex Nomenclature

Naming coordination compounds follows specific rules that help identify the components and their arrangement within the complex. Understanding nomenclature is vital for anyone learning chemistry to easily communicate and recognize compounds. Here's a simple approach:

• Ligands are listed first in the name, in alphabetical order, regardless of their charge.
• Prefixes are used to indicate the number of each type of ligand (e.g., "di-" for two, "tri-" for three).
• The name of the central metal follows, with its oxidation state provided in Roman numerals in parentheses. For the given problem, chromium's oxidation state +3 is denoted as (III).
• If the complex ion is a cation, the metal name is stated as is. If it's an anion, the metal's name ends with "-ate."

In this way, the compound given in the exercise is named as dibromidobis(ethylene diamine) chromium (III) bromide, accurately reflecting its structure and composition.