Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is not a pi-acceptor ligands among the following ligands \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}, \mathrm{NO}^{+}, \mathrm{CN}^{-}\)and \(\mathrm{I}_{3}^{-1}\) ? (a) \(\mathrm{CN}^{-}\) (b) \(\mathrm{I}_{3}^{-}\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}\) (d) \(\mathrm{NO}^{+}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{I}_3^-\) is not a pi-acceptor ligand.

Step 1: Understanding Pi-acceptor Ligands

Pi-acceptor ligands have the ability to accept electrons via their empty orbitals, often through back-bonding with metal d-orbitals. Ligands that can act as pi-acceptors typically have vacant orbitals of suitable symmetry.

Step 2: Evaluating Triiodide Ion 

Examine \(\mathrm{I}_3^-\): As a polyatomic ion, \(\mathrm{I}_3^-\)does not have empty orbitals capable of accepting electron density, making it unlikely to be a pi-acceptor ligand.

Step 3: Evaluating Nitrosyl Cation

Examine \[\mathrm{NO}^+\]: This is a well-known pi-acceptor ligand because the nitrogen has an empty orbital (due to the positive charge) that can accept electron density from metal centers.

Step 4: Evaluating Cyanide

Examine \(\mathrm{CN}^-\): Despite being negatively charged, \(\mathrm{CN}^-\) can act as a pi-acceptor through back-bonding as it has suitable empty orbitals perpendicular to the bond axis.

Step 5: Evaluating Trimethylphosphine

Examine \((\mathrm{CH}_3)_3P\): Phosphines typically act as sigma-donors rather than pi-acceptors, as they lack accessible low-energy empty orbitals for back-bonding.

Step 6: Conclusion about Pi-acceptor Ligands

From the analysis, \(\mathrm{I}_3^-\)is not a pi-acceptor because it does not have the required vacant orbitals, unlike the other choices.

Key Concepts

Back-bonding

Back-bonding is a fascinating concept in the chemistry of ligands. It occurs when an electron donor and an electron acceptor share electrons in a two-way exchange. In the context of ligands and metal complexes, back-bonding involves the transfer of electron density from a metal with available d-orbitals to empty π* orbitals on a ligand.
This interaction stabilizes the complex and can lead to shorter bond lengths between the metal and the ligand.
Back-bonding is especially significant in ligands with available π* orbitals such as carbon monoxide ( ext{CO} ), where it contributes to strong metal-ligand bonds.
It plays a key role in determining the properties of metal complexes, such as their reactivity and bond strength.

Ligands

Ligands are molecules or ions that bind to central metal atoms or ions, forming metal-ligand complexes.
They play an essential role in coordination chemistry, influencing the properties and reactivity of metal centers.
Ligands can be classified based on the type of electron donation:

• Sigma-donors: These ligands donate electron pairs from their highest occupied molecular orbital (HOMO) to the metal atom.
• Pi-acceptors: These ligands can accept electron density into their lowest unoccupied molecular orbital (LUMO), often participating in back-bonding.
• Bidentate or polydentate: These ligands have multiple "teeth" or binding sites, allowing them to form several bonds with the metal atom, adding to the stability of the complex.

By understanding the type of bonding a ligand can offer, chemists can predict the behavior of metal complexes in catalysis, biochemical systems, and material science applications.

Orbital Theory

Orbital theory is a cornerstone in understanding how ligands interact with metal centers.
It provides a framework to visualize and predict the bonding and behavior of atoms in chemical interactions.
In the context of pi-acceptor ligands, orbital theory helps elucidate how back-bonding occurs and which orbitals are involved. For instance:

• One can visualize the empty π* orbitals of a ligand that can accept electron density.
• The overlapping of metal d-orbitals with ligand π* orbitals can be explained using Molecular Orbital (MO) theory.

These interactions not only reinforce the metal-ligand bond but also affect the electronic configuration and geometry of the complex. Understanding orbital theory is crucial for designing new complexes with specific properties tailored to industrial or biological applications.

Chemistry of Ligands

The chemistry of ligands is rich and diverse, offering a multitude of operations and interactions in metal complexes.
Ligands are characterized by their electron donation and acceptance abilities, which affect their bonding patterns and overall complex properties.
Some well-known pi-acceptor ligands include:

• Carbon monoxide ( CO ): Known for its ability to accept electron density from metal centers.
• Nitrosyl ( NO^+ ): A classic example of a pi-acceptor ligand with positive charge facilitating electron-pair acceptance.

In contrast, some ligands like phosphines (e.g., ( CH_3 )_3P ) primarily act as sigma-donors and are not usually pi-acceptors due to the lack of appropriate vacant orbitals.
Understanding these distinctions aids in the strategic choice of ligands for applications in synthesis and catalysis, where specific interaction patterns are desired.