Problem 39 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Geometrical shapes of the complexes formed by the reaction of \(\mathrm{Ni}^{2+}\) with \(\mathrm{Cl}^{-}, \mathrm{CN}^{-}\)and \(\mathrm{H}_{2} \mathrm{O}\), respectively, are [2011] (a) octahedral, tetrahedral and square planar (b) tetrahedral, square planar and octahedral (c) square planar, tetrahedral and octahedral (d) octahedral, square planar and octahedral

Step-by-Step Solution

Answer

(b) tetrahedral, square planar, and octahedral.

Step 1: Understand the Coordination Chemistry

Coordination chemistry involves the study of chemical compounds formed between metal ions and ligands. A metal complex consists of a central metal atom or ion bonded to surrounding molecules or ions (ligands). The geometry of a complex is determined by the number of ligands and their spatial arrangement around the metal center.

Step 2: Analyze the Reaction with \(\mathrm{Cl}^{-}\)

\(\mathrm{Ni}^{2+}\) forms complexes with different ligands. When it reacts with \(\mathrm{Cl}^{-}\), four \(\mathrm{Cl}^{-}\) ions coordinate with the nickel ion, forming a complex \([\mathrm{NiCl}_4]^{2-}\). This complex is well-known to have a tetrahedral geometry due to the presence of four ligands.

Step 3: Analyze the Reaction with \(\mathrm{CN}^{-}\)

The reaction of \(\mathrm{Ni}^{2+}\) with \(\mathrm{CN}^{-}\) results in the formation of the complex \([\mathrm{Ni(CN)}_4]^{2-}\). This complex typically results in a square planar geometry due to electronic configuration and crystal field stabilization involving \(d^8\) systems.

Step 4: Analyze the Reaction with \(\mathrm{H}_2 \mathrm{O}\)

Nickel ions \(\mathrm{Ni}^{2+}\) in an aqueous solution form an octahedral complex \([\mathrm{Ni(H_2O)}_6]^{2+}\). There are six \(\mathrm{H}_2\mathrm{O}\) molecules coordinating with the ion to form the octahedral geometry typical for complexes with six ligands.

Step 5: Conclusion with Correct Option

Comparing the geometries from the previous steps, the geometries for complexes formed with \(\mathrm{Ni}^{2+}\) are: tetrahedral with \(\mathrm{Cl}^{-}\), square planar with \(\mathrm{CN}^{-}\) and octahedral with \(\mathrm{H}_2\mathrm{O}\). Therefore, the correct sequence of geometries is given in answer (b) tetrahedral, square planar and octahedral.

Key Concepts

Complex Geometry

In coordination chemistry, the geometry of a complex refers to the three-dimensional arrangement of ligands around a central metal ion. The geometry is crucial as it affects the properties and reactivity of the complex. Common geometries include:

• Tetrahedral: Four ligands symmetrically arrange around the central metal ion, resulting in a shape similar to a pyramid. An example is the nickel complex with chloride ions, \([\mathrm{NiCl}_4]^{2-}\).


• Square Planar: Here, four ligands lie in the same plane, forming a square around the central metal. A typical example is the nickel complex with cyanide ions, \([\mathrm{Ni(CN)}_4]^{2-}\).


• Octahedral: Six ligands are symmetrically arranged around the central ion, resembling an octahedron. An example is the nickel complex with water molecules, \([\mathrm{Ni(H_2O)}_6]^{2+}\).

Understanding these shapes helps in predicting the behavior and reactivity of metal complexes in different chemical environments.

Metal-Ligand Interactions

Metal-ligand interactions are pivotal in determining the structure and properties of a complex. These interactions involve the sharing of electrons between the central metal ion and the surrounding ligands. Different types of ligands can affect the metal center in various ways:

• Chloride Ion: It typically forms weaker interactions with metal ions like \(\mathrm{Ni}^{2+}\), promoting the formation of a tetrahedral complex such as \([\mathrm{NiCl}_4]^{2-}\).


• Cyanide Ion: Strong field ligands like \(\mathrm{CN}^{-}\) lead to more stable and lower energy configurations, often resulting in square planar geometries for \(d^8\) metals like nickel, as seen in \([\mathrm{Ni(CN)}_4]^{2-}\).


• Water: A common ligand that often forms octahedral complexes with transition metals such as \([\mathrm{Ni(H_2O)}_6]^{2+}\). Water acts as a neutral ligand, coordinating through its oxygen atom.

The type of metal-ligand interaction influences the complex's overall geometry and stability.

Octahedral Complexes

Octahedral complexes are a prevalent and significant geometry in coordination chemistry. These complexes have six ligands symmetrically arranged around a central metal ion. A classical example is the nickel complex \([\mathrm{Ni(H_2O)}_6]^{2+}\), where each water molecule coordinates with the metal through lone pairs on their oxygen atoms.There are a few key points to note about octahedral complexes:

• Commonly found with transition metals, due to their ability to accommodate multiple ligands.


• This geometry arises when the coordination number of the metal is six.


• Ligands are positioned at the vertices of an octahedron around the metal center.

Understanding octahedral geometry is essential because it is one of the cornerstones of coordination chemistry. It helps predict the behavior of complexes in reactions and their spectroscopic properties.