Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number and oxidation number of \(\mathrm{M}\) in the compound \(\left[\mathrm{M}\left(\mathrm{SO}_{4}\right)\left(\mathrm{NH}_{3}\right)_{5}\right]\) will be (a) 6 and 3 (b) 2 and 6 (c) 6 and 2 (d) 3 and 6

Step-by-Step Solution

Answer

Coordination number is 6 and oxidation number is 2 (option c).

Step 1: Understanding the Compound

The compound given is \([\mathrm{M}(\mathrm{SO}_{4})(\mathrm{NH}_{3})_{5}]\). In coordination chemistry, the central metal atom, \(\mathrm{M}\), forms coordination bonds with ligands. Here, SO₄ and NH₃ are the ligands.

Step 2: Identifying the Coordination Number

Coordination number is the total number of coordinate bonds from ligands to the metal atom. In the complex \([\mathrm{M}(\mathrm{SO}_{4})(\mathrm{NH}_{3})_{5}]\), the ligands are one \(\mathrm{SO}_{4}^{2-}\) and five \(\mathrm{NH}_{3}\). With \(\mathrm{NH}_{3}\) acting as a monodentate ligand and \(\mathrm{SO}_{4}^{2-}\) potentially acting as a monodentate as well, coordination number of M is 6.

Step 3: Identifying the Oxidation Number

Oxidation number of the metal can be calculated by considering the charges. Assume the oxidation state of \(\mathrm{M}\) is \(x\). The sulfate ligand, \(\mathrm{SO}_{4}\), has a charge of -2, and ammonia, \(\mathrm{NH}_{3}\), being neutral, doesn't contribute to the charge. The entire complex is neutral, so the equation is \(x - 2 = 0\). Solving for \(x\), we get \(x = 2\). Hence, the oxidation number of \(\mathrm{M}\) is 2.

Key Concepts

Coordination Number

In coordination chemistry, the coordination number is a crucial concept. It refers to the number of ligand donor atoms that are directly attached to the central metal atom or ion through coordination bonds. In simpler terms, it's the number of places where ligands "hold on" to the metal atom.
For example, in the complex compound \[[\mathrm{M}(\mathrm{SO}_{4})(\mathrm{NH}_{3})_{5}]\]we need to count the number of bonds between the ligands and the metal center M. Here, we have one \(\mathrm{SO}_{4}^{2-}\) and five \(\mathrm{NH}_{3}\) molecules.- Ammonia \((\mathrm{NH}_{3})\) is a typical monodentate ligand, meaning each ammonia molecule forms one bond with the central metal.- \(\mathrm{SO}_{4}^{2-}\) is a bit trickier; while sulfate can act as polydentate, in this context, it is often considered monodentate.With these bindings, the coordination number in this complex is 6, indicating there are six bonding connections involving the ligands.

Oxidation Number

The oxidation number of an atom in a compound signifies the number of electrons lost or gained by that atom to form the compound. It's a theoretical concept but highly useful for understanding the degree of oxidation of an atom.
Determining the oxidation number is straightforward. Consider the complex:\[[\mathrm{M}(\mathrm{SO}_{4})(\mathrm{NH}_{3})_{5}]\]In this compound:- \(\mathrm{NH}_{3}\) is neutral, contributing no charge.- \(\mathrm{SO}_{4}^{2-}\) carries a charge of -2.Since the entire complex is electrically neutral, the oxidation number \(x\) of the metal \(\mathrm{M}\) can be calculated by balancing the charges. Solve:\[x + 0 - 2 = 0\]This results in \(x = 2\). Therefore, the oxidation number of \(M\) in this compound is 2, indicating it is in a relatively low oxidation state.

Ligands

Ligands are ions or molecules that coordinate or attach to a central metal atom to form a coordination complex. They play a pivotal role in defining the structure and properties of the complex.- **Types of Ligands**: Ligands can be classified based on the number of sites available for attachment to the central atom. For example: - **Monodentate**: These ligands have just one site or atom through which they bind to the metal. Ammonia \((\mathrm{NH}_{3})\) is a classic monodentate ligand. - **Polydentate**: These ligands, such as ethylenediamine, have multiple sites and can form multiple bonds with the central metal.- **Roles of Ligands**: Ligands determine the coordination number, the geometry, and sometimes even the stability of the coordination compound. They do this by donating electron pairs to the metal center, forming what are called coordinate covalent bonds.In our given compound, the ligands \(\mathrm{NH}_{3}\) and \(\mathrm{SO}_{4}^{2-}\) work together to define the structure, arranging themselves around the metal center to complete the coordination sphere.