Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

The complex ion which has no d electrons in the central metal atom is (Atomic number \(\mathrm{Cr}=24\), \(\mathrm{Mn}=25, \mathrm{Fe}=26, \mathrm{Co}=27)\) (a) \(\left[\mathrm{MnO}_{4}\right]\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) (c) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3}\) (d) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3^{+}}\)

Step-by-Step Solution

Answer

The complex ion \([\mathrm{MnO}_{4}]^{-}\) has no d electrons in the central metal atom.

Step 1: Determine Oxidation State

For each complex ion, determine the oxidation state of the central metal atom. This is done by considering the charges of the ligands and the overall charge on the ion. For example, in \([\mathrm{MnO}_{4}]^{-}\), the overall charge is -1, and each \(\mathrm{O}\) is -2, resulting in \(\mathrm{Mn}\) having an oxidation state of +7.

Step 2: Electronic Configuration of Isolated Atoms

Write the electronic configuration of each metal atom in its neutral state. For example, \(\mathrm{Mn}\) with atomic number 25 has a configuration of \([\mathrm{Ar}]\, 3d^5 4s^2\).

Step 3: Subtract Electrons Lost Due to Oxidation

From the electronic configuration calculated in step 2, subtract electrons according to oxidation state. For \(\mathrm{Mn}^{7+}\), we remove 7 electrons, resulting in \([\mathrm{Ar}]\). This means it has no d electrons. Repeat this for each complex ion.

Step 4: Compare Electron Configurations

Each central metal atom's resulting electron configuration from step 3 will show if it has any d electrons left. Compare each configuration to determine which complex ion has no d electrons remaining.

Key Concepts

Oxidation States

The oxidation state of an element in a compound provides essential information about its electron loss or gain. In coordination chemistry, determining the oxidation state of a central metal atom within a complex ion is crucial. Begin with the overall charge of the complex ion, consider the charges of accompanying ligands, and solve the oxidation state's mathematical equation. For instance:

• In \([\mathrm{MnO}_{4}]^{-}\), each oxygen has an oxidation state of -2.
• The total charge on the oxygen ligands is -8 (since there are four oxygens).
• The total charge of the complex is -1. From this, deduce Mn's oxidation state as +7.

Discovering the oxidation state helps transition to understanding the electron configuration and bonding nature within the complex.

Electronic Configuration

An atom's electronic configuration is its arrangement of electrons across different energy levels or orbitals. This configuration dictates how atoms can bond in complex ions. Begin by identifying the electronic configuration for each element in its neutral state. For example:

• Manganese (Mn), with atomic number 25, has an electronic configuration of \[\mathrm{[Ar]}\ 3d^5\ 4s^2\] when neutral.
• This arrangement allows for predictions about how Mn will behave when ionized.

Once the oxidation state is known, remove electrons accordingly:

• For \({\mathrm{Mn}}^{7+}\), remove 7 electrons. Hence, it loses its 3d and 4s electrons.
• The remaining configuration is \[\mathrm{[Ar]}\], indicating that manganese in this ionization state has no d electrons left.

Electron configurations help understand the electronic foundation of bonding in complex ions.

Complex Ions

Complex ions consist of a central metal ion bonded to surrounding molecules or ions, known as ligands. The study of complex ions is a significant part of coordination chemistry, revealing insights into the central metal's oxidation state and electronic structure.Key aspects include:

• Coordination number: The number of ligands directly attached to the central metal.
• Geometry: Determines the spatial arrangement of ligands around the metal.

For example, \([\mathrm{MnO}_{4}]^{-}\) is a complex ion where the central manganese is bonded to four oxygen ligands, giving it a tetrahedral geometry. Understanding complex ions involves analyzing how ligands, charge, and electron configuration of the central metal interlink to create unique chemical behaviors in these entities.

Transition Metals

Transition metals are elements found in the d-block of the periodic table. They are defined by their ability to form various oxidation states and complex ions due to their unique electronic configurations. These metals, such as iron (Fe), cobalt (Co), and chromium (Cr), have partially filled d orbitals that enable flexibility in electron arrangements.Characteristics of transition metals include:

• Exhibit multiple oxidation states, facilitating diverse chemical reactions.
• Form colored compounds due to d-d electron transitions.
• Act as good catalysts in industrial processes, enhancing reaction rates.

For example, in the complex ion \([\mathrm{Cr}(\mathrm{H}_{2}\mathrm{O})_{6}]^{3+}\), chromium displays its ability to have a +3 oxidation state, with water molecules coordinated as ligands. This adaptability is a hallmark of transition metals, providing versatility in bonding and functionality across chemical systems.