Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

If \(\mathrm{NH}_{4} \mathrm{OH}\) is added to the \(\left(\mathrm{PtCl}_{4}\right)^{2-}\) ion, the complex formed represents (a) zero dipole (b) infinite dipole (c) finite dipole (d) all of these

Step-by-Step Solution

Answer

Option (c), finite dipole, is correct.

Step 1: Understanding the Problem

We are given the complex ion \(\text{PtCl}_4^{2-}\) and need to determine the nature of the dipole moment when \(\text{NH}_4\text{OH}\) is added to it. The options are zero dipole, infinite dipole, finite dipole, or all of these.

Step 2: Identify the Complex Formation

Upon the addition of \(\text{NH}_4\text{OH}\), which contains \(\text{OH}^-\), the complex \(\text{PtCl}_2(\text{NH}_3)_2\) or \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\) can form, known as cis-diamminedichloroplatinum(II).

Step 3: Analyzing the Geometry

The formed complex \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\) is planar with four ligands around the central platinum ion in a square planar arrangement. The cis and trans isomers have different dipole moments.

Step 4: Determine the Dipole Moment

In the cis-isomer, due to its shape, it has a finite dipole moment. The trans-isomer, however, has a symmetrical arrangement, making its net dipole zero.

Step 5: Select the Correct Option

Since a planar square complex can have either a cis or trans arrangement, and the cis has a finite dipole while trans has zero dipole, option (c), finite dipole, is correct.

Key Concepts

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges within a molecule. It indicates how polar the molecule is. Molecules with a dipole moment interact with electric fields and can affect how the substance behaves physically and chemically.
A dipole moment occurs when there is an asymmetrical distribution of electron density. For instance, in the cis isomer of a square planar complex like \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\), the \(\text{NH}_3\) and \(\text{Cl}^-\) ligands are positioned adjacent to each other, creating a separation of charge that results in a finite dipole moment. Conversely, the trans isomer has ligands opposite each other, allowing the dipole contributions to cancel out and resulting in a zero dipole moment.

Square Planar Geometry

Square planar geometry is a type of molecular shape that plays a crucial role in the properties of complexes, particularly in transition metals. In a square planar arrangement, a central metal atom is surrounded by four ligands positioned at the corners of a square plane. This structure is common for metal complexes with a \(d^8\) electron configuration, like platinum(II) complexes.
This geometry often leads to unique stereoisomers, such as cis and trans forms. Factors that affect the stability and formation of square planar complexes include the nature of the metal, as well as the electronic and steric properties of the ligands. Understanding this arrangement helps in predicting the behavior of these complexes in reactions, such as the formation of \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\) when \(\text{NH}_{4}\text{OH}\) is added to \(\text{PtCl}_4^{2-}\).

Cis and Trans Isomers

Cis and trans isomers are variations of a molecule that differ only in the spatial arrangement of ligands around a central atom. This is especially significant in square planar complexes, where the position of the ligands dramatically influences molecular properties.
In cis isomers, similar ligands are adjacent to each other. This arrangement can lead to properties like polar behavior given the asymmetrical distribution, leading to a finite dipole moment. In trans isomers, similar ligands are across from each other, allowing dipoles to effectively cancel, resulting in a non-polar molecule with a zero dipole moment.
An excellent example is the complex \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\), where the cis form is used in chemotherapy as cisplatin, a drug that targets cancer cells due to its ability to bind DNA in cells. The trans isomer does not have the same therapeutic effect, illustrating how minor structural differences can lead to significantly different chemical properties and uses.