Question

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

Short answer

Option (c): N is pyramidal, sp^3; B is tetrahedral, sp^3.

Step-by-step solution

Understanding BF3-NH3 Complex

The reaction between \( ext{BF}_3\) and \( ext{NH}_3\) forms a 1:1 complex where the lone pair on the nitrogen of \( ext{NH}_3\) makes a dative bond with the boron of \( ext{BF}_3\). This reaction addresses the electron-deficient nature of the boron atom.

Determine Geometry and Hybridization of NH3

\( ext{NH}_3\) has a pyramidal shape due to the lone pair on nitrogen. Nitrogen forms three sigma bonds with hydrogen atoms using \(sp^3\) hybrid orbitals, with the lone pair occupying one of these \(sp^3\) orbitals, resulting in a trigonal pyramidal shape.

Determine Geometry and Hybridization of BF3

Before complexation, \( ext{BF}_3\) is planar and the boron is \(sp^2\) hybridized. However, upon accepting the lone pair from \( ext{NH}_3\), boron undergoes \(sp^3\) hybridization, forming a fourth covalent bond. This changes the geometry around boron to tetrahedral.

Analyzing the Whole Complex

After forming the \( ext{BF}_3\)-\( ext{NH}_3\) complex, nitrogen remains pyramidal \(sp^3\), while boron becomes tetrahedral \(sp^3\). Nitrogen donates its lone pair, but its geometry does not significantly change.

Key concepts

Coordinate Geometry

Coordinate geometry helps understand the spatial arrangement of atoms in molecules and complexes. In the context of the \(\mathrm{BF}_3-\mathrm{NH}_3\) complex, it refers to the three-dimensional arrangement around the central atoms, nitrogen (N) and boron (B).
- **Before Complexation:** - Boron in \(\mathrm{BF}_3\) is in a planar configuration using \(sp^2\) hybridization. - Nitrogen in \(\mathrm{NH}_3\) is pyramidal due to its \(sp^3\) hybridization with a lone pair that pushes the hydrogen atoms down.
- **After Complexation:** - Boron accepts a lone pair from nitrogen, changing its geometry from planar to tetrahedral as it shifts to \(sp^3\) hybridization. - Coordinate geometry enables prediction of molecular shape, impacting the molecule's physical properties and reactivity.

Hybridization

Hybridization is a critical concept to understand molecular bonding and geometry. It represents the mixing of atomic orbitals to form new hybrid orbitals suited for bonding.
- **In \(\mathrm{NH}_3\):** - Nitrogen undergoes \(sp^3\) hybridization, which involves combining one \(s\) and three \(p\) orbitals. - This results in four equivalent \(sp^3\) hybrid orbitals, fitting one lone pair and three bonding pairs with hydrogen.
- **In \(\mathrm{BF}_3\):** - Initially, boron is \(sp^2\) hybridized with three hybrid orbitals forming bonds with fluorine. - Post-complexation (after accepting nitrogen's lone pair), boron's hybridization transitions to \(sp^3\), leading to a new bonding orbital configuration and achieving a tetrahedral shape.
Understanding hybridization helps streamline the understanding of bond angles and spatial arrangements in molecules.

Molecular Complexes

Molecular complexes like \(\mathrm{BF}_3-\mathrm{NH}_3\) exhibit unique properties and structures due to their component interactions. The complex demonstrates how molecules interact through covalent and coordinate bonds.

• **Dative Bond Formation:** - Nitrogen in \(\mathrm{NH}_3\) donates its lone pair to electron-deficient boron in \(\mathrm{BF}_3\), forming a dative bond.
• **Geometric Influence:** - The coordinate bond changes boron's geometry from planar to tetrahedral, illustrating how bonding influences molecular shape.

Through this interaction, the \(\mathrm{BF}_3-\mathrm{NH}_3\) complex showcases the dynamic nature of molecular complexes and their importance in chemistry, notably in catalysis and material sciences.

Lewis Structures

Lewis structures are a simple yet powerful tool for representing molecular bonding and electron arrangement. By using dots and lines, they depict the relationships between different elements within a molecule or complex.
- **Lewis Structure of \(\mathrm{NH}_3\):** - Nitrogen atom is at the center, surrounded by three hydrogen atoms, with a pair of dots representing a lone pair.
- **Lewis Structure of \(\mathrm{BF}_3\):** - Boron is at the center bonded to three fluorine atoms, with no lone pairs initially.
- **In the \(\mathrm{BF}_3-\mathrm{NH}_3\) Complex:** - The lone pair from \(\mathrm{NH}_3\) is used to form a bond with boron.
Using Lewis structures helps visualize electron sharing and lone pairs, playing a crucial role in predicting molecular geometry and reactivity as seen in this complex.