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

(a) \\([\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}] \mathrm{Br}\\) is the correct complex.

Step 1: Analyze the Compound Name

The compound name is dibromidobis (ethylene diamine) chromium (III) bromide. This indicates a complex with two bromide ions ("dibromido") and two ethylene diamine ("bis ethylene diamine") ligands.

Step 2: Define the Complex Structure

From the name 'dibromidobis (ethylene diamine),' we determine that in the coordination sphere of the complex, there are 2 bromide ions (2 Br-) and 2 ethylene diamine ligands.

Step 3: Check the Oxidation State of Chromium

The name chromium (III) bromide indicates that the chromium ion has a +3 oxidation state. This condition must be matched in the correct formula.

Step 4: Identify the Overall Charge

To balance the +3 charge from chromium, the coordination complex must have a charge of -1 to complement the extra Br- ion, which is outside the complex.

Step 5: Match with Given Options

The complex that fits this description is \([\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}] \mathrm{Br}\), which is mentioned in option (a). This structure has two ethylene diamine ligands, two bromide ions inside the coordination sphere, and one bromide ion outside, matching the overall charge and description.

Key Concepts

Ligand Naming

In coordination chemistry, naming ligands properly is crucial to identifying coordination compounds accurately. Ligands are the ions or molecules bound to a central metal atom in a coordination complex. They are the key players in forming the overall structure of the complex. When naming a ligand, certain rules are followed to ensure that the name reflects both the number and the type of ligands connected to the metal center.

For instance, prefixes like "di-", "tri-", and "tetra-" indicate the number of a specific type of ligand in the complex. In our exercise, the term "dibromido" signals two bromide ions. Similarly, "bis(ethylene diamine)" denotes the presence of two ethylene diamine ligands, where "bis" is employed to prevent confusion when the ligand name itself already includes a numerical prefix (e.g., "ethylene diamine"). Remembering these naming conventions aids in efficiently determining the composition of coordination compounds.

Oxidation State

The oxidation state of a metal in a coordination compound reveals what charge the metal ion carries. This is crucial as it impacts the complex's structure and reactivity. It's typically denoted in Roman numerals within parentheses, right after the metal's name in the compound name.

In the original exercise, "chromium (III) bromide" indicates that the chromium has an oxidation state of +3. When unpacking this, it tells us how electrons are shared between the metal and its ligands inside the complex. Calculating oxidation states is a key skill in coordination chemistry.

To determine the oxidation state, consider the known charges of the ligands and balance them with the overall complex charge. Each bromide ion, for example, carries a -1 charge, which contributes to calculating the overall charge balance in the coordination sphere.

Coordination Compounds

Coordination compounds play a pivotal role in chemistry, composed of a central metal atom or ion bonded to surrounding ligands. They are not only fascinating in their own right but are also essential in various biological and industrial processes.

These compounds are identified by the coordination number, which represents the number of ligand attachments to the metal. In our specific coordination compound \(\left[\text{Cr(en)}_{2}\text{Br}_{2}\right]\text{Br}\), the coordination number of chromium is 6. This comes from the fact that the ligands ethylene diamine are bidentate, meaning they can attach at two sites per ligand. Coordination compounds exhibit diverse properties, such as varied solubility and complex stability levels, shaped by their constituent ligands and metal centers.

Exploring the nature of these compounds provides insight into advanced topics such as stereochemistry and electronic configurations of the metal-ligand interaction.

Complex Ion Structures

Understanding complex ion structures enables us to predict the spatial arrangement of ligands around the central metal atom. Structures range widely from simple to geometrically intricate forms.

For the complex \(\left[\text{Cr(en)}_{2}\text{Br}_{2}\right]\text{Br}\), we have an octahedral configuration. This arrangement is quite common, especially in coordination numbers of 6, where six ligands or binding sites are symmetrically arranged around the metal center. Such structures have implications in stereochemical geometry, affecting properties like color, magnetic behavior, and reactivity.

Knowing about these geometric configurations helps in visualizing molecule shapes and their consequent chemical behavior—fundamental for applications ranging from material science to biochemistry.