Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

The ion which is not tetrahedral in shape is (a) \(\mathrm{BF}_{4}^{-}\) (b) \(\mathrm{NH}_{4}^{+}\) (c) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) (d) \(\mathrm{NiCl}_{4}^{2-}\)

Step-by-Step Solution

Answer

\( \left[ \mathrm{Ni}(\mathrm{CN})_{4} \right]^{2-} \) is not tetrahedral; it is square planar.

Step 1: Determine the Structure of Each Ion

The first step is to evaluate the geometric structure of each ion. The tetrahedral shape is characterized by having four peripheral atoms equally spaced around a central atom, creating bond angles of approximately 109.5 degrees.

Step 2: Analyze \\( \mathrm{BF}_{4}^{-} \\)

The ion \( \mathrm{BF}_{4}^{-} \) consists of one boron atom surrounded by four fluorine atoms. Boron typically forms covalent bonds with four electron pairs, adopting a tetrahedral geometry. Thus, \( \mathrm{BF}_{4}^{-} \) is tetrahedral.

Step 3: Analyze \\( \mathrm{NH}_{4}^{+} \\)

The ammonium ion \( \mathrm{NH}_{4}^{+} \) consists of one nitrogen atom bonded to four hydrogen atoms. Nitrogen here exhibits a tetrahedral arrangement of bonds. Therefore, \( \mathrm{NH}_{4}^{+} \) is also tetrahedral.

Step 4: Analyze \\( \left[ \mathrm{Ni}(\mathrm{CN})_{4} \right]^{2-} \\)

For the ion \( \left[ \mathrm{Ni}(\mathrm{CN})_{4} \right]^{2-} \), nickel forms a complex with cyanide ions. This is a square planar complex, typically for such transition metal complexes, as a common geometry for nickel in a tetracoordinate setting. Therefore, this ion is not tetrahedral.

Step 5: Analyze \\( \mathrm{NiCl}_{4}^{2-} \\)

The \( \mathrm{NiCl}_{4}^{2-} \) ion has nickel bonded to four chlorine atoms. In this complex, nickel can form a tetrahedral geometry with chloride ions, common for nickel complexes with larger ligand fields like chloride. Thus, \( \mathrm{NiCl}_{4}^{2-} \) is tetrahedral.

Step 6: Conclusion: Identify the Non-Tetrahedral Ion

From the analysis, \( \left[ \mathrm{Ni}(\mathrm{CN})_{4} \right]^{2-} \) is the only ion that adopts a square planar structure rather than a tetrahedral one, so it is the answer to the given problem.

Key Concepts

Tetrahedral Geometry

In molecular geometry, a tetrahedral structure is one of the most common arrangements. It occurs when a central atom is surrounded by four atoms, each located at the corners of an imaginary tetrahedron. The defining feature of a tetrahedral geometry is the bond angle, which is approximately 109.5 degrees between the atoms. This arrangement is due to the equal repulsion between electron pairs, whether they form bonds or are unshared.

• Typical examples of molecules adopting this geometry are \(\mathrm{CH}_{4}\) (methane) and \(\mathrm{BF}_{4}^{-}\). Here, a central atom forms bonds with four other atoms, spreading them evenly in space.
• The geometry helps minimize electron pair repulsion, which is why it is so common in chemistry.

This geometry arises frequently in covalent compounds and ions where the central atom has no lone pairs or exhibits low steric demand.

Square Planar Complexes

Square planar geometry is an important concept especially within the context of coordination complexes. It is less common than tetrahedral shape but very significant in chemistry of transition metals. In this arrangement, a central atom is bonded to four ligands located at the corners of a square plane, effectively creating 90-degree bond angles.

• Square planar geometry is characteristic of some transition metal complexes like \(\left[ \mathrm{Ni}(abla)_4 \right]^{2-}\).
• Such complexes often involve elements like Nickel (Ni), Platinum (Pt), and Palladium (Pd), where a distinctive electronic configuration favors not having ligands in the axial positions.
• The stability of square planar complexes comes largely from ligand field stabilization and hybridization involved in these structures.

This form is prevalent in molecules with a 16 or 18 electron count around metal centers, which dictates their shape and bonding properties.

Coordination Chemistry

Coordination chemistry is a fascinating field that explores the interaction between metal ions and ligands to form complexes. These complexes can exhibit a range of geometries such as tetrahedral and square planar, depending on factors like the type of metal and ligands involved.

• Coordination chemistry is fundamental for understanding catalysts, biological systems, and materials science.
• The number of ligand atoms directly bonded to the central atom is known as the coordination number, which guides the complex's geometry.
• Ligands act like arms reaching out to bind the metal, thereby stabilizing the system through donating electron pairs.

The study of such complexes not only reveals the geometry but also in-depth understanding of their electronic properties, reactivity, and potential applications in various domains of science and technology.