Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following forms, with an excess of \(\mathrm{CN}^{-}\), a complex having coordination number two? (a) \(\mathrm{Ni}^{2+}\) (b) \(\mathrm{Cu}^{+}\) (c) \(\mathrm{Ag}^{+}\) (d) \(\mathrm{Fe}^{2+}\)

Step-by-Step Solution

Answer

The complex having coordination number two is formed by c '\mathrm{Ag}^{+}\', which forms c '\mathrm{[Ag(CN)_2]^{-}\'.

Step 1: Understand coordination number

The coordination number refers to the number of ligand donor atoms to which the central atom is directly bonded. In this question, we seek a complex with coordination number two.

Step 2: Ligand characteristics

The ligand \(\mathrm{CN}^{-}\) is a strong field ligand, which tends to form complexes with transition metal ions. We need to identify which metal ion, when combined with \(\mathrm{CN}^{-}\), can result in coordination number two.

Step 3: Analyze metal ions

Each metal ion option provided can form complexes with cyanide ions, because \(\mathrm{CN}^{-}\) is a strong and small ligand able to cause certain metals to exhibit lower coordination numbers.

Step 4: Specific analysis of Ag^{+}

The silver ion \(\mathrm{Ag}^{+}\) is known to readily form a linear complex with \(\mathrm{CN}^{-}\) due to its preference for a two-coordinated structure, forming \(\mathrm{[Ag(CN)_2]^{-}\) with coordination number two.

Step 5: Conclusion

From the analysis, \(\mathrm{Ag}^{+}\) forms a complex \(\mathrm{[Ag(CN)_2]^{-}\) which has a coordination number of two, confirming it as the correct answer.

Key Concepts

Coordination Number

In coordination chemistry, the coordination number of a complex refers to the number of ligand donor atoms bonded directly to the central metal atom or ion. Understanding this concept is crucial when studying complexes, as it influences the geometry and stability of the formed complex.

• A coordination number of two often results in a linear geometry.
• Common coordination numbers range from two to twelve, with most compounds having a coordination number between four and six.

Knowing the coordination number helps scientists predict the shapes of the molecules.

Strong Field Ligand

A strong field ligand greatly affects the properties and behavior of transition metal complexes. By causing significant splitting of the d-orbitals in the metal center, these ligands play a crucial role in the determination of the coordination number and magnetic properties.

• Strong field ligands can cause low-spin complexes by pairing electrons in lower energy orbitals.
• They can alter the oxidation state and electronic configuration of a metal, influencing the coordination number and geometry.

The cyanide ion (\( ext{CN}^{-} \)), being a strong field ligand, often leads to complexes with lower coordination numbers, as seen in the formation of \( [ ext{Ag(CN)}_2]^{-} \).

Transition Metal Complexes

Transition metal complexes are fascinating because they display a wide range of colors, magnetic properties, and reactivities. These complexes are formed by the combination of transition metals with ligands, where the metal is typically located in the center.

• The variety of possible ligands allows for diverse coordination environments.
• The properties of these complexes are largely dictated by the nature of the metal and the ligands.

Understanding how these complexes form and behave helps in various applications, such as catalysis and materials science.

Cyanide Ion Ligand

The cyanide ion (\( ext{CN}^{-} \) ) is a highly significant ligand in coordination chemistry. Due to its ability to act as a strong field ligand, it dramatically influences the bonding and properties of the resulting complexes.

• It often forms stable, linear complexes with coordination number two.
• CYanide behaves as a monodentate ligand, meaning it bonds through a single donor atom.

A classic example is the \( [ ext{Ag(CN)}_2]^{-} \) complex, where silver ions form a stable two-coordinate linear structure.