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

Question

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

Step-by-Step Solution

Answer

The coordination number is 4 and the oxidation number is +2.

Step 1: Understand Coordination Number

The coordination number of an atom in a complex is the number of ligand donor atoms to which the metal is directly bonded. In the compound \( [\mathrm{M}(\mathrm{SO}_{4})(\mathrm{NH}_{3})_{3}] \), the ligands attached are sulfate (\( \mathrm{SO}_{4}^{2-} \)) and three ammonia (\( \mathrm{NH}_{3} \)) molecules. Both sulfate and ammonia act as monodentate ligands in this complex, meaning each contributes one coordinate bond. Therefore, there are 4 ligands (1 sulfate and 3 ammonias), making the coordination number 4.

Step 2: Identify Oxidation Number

The oxidation number of the metal can be deduced by considering the charge of the ligands and the overall neutrality of the complex. \( \mathrm{NH}_{3} \) is a neutral ligand, contributing no charge. \( \mathrm{SO}_{4}^{2-} \) has a charge of -2. Let the oxidation number of \( \mathrm{M} \) be \( x \). The sum of the oxidation numbers in the complex should equal zero, leading to the equation: \( x - 2 = 0 \). Solving for \( x \), we find \( x = +2 \).

Step 3: Conclusion

Based on the determined coordination number and oxidation number, we can confirm that the coordination number is 4 and the oxidation number is +2.

Key Concepts

Coordination Number

The coordination number of a metal ion in a coordination compound is a fascinating concept that offers insight into the compound's structure and bonding. Simply put, it represents the number of ligand donor atoms bonded directly to the central metal ion. These donor atoms form coordinate bonds with the metal, stabilizing the entire complex structure.

For example, in the compound \([\text{M}(\text{SO}_{4})(\text{NH}_{3})_{3}]\), identifying the coordination number involves evaluating its ligands. Here, we have one sulfate ion \(( \text{SO}_4^{2-})\) and three ammonia molecules \(( \text{NH}_3)\). Since both are monodentate ligands (each forms one bond with the metal), the total number of bonds is simply the sum of these individual interactions. In this case, 1 bond from sulfate and 3 from ammonia molecules give us a coordination number of 4. Snugly packed, these bonds wrap around the metal, creating its unique geometrical structure.

Oxidation Number

The oxidation number concept is crucial when analyzing coordination compounds, helping us understand charge distributions and balance in a chemical entity. This number indicates how many electrons have been effectively lost or gained by the metal ion due to its interactions within the compound.

To determine the oxidation number of the metal \( \text{M} \) in \([\text{M}(\text{SO}_{4})(\text{NH}_{3})_{3}]\), we focus on the charges of the ligands. The sulfate ion carries a charge of \(-2\), whereas ammonia is neutral, contributing no charge. Since the overall charge of the complex is neutral, the metal must offset the sulfate's \(-2\) charge. We denote the oxidation number of \( \text{M} \) as \( x \). Therefore, the equation becomes:\[x - 2 = 0\]Solving gives \( x = +2 \).

Hence, \( \text{M} \) has an oxidation number of +2, balancing the internal charge distribution neatly.

Monodentate Ligands

Ligands are fundamental players in coordination chemistry. They are molecules or ions that bind to the central metal ion, forming complex structures. Monodentate ligands are a type that form a single coordinate bond with the metal.

Consider the ligands in \([\text{M}(\text{SO}_{4})(\text{NH}_{3})_{3}]\). Both sulfate \((\text{SO}_4^{2-})\) and ammonia \((\text{NH}_{3})\) are classic examples of monodentate ligands. The sulfate ion, although potentially confusing due to multiple atoms, behaves as a single-point donor in this context. Similarly, ammonia uses its lone pair to create one stable bond with the metal.

Monodentate ligands, despite their single bonding capabilities, are incredibly versatile and can participate in forming diverse geometrical arrangements around the metal. This single bond property provides the foundation for creating more complex multi-ligand systems.