Problem 32 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The common features among the species \(\mathrm{CN}^{-}, \mathrm{CO}\) and \(\mathrm{NO}^{+}\)are [2001S] (a) bond order three and isoelectronic (b) bond order three and weak field ligands (c) bond order two and \(\pi\)-acceptors (d) isoelectronic and weak field ligands

Step-by-Step Solution

Answer

(a) bond order three and isoelectronic.

Step 1: Understand the Molecular Structures

We first need to understand the number of electrons in each of the given species: \(\mathrm{CN}^{-}, \mathrm{CO}, \mathrm{NO}^{+}\). The species \(\mathrm{CN}^{-}\) has \(14\) electrons, \(\mathrm{CO}\) has \(14\) electrons, and \(\mathrm{NO}^{+}\) also has \(14\) electrons. These are all isoelectronic species, meaning they have the same number of electrons.

Step 2: Calculate Bond Order

Bond order is determined by the formula: \(\text{Bond Order} = \frac{(\text{Number of bonding electrons} - \text{Number of antibonding electrons})}{2}\). In molecular orbital theory, bond order for these isoelectronic species can be calculated based on their standard electron configurations. For \(\mathrm{CN}^{-}, \mathrm{CO}, \mathrm{NO}^{+}\), the bond order is calculated to be 3.

Step 3: Evaluate Field Strength and Electron Acceptance

To determine ligand strength and \(\pi\)-acceptor characteristics, it's essential to note that both \(\mathrm{CN}^{-}\) and \(\mathrm{CO}\) are strong field ligands and good \(\pi\)-acceptors. While \(\mathrm{NO}^{+}\) is generally considered a strong field ligand as well, in some contexts it can behave differently. However, for the purpose of this problem where common features are sought, focus remains on similarity in bond order and electronic structure.

Step 4: Choose the Correct Option

Given that \(\mathrm{CN}^{-}, \mathrm{CO}, \mathrm{NO}^{+}\) are isoelectronic and possess bond order of 3, the option that best fits is (a) bond order three and isoelectronic. The other options either misidentify field strength characteristics or fail to capture the correct bond order.

Key Concepts

Bond Order

Bond order is a fundamental concept in molecular orbital theory that reveals the stability and strength of a chemical bond. It is calculated by determining the difference between the number of bonding and antibonding electrons, then dividing by two.
In the molecular orbital configuration:

• The bonding electrons stabilize the molecule, while antibonding electrons destabilize it.
• A higher bond order indicates a stronger, more stable bond.

For example, species like \[ \text{CN}^-, \text{CO}, \text{and}\, \text{NO}^{+} \] are isoelectronic, meaning they have the same number of electrons, specifically 14 in this case.
Consequently, they also share the same bond order, calculated to be three. A bond order of three suggests that each molecule has a triple bond. Understanding bond order helps us predict the stability and reactivity of molecules.

Isoelectronic Species

The term 'isoelectronic species' refers to atoms, ions, or molecules that contain the same number of electrons or exhibit similar electronic configurations. This implies that although their elemental composition may differ, their total number of electrons contributes to comparable chemical behavior.
Flipping back to our previous example, the species \[ \text{CN}^-, \text{CO}, \text{and}\, \text{NO}^{+} \] are isoelectronic, each containing 14 electrons.

• Being isoelectronic means these species will have identical or very similar properties.
• They will likely form bonds of similar types and strengths.

This allows for predictions about molecular properties across diverse species sharing this characteristic. The study of isoelectronic species is crucial for understanding the underlying similarities in chemical and physical properties.

Pi-Acceptors

Pi-acceptors, often encountered in ligand field theory, are ligands that can accept electron density from metal atoms via their empty pi* (antibonding pi) orbitals. They participate in backbonding by accepting electrons from the metal's d orbitals.

• This interaction leads to stabilization of the metal complex.
• It typically increases the metal-ligand bond strength.

Consider \[ \text{CN}^- \text{and} \text{CO} \] -- both renowned pi-acceptor ligands due to their efficient electron acceptance via pi* orbitals.
These characteristics make them strong field ligands, enhancing the splitting of d-orbitals in metal complexes. Understanding pi-acceptors is important for delving into transition metal chemistry and the properties of metal-ligand complexes.