Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

Specify the coordination geometry and hybridization of \(\mathrm{N}\) and \(\mathrm{B}\) atoms in a \(1: 1\) complex of \(\mathrm{BF}_{3}\) and \(\mathrm{NH}_{3}\) (a) N: tetrahedral, sp; B: tetrahedral, sp \(^{3}\) (b) N: pyramidal, sp \(^{3} ;\) B: pyramidal, sp \(^{3}\) (c) N: pyramidal, sp \(^{3} ;\) B: planar, sp \(^{2}\) (d) N: pyramidal, sp \(^{3} ;\) B: tetrahedral, sp^{3}

Step-by-Step Solution

Answer

(c) N: pyramidal, sp\(^{3}\); B: planar, sp\(^{2}\).

Step 1: Analyze the Compound

In a \(1:1\) complex of \( \mathrm{BF}_3 \) and \( \mathrm{NH}_3 \), the \( \mathrm{NH}_3 \) molecule acts as a Lewis base donating a pair of electrons, and \( \mathrm{BF}_3 \) acts as a Lewis acid accepting those electrons. This creates a new coordinate bond.

Step 2: Determine Coordination Geometry of \(\mathrm{NH}_3\)

\(\mathrm{NH}_3\) is a pyramidal molecule in its stable form, with nitrogen at the center of a tetrahedron formed by three hydrogen atoms and a lone pair of electrons. The geometry remains pyramidal as nitrogen donates its lone pair.

Step 3: Determine Hybridization of \(\mathrm{NH}_3\)

The nitrogen atom in \(\mathrm{NH}_3\) undergoes \(\text{sp}^3\) hybridization, as it forms three \(\sigma\) bonds with hydrogen atoms and maintains a lone pair, consistent with tetrahedral electronic geometry.

Step 4: Determine Coordination Geometry of \(\mathrm{BF}_3\)

In the \(1:1\) complex, \(\mathrm{BF}_3\) shifts from planar triangular to tetrahedral upon accepting the lone pair from nitrogen, although \(\mathrm{BF}_3\) itself is originally planar.

Step 5: Determine Hybridization of \(\mathrm{BF}_3\)

Originally, \(\mathrm{BF}_3\) is \(\text{sp}^2\) hybridized as it forms three \(\sigma\) bonds and no lone pairs, resulting in a planar geometry. Even though the electron geometry changes upon complexation, the hybridization remains \(\text{sp}^2\).

Key Concepts

Coordination Geometry

In coordination chemistry, coordination geometry describes how atoms or groups are arranged around a central atom within a molecule or complex. When considering the 1:1 complex of BF_3 and NH_3, the coordination geometry gives insight into the molecular shape and orientation.
- For NH_3, the nitrogen atom exhibits a pyramidal geometry. It is situated at the center, surrounded by three hydrogen atoms and a lone pair of electrons. The lone pair occupies one vertex of a theoretical tetrahedron, which emphasizes the pyramidal nature of the molecule.
- On the other hand, BF_3 in its unbound state is planar due to the three fluorine atoms symmetrically arranged in a triangle around the central boron atom. However, upon forming a complex with NH_3, it adopts a more tetrahedral-like configuration as it accepts the lone pair from nitrogen.

Hybridization

Hybridization is the process where atomic orbitals mix to form new orbitals, which can affect molecular geometry. It provides an explanation for the arrangement of bonds in terms of their energy and shape.
- In NH_3, nitrogen undergoes sp^3 hybridization. This means one s orbital mixes with three p orbitals to form four sp^3 hybrid orbitals. These evolve into three sigma bonds with hydrogen and one remaining orbital holds a lone pair, accounting for its pyramidal shape.
- In the case of BF_3, boron is initially sp^2 hybridized in its planar form. This involves one s and two p orbitals mixing, which results in the formation of three sigma bonds with the fluorine atoms. Even in the complex form with NH_3, the hybridization remains sp^2, though the geometry adjusts to accept the electron pair from nitrogen.

Lewis Acid-Base Reaction

In a Lewis acid-base reaction, a Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. This interaction is essential in the formation of complexes like BF_3 and NH_3.
- NH_3 acts as the Lewis base by donating its lone pair of electrons. This pair is readily available due to its location in an orbital with sufficient overlap potential with the boron center.
- Conversely, BF_3 functions as the Lewis acid. It accepts the lone pair from NH_3 into an empty p orbital, allowing it to complete its valence shell configuration, enhancing molecular stability.
This electron pair transfer is pivotal in forming a coordinate covalent bond, transforming the composite structure from two separate entities into a unified complex.

Molecular Geometry

Molecular geometry takes into account the three-dimensional arrangement of atoms within a molecule, playing a significant role in defining physical and chemical properties. The geometry impacts molecule interactions, reactivity, and overall stability.
- NH_3, with its sp^3 hybridized nitrogen, retains a pyramidal geometric structure. Its slightly distorted tetrahedral shape results from the repulsion among the hydrogen atoms and the lone pair on nitrogen. This leads to bond angles less than the ideal 109.5^ degrees of a perfect tetrahedron.
- For BF_3, even though boron retains an sp^2 hybridization, its coordination with NH_3 alters its spatial arrangement from planar to more tetrahedron-like. This shift supports the electron pair from NH_3, enabling more stable interactions between boron and the surrounding atoms.
Thus, understanding molecular geometry is essential for anticipating reaction behaviors and molecular interactions, especially in complex formation.