Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following species has a square planar structure? (a) \(\left[\mathrm{SF}_{6}\right]\) (b) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) (c) \(\left[\mathrm{BeF}_{4}\right]^{2-}\) (d) \(\left[\mathrm{Hg}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The species \([\mathrm{Pt}(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}]\) has a square planar structure.

Step 1: Identify the Geometry of SF6

The molecule \([\mathrm{SF}_{6}]\) is sulfur hexafluoride. It has an octahedral geometry due to the presence of six bonding pairs of electrons around the central sulfur atom.

Step 2: Identify the Geometry of Pt(NH3)2Cl2

The complex \([\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}]\) is known as \'cisplatin\'. Platinum in this complex is coordinated to four ligands in a plane, adopting a square planar configuration due to the \(d^8\) electronic configuration of \(\mathrm{Pt}^{2+}\).

Step 3: Identify the Geometry of BeF4^{2-}

The species \[\left[\mathrm{BeF}_{4}\right]^{2-}\] is a complex where beryllium is surrounded by four fluoride ions. This arrangement results in a tetrahedral geometry, not square planar.

Step 4: Identify the Geometry of Hg(NH3)2Cl2

The compound \([\mathrm{Hg}\left(\mathrm{NH}_{3}\right)_{2}] \mathrm{Cl}_{2}\) typically forms a linear structure due to the presence of two ammonia ligands and a mercury ion, and the chlorides acting as counterions.

Key Concepts

Square Planar Complex

In the world of molecular geometry, a square planar complex is a fascinating arrangement of atoms. This configuration occurs when a central metal atom or ion is surrounded by four ligands. The atoms or groups are positioned at the four corners of a square, all lying in the same plane.

One reason why this arrangement forms is due to the electronic configuration of the metal center, specifically in metals with a \(d^8\) configuration. These metals have a natural tendency to minimize electronic repulsions and achieve lower energy states. A well-known example is platinum(II) complexes, such as cisplatin.

In square planar geometry, you can imagine the molecules as flat and 2-dimensional, differing from the more familiar 3D geometries. The ligands are spaced at 90-degree angles, allowing for unique reactivity and properties. These characteristics make square planar complexes interesting in designing drugs and catalysts.

Cisplatin

Cisplatin is a famous example of a square planar complex with far-reaching implications in medicine. Its chemical makeup is \([\text{Pt}\left(\text{NH}_3\right)_2 \text{Cl}_2]\), describing a platinum atom coordinated to two ammonia molecules and two chloride ions.

This coordination in a square planar geometry gives cisplatin its remarkable properties, especially in cancer treatment. Cisplatin can interfere with DNA replication in cancer cells, leading to cell death, thus acting as a potent chemotherapeutic agent. Its square planar arrangement ensures a specific alignment that is crucial for its biological activity.

When visualizing cisplatin, it's useful to picture the platinum atom at the center of the square with its ligands perfectly aligned at 90 degrees. Understanding its geometry helps in comprehending how it interacts with other biological molecules, showcasing its importance beyond the chemistry lab.

Octahedral Geometry

Octahedral geometry is a common molecular shape where six atoms or ligands surround a central atom, resembling an eight-faced shape. In terms of chemistry, this arrangement is stable and prevalent among coordination compounds.

A prime example of octahedral geometry is sulfur hexafluoride, \( \text{SF}_6 \), where six fluorine atoms are symmetrically distributed around a central sulfur atom. The fluorine atoms are equidistant from one another, creating 90-degree bond angles. This geometry ensures maximum ligand separation, reducing repulsion and stabilizing the molecule.

Visualizing octahedral geometry can be likened to two square pyramids joined at their bases, allowing students to grasp the 3D aspect of these molecules. This structural configuration affects physical and chemical properties, making it a key topic in studying inorganic chemistry.

Tetrahedral Geometry

Tetrahedral geometry is a fundamental three-dimensional shape occurring when a central atom bonds with four ligands. The atoms are arranged at the corners of an ideal tetrahedron, resulting in bond angles of approximately 109.5 degrees.

In the molecule \( \left[\text{BeF}_4\right]^{2-} \), you find tetrahedral geometry. The beryllium atom at the center is surrounded by four fluoride ions at equal distances, creating a symmetrical shape. This spatial arrangement offers maximal separation between bonds, reducing electron pair repulsion according to VSEPR theory.

By visualizing a tetrahedron, students can better understand how this geometry contrasts with others like square planar or linear geometries. Tetrahedral geometry is widespread in both organic and inorganic chemistry, forming the basis of many structural analyses and synthesis.