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

8989 is not a pi-acceptor ligand.

Step 1: Understand Pi-Acceptor Ligands

Pi-acceptor ligands are those that can accept electron density from metal d orbitals via back-bonding. This ability typically stems from the ligand having empty orbitals, often related to anti-bonding pi orbitals, allowing it to accept electrons into these orbitals.

Step 2: Analyze Each Ligand

Review each ligand for its ability to serve as a pi-acceptor: 1. 89−89: Known for strong pi-acceptor properties due to its vacant 89 orbitals. 2. 89: Known for pi-acceptor capability through its empty  (sigma* antibonding) orbitals. 3. 89: Contains  electrons and has the ability to accept electrons into its vacant orbitals. 4. 89:89: Does not have vacant orbitals or the ability to accept pi-back bonding effectively.

Step 3: Identify the Non Pi-Acceptor

Given the descriptions, determine which ligand lacks the ability to act as a pi-acceptor. 8989 does not possess empty orbitals suitable for pi-back bonding, making it the odd one out.

Key Concepts

Back-Bonding

Back-bonding is a fascinating concept in chemistry that involves a form of electron sharing between a ligand and a metal atom.
This phenomenon occurs when a metal donates electrons from its filled orbitals into the vacant orbitals of a ligand. The opposite interaction also occurs, where electrons from the ligand can enter into the partly-filled orbitals of the metal.
This mutual exchange can enhance stability and is fundamental in understanding coordination chemistry. One key aspect of back-bonding is its effectiveness in stabilizing metal complexes.

• It allows for a synergistic interaction between the ligand and the metal.
• It can influence the bonding properties, reactivity, and structure of the complex.

This electron transfer process is particularly prominent in ligands with vacant antibonding orbitals.

Electron Density

Electron density is the probability of finding an electron at a particular part of a molecule.
In the context of pi-acceptor ligands, electron density is crucial because it determines the ability of a ligand to participate in back-bonding. For pi-acceptor ligands, the electron density is often manipulated through the donation and acceptance of electrons from the metal.
As ligands accept electron density from the metal, they create a stronger bond between the ligand and the metal atom, enhancing the stability of the complex.
Conversely, when they donate electron density, they can significantly alter the reactivity of the complex. Tracking electron density changes helps chemists understand and predict the behavior of these molecule structures.

Metal d Orbitals

Metal d orbitals play an integral role in forming bonds with ligands.
These orbitals in transition metals can both donate and accept electron density, making them central in understanding back-bonding interactions in complexes. The five d orbitals each have specific shapes and capabilities to overlap with ligand orbitals.

• The availability of these orbitals allows metals to participate in a variety of interactions, such as sigma bonding and pi back-bonding.
• When metal d orbitals donate electron density, they engage in forming stronger and more stable bonds with ligands that have vacant orbitals.

This dual capacity is a pivotal factor in many industrial applications, including catalysis, as it allows for diverse chemical transformations.

Vacant Orbitals

Vacant orbitals in ligands are unoccupied orbitals ready to accept electrons.
For pi-acceptor ligands, having vacant orbitals, particularly empty pi or antibonding orbitals, is essential for successful back-bonding interactions.Ligands like \(\mathrm{CN}^{-}\) are known to have these characteristics, allowing them to readily accept electron density from metal d orbitals.
This property contributes to the formation of stable complexes as the electrons from the metal fill these empty spaces.

• The presence of these vacant orbitals also influences the binding strength between the metal and the ligand.
• It is a key feature that chemists look for when predicting and designing new complexes in research.