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 \(\mathrm{I}_{3}^{-2}\) (a) \(\mathrm{CN}^{-}\) (b) \(\mathrm{I}_{3}-\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}\) (d) \(\mathrm{NO}^{+}\)

Step-by-Step Solution

Answer

(c) \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{P} \) is not a pi-acceptor ligand.

Step 1: Understanding Pi Acceptor Ligands

Pi acceptor ligands are ligands that can accept electron density from the metal into their empty anti-bonding orbitals in a process called back-bonding. This requires the ligand to have empty pi* orbitals.

Step 2: Examine Each Ligand

We will examine each ligand to determine if it can act as a pi-acceptor. 1. \( \left( \mathrm{CH}_{3}\right)_{3} \mathrm{P} \) - Typically acts via sigma donation, not known for pi-acceptor capabilities. 2. \( \mathrm{NO}^{+} \) - Known as a strong pi-acceptor because it has empty pi* orbitals. 3. \( \mathrm{CN}^{-} \) - It is a well-known pi-acceptor. 4. \( \mathrm{I}_{3}^{-2} \) - Does not have suitable empty orbitals for pi-acceptance and behaves as sigma-only donor due to its large size.

Step 3: Identify the Non Pi-Acceptor Ligand

Among the ligands listed, \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{P} \) and \( \mathrm{I}_{3}^{-2} \) are less likely to be pi-acceptors because they lack the suitable empty orbitals. Particularly, \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{P} \) is typical for sigma donation without pi back-bonding.

Step 4: Select the Correct Answer

The correct answer is that \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{P} \) is not a pi-acceptor. Therefore, option (c) is the correct answer.

Key Concepts

Pi Acceptor Ligands

In coordination chemistry, pi acceptor ligands play a crucial role by allowing electron transfer from a metal's d orbitals into their empty pi* (pi-star) orbitals. This means they can accept electrons from a metal center, stabilizing certain complexes.

Pi acceptor ligands are vital for creating stable complexes through a process known as "back bonding," where the metal completes the transfer of electron density. This strengthens the bond between the metal and the ligand.

• Common pi acceptor ligands include carbon monoxide (CO) and nitric oxide (NO⁺), which have empty pi* orbitals ready to accept electrons.
• These ligands often enhance the stability and reactivity of metal complexes due to the strong metal-ligand interactions.

Back Bonding

Back bonding is an interesting phenomenon where electrons move backwards, from metal to ligand. This opposite direction of electron flow is essential for creating robust metal-ligand bonds.

For back bonding to occur, the ligand must feature empty orbitals that can receive electron density, commonly recognized as the pi* orbitals.

• This transfer effectively shortens and strengthens the metal-ligand bond, improving the stability of a coordination complex.
• Typical examples of ligands involved in back bonding are CO, CN^-, and NO⁺.

Back bonding is a key concept for students to understand the dynamics in organometallic and metal complex chemistry.

Sigma Donation

Sigma donation refers to the typical ligand-metal interaction where a ligand donates its lone pair of electrons to form a sigma bond with a metal center. This is the most straightforward type of bonding in coordination compounds.

Ligands such as ammonia (NH₃) and trimethylphosphine ((CH₃)₃P) primarily demonstrate sigma donation.

• These donors provide electron density, which can stabilize the positively charged metal centers.
• Sigma donation can work in tandem with pi back-bonding in advanced compounds, balancing donation with back bonding to optimize complex stability.

Although sigma donation is more straightforward than pi acceptance or back bonding, it's arguably the foundation of ligand interactions in transition metal chemistry.

Antibonding Orbitals

Antibonding orbitals, often referred to in pi acceptor discussions, are orbitals that exist above the bond order. They form when atomic orbitals in a molecule overlap in a manner that destabilizes a chemical bond.

• The antibonding orbitals (*) are key for back bonding since they can overlap with metal d orbitals to accept electrons.
• This overlap reduces electron pressure on the metal, stabilizing the metal-ligand complex.

Understanding the role of antibonding orbitals is critical for comprehending how molecules bond and how they can be manipulated to stabilize metal complexes. Their role highlights the nuanced and strategic construction of metal-ligand interactions in coordination compounds.