Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following ions the \(\mathrm{p} \pi\) - \(\mathrm{d} \pi\) overlap could be present in (a) \(\mathrm{NO}_{2}^{-}\) (b) \(\mathrm{NO}_{3}\) (c) \(\mathrm{PO}_{4}^{3-}\) (d) \(\mathrm{CO}_{3}^{2-}\)

Step-by-Step Solution

Answer

The ion \(\mathrm{PO}_{4}^{3-}\) can have a pπ-dπ overlap.

Step 1: Identify the possibility of pπ-dπ bonding

The first step is to determine which ions allow the presence of pπ-dπ bonding. This type of bonding can occur when a central atom in a molecule or ion has vacant d-orbitals that can participate in π-bonding with the p-orbitals of a terminal atom. Common examples of elements that can have vacant d-orbitals include third-period or heavier elements, like phosphorus (P) or sulfur (S), while lighter elements like nitrogen (N) and carbon (C) typically do not have accessible d-orbitals for pπ-dπ interactions.

Step 2: Analyze each ion for pπ-dπ overlap

Now, analyze each of the given ions:- **(a) \(\mathrm{NO}_{2}^{-}\)**: Nitrogen is the central atom and being a second-period element, it lacks d-orbitals.- **(b) \(\mathrm{NO}_{3}\)**: Similar to \(\mathrm{NO}_{2}^{-}\), nitrogen is central and lacks d-orbitals.- **(c) \(\mathrm{PO}_{4}^{3-}\)**: Phosphorus is the central atom, and as a third-period element, it has available d-orbitals, allowing pπ-dπ bonding.- **(d) \(\mathrm{CO}_{3}^{2-}\)**: Carbon, like nitrogen, is a second-period element and thus lacks d-orbitals.

Step 3: Conclude which ion allows pπ-dπ bonding

Based on the analysis, \(\mathrm{PO}_{4}^{3-}\) is the only ion where the central atom, phosphorus, has available d-orbitals, making pπ-dπ overlap feasible. The other ions have nitrogen or carbon as the central atom, which do not have d-orbitals for such bonding.

Key Concepts

pπ-dπ Overlap

The concept of pπ-dπ overlap is a fascinating aspect of chemical bonding where we see the interplay between different atomic orbitals. In order for pπ-dπ overlap to occur, we need two primary components:

• The presence of a central atom with vacant d-orbitals, typically found in the third period or beyond in the periodic table.
• The ability to form overlapping bonds with p-orbitals from another atom, such as oxygen, which commonly donates its p-orbitals in such arrangements.

This overlap results in a more extended electron cloud, which can influence the molecule's stability and reactivity. In practice, pπ-dπ overlap is crucial in understanding bonding in biochemical and industrial compounds. Only central atoms from elements like phosphorus or sulfur often partake due to their available d-orbitals. Such overlaps are not possible with lighter elements like nitrogen and carbon, due to their lack of accessible d-orbitals in the second-period elements.

Vacant d-Orbitals

Vacant d-orbitals play a pivotal role in making pπ-dπ overlaps possible. In the context of the periodic table, these orbitals usually become relevant starting from the third period. Here's why that's important:

• Elements in the third and higher periods have these d-orbitals available to participate in bonding, adding to their versatility.
• These orbitals can accept electron density from p-orbitals of other atoms, a process crucial for forming stable coordination compounds.

For example, phosphorus in (\(\mathrm{PO}_{4}^{3-}\)) showcases how these d-orbitals can interact and form pπ-dπ overlaps. The availability of vacant d-orbitals allows phosphorus to engage in complex bonding structures that lighter elements like nitrogen, due to their lack of d-orbitals, simply cannot.

Phosphates

Phosphates, such as (\(\mathrm{PO}_{4}^{3-}\)), are prime candidates for exhibiting pπ-dπ overlaps due to the nature of their central phosphorus atom. Phosphorous, found in the third period, can utilize its vacant d-orbitals to engage in bonds with oxygen atoms. This unique capability contributes to the versatility of phosphates in both biological systems and industrial applications.

• The ability of phosphates to form extensive networks through pπ-dπ overlaps makes them crucial in biological molecules, like DNA, where they link nucleotides.
• In agriculture, phosphates are pivotal components of fertilizers, driving the growth of plants through their ability to transport nutrients.

Understanding the pπ-dπ bonding in phosphates highlights how chemical bonding extends beyond simple electron sharing, inviting us to consider the spatial and energetic dynamics of d-orbitals in creating complex structures.

Second-Period Elements

Second-period elements, such as nitrogen and carbon, present a unique limitation in terms of chemical bonding. Unlike their counterparts in higher periods, second-period elements do not possess d-orbitals. Here's why that matters:

• Due to their lack of d-orbitals, nitrogen and carbon cannot engage in pπ-dπ interactions, limiting their ability to form such complex bonding structures.
• This influences the type of molecules they can create, often leading to simpler bonding geometries compared to heavier elements with d-orbitals.

For instance, neither (\(\mathrm{NO}_{2}^{-}\)) nor (\(\mathrm{CO}_{3}^{2-}\)) can display pπ-dπ overlap due to their central atoms—nitrogen and carbon, respectively. Both elements are integral in countless organic and inorganic structures, yet they achieve stability through different means, relying primarily on pπ-pπ bonding when engaging with elements like oxygen.