Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

The common features among the species \(\mathrm{CN}^{-}, \mathrm{CO}\) and \(\mathrm{NO}^{+}\)are (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: Determine Electron Count

First, count the number of valence electrons in each species. For \(\mathrm{CN}^-\), there are 14 electrons (5 from C, 5 from N, and 1 extra for the negative charge). For \(\mathrm{CO}\), we have 14 electrons (4 from C and 6 from O), and for \(\mathrm{NO}^+\), there are also 14 electrons (5 from N, 5 from O, and subtract 1 for the positive charge).

Step 2: Isoelectronic Check

All species, \(\mathrm{CN}^-\), \(\mathrm{CO}\), and \(\mathrm{NO}^+\) have 14 electrons, making them isoelectronic.

Step 3: Determine Bond Order

For a diatomic molecule with 14 electrons, the bond order can be determined using the formula: \( \text{Bond Order} = \frac{1}{2} (\text{Number of electrons in bonding orbitals} - \text{Number of electrons in antibonding orbitals}) \). Each species fits the electronic configuration of a bond order of 3.

Step 4: Identify Ligand Strength

Examine if these species act as strong or weak field ligands. In organometallic chemistry, \(\mathrm{CN}^-\), \(\mathrm{CO}\), and \(\mathrm{NO}^+\) are known as good \(\pi\)-acceptors, often regarded as strong field ligands.

Key Concepts

Isoelectronic Species

The term "isoelectronic" refers to molecules or ions that have the same number of electrons. This concept is very useful in chemistry because it allows us to compare different species that have similar electronic structures and predict their properties. In the case of

• \(\mathrm{CN}^-\)
• \(\mathrm{CO}\)
• \(\mathrm{NO}^+\)

all three have 14 valence electrons. This makes them isoelectronic with one another. Although they may consist of different atoms, the electron count is the same. This means they should exhibit similar bond orders and possibly similar chemical behavior. Understanding which species are isoelectronic helps predict properties like shape, reactivity, and magnetic behavior. While isoelectronic species share the same electron count, the way they bond might differ due to the identities of the atoms involved.

Bond Order

Bond order is a concept that helps to describe the bonding and structure in molecules. It is calculated using the formula: \[\text{Bond Order} = \frac{1}{2} (\text{Number of electrons in bonding orbitals} - \text{Number of electrons in antibonding orbitals})\]In the context of Molecular Orbital Theory, bond order gives an indication of the stability and strength of a bond. A higher bond order typically indicates a stronger and shorter bond. For the species

• \(\mathrm{CN}^-\)
• \(\mathrm{CO}\)
• \(\mathrm{NO}^+\)

all are calculated to have a bond order of 3, meaning they each have strong triple bonds. This high bond order explains why these molecules are quite stable and have similar properties despite their different atoms. Understanding bond order helps us grasp the concept of molecular stability and reactivity.

Ligand Field Strength

Ligand field strength is crucial when discussing how molecules interact with metals, particularly in transition metal chemistry. It relates to how much the presence of a ligand can split the d-orbitals in a metal ion when forming a complex. The ligands

• \(\mathrm{CN}^-\)
• \(\mathrm{CO}\)
• \(\mathrm{NO}^+\)

are recognized as strong field ligands, which means they can cause a large splitting of the d-orbitals. These species are also known as good \(\pi\)-acceptors. When a ligand is a good \(\pi\)-acceptor, it means it can backbond with metals, accepting electron density from the metal into its own \(\pi^*\) orbitals. This capability strengthens the interaction between the metal and ligand, typically resulting in low-spin complexes. Understanding ligand field strength is essential for predicting magnetic properties and the color of metal complexes.