Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

In \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{2}\), the number of \(\pi\) bonds is (a) 2 (b) 3 (c) 4 (d) 6

Step-by-Step Solution

Answer

The number of \(\pi\) bonds is 4.

Step 1: Understanding the Problem

We need to find the number of \(\pi\) bonds in the complex ion \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{2-}\). The complex consists of a central metal atom (Ag) and two cyanide (CN\(^-\)) groups bonded to it.

Step 2: Identifying Components Within the Complex

The cyanide ion, \(\mathrm{CN}^-\), consists of carbon and nitrogen. The structure of cyanide involves a carbon triple-bonded to nitrogen (C\equiv N), which includes one \(\sigma\) bond and two \(\pi\) bonds between C and N within each \(\mathrm{CN}^-\) unit.

Step 3: Counting the Number of \(\pi\) Bonds

Since each \(\mathrm{CN}^-\) ion has two \(\pi\) bonds and there are two \(\mathrm{CN}^-\) ions in the complex, you multiply the two \(\pi\) bonds per cyanide ion by the number of cyanide ions: \(2 \times 2 = 4\).

Step 4: Conclusion

Therefore, the total number of \(\pi\) bonds in the complex \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{2-}\) is 4.

Key Concepts

Coordination Chemistry

Coordination chemistry is a fascinating area of chemistry that focuses on the study of complex compounds. These are structures where a central metal atom or ion is surrounded by molecules or anions, sometimes called ligands, which are bonded to the metal through coordinate covalent bonds.
To better understand, let's consider [Ag(CN)_2]^{2-} as an example. In this coordination complex, silver (Ag) is the central atom and cyanide ions (CN^-) act as the ligands. A key aspect of coordination chemistry is the concept of coordination numbers, which refers to the number of ligand donor atoms that surround a central metal ion.

• For the [Ag(CN)_2]^{2-} ion, the coordination number is 2, indicating there are two cyanide ions bonded to the silver atom.
• This brings about certain geometric arrangements; in this case, it is linear due to the linear geometry of the cyanide ion.

Coordination chemistry helps to explain the bonding behavior of complex ions, their geometric structures, and their acid-base chemistry among other properties. Understanding these complex interactions is crucial for explaining numerous biological, environmental, and industrial processes.

Cyanide Ion

The cyanide ion, denoted as CN^- is a simple but significant anion with interesting bonding properties. It consists of a carbon atom triple-bonded to a nitrogen atom, written as C≡N . This linear structure is strong and exhibits multiple bond types.

• The bonds involve one sigma bond and two pi bonds between the carbon and nitrogen atoms.
• This results in a very stable molecular structure that contributes to its ability to act as a ligand in forming coordination complexes like [Ag(CN)_2]^{2-} .

In coordination complexes, cyanide ions donate a pair of electrons from the carbon atom to the metal center, forming a coordinate covalent bond. This donation property is due to the lone pair of electrons that resides on the nitrogen atom. The wide involvement of cyanide ions in chelation (binding) makes them valuable in processes such as electroplating, metal extraction, and organic synthesis. However, they must be handled with care due to their high toxicity.

Chemical Bonding

Chemical bonding forms the foundation of molecular structures and interactions. It involves the attraction forces that hold atoms together in compounds. In coordination chemistry, several types of bonding are at play, especially when considering complex ions like [Ag(CN)_2]^{2-} .

• Covalent bonds occur when atoms share one or more pairs of electrons. In the case of CN^-, the carbon and nitrogen atoms are connected by a covalent triple bond, comprising one sigma and two pi bonds.
• Coordinate covalent bonds, sometimes called dative bonds, are a crucial part of how ligands bind to central metal ions. Here, one atom donates a pair of electrons to form the bond—a hallmark of coordination complexes.

Understanding these bonds not only explains the formation of complex ions but also elaborates on their properties and reactivity. This knowledge is critical for fields ranging from biochemistry to materials science, where control over chemical reactions and the behavior of materials is essential.