Question

Comment on each of the following observations: (a) \(\mathrm{XeF}_{6}\) has a distorted octahedral structure. (b) \(\left[\mathrm{N}_{3}\right]^{-}\) is linear with equal \(\mathrm{N}-\mathrm{N}\) bond lengths; \(\left[\mathrm{N}_{5}\right]^{\prime}\) is bent at the central \(\mathrm{N}\) atom. (c) \(\mathrm{NOF}_{3}\) is well established; it has \(\mathrm{N}-\mathrm{O}\) and \(\mathrm{N}-\mathrm{F}\) bond lengths of 116 and \(146 \mathrm{pm}\) respectively. (d) \(\mathrm{BH}_{3}\) can accept a pair of electrons to form compounds such as \(\mathrm{H}_{3} \mathrm{BNMe}_{3}\) in which the \(\mathrm{B}\) atom is tetrahedral.

Short answer

(a) Lone pair causes distortion; (b) Equivalent resonance in \([\mathrm{N}_{3}]^{-}\), bent in \([\mathrm{N}_{5}^{-}]\); (c) Reflects bond order; (d) Tetrahedral due to electron acceptance.

Step-by-step solution

Understanding Xenon Hexafluoride Structure

The molecule \( \mathrm{XeF}_{6} \) has a distorted octahedral structure due to the presence of a lone pair on the Xenon atom. In a perfect octahedral structure, there are six bond pairs and no lone pairs. In the case of \( \mathrm{XeF}_{6} \), there are six \( \mathrm{Xe-F} \) bonds and one lone pair causing a distortion from the ideal geometry.

Investigating the Azide Ion Configuration

The azide ion \( \left[\mathrm{N}_{3}\right]^{-} \) is linear because it follows a resonance structure with two equivalent forms, which average out to equal \( \mathrm{N}-\mathrm{N} \) bond lengths. However, \( \left[\mathrm{N}_{5}\right]^{-} \) has a central \( \mathrm{N} \) atom connected to other \( \mathrm{N} \) atoms, leading to a bent configuration due to lone pair-bond pair repulsion at the central nitrogen.

Analyzing Nitrosyl Fluoride Structure

In \( \mathrm{NOF}_{3} \), the nitrogen atom forms bonds with both oxygen and fluorine. The bond lengths given are \( 116 \mathrm{pm} \) for \( \mathrm{N}-\mathrm{O} \) and \( 146 \mathrm{pm} \) for \( \mathrm{N}-\mathrm{F} \), indicating a single \( \mathrm{N}-\mathrm{F} \) bond and a double \( \mathrm{N}-\mathrm{O} \) bond. The variation in bond lengths reflects the different electronegativities and bond orders.

Understanding Borane Electron Acceptance

\( \mathrm{BH}_{3} \) is a Lewis acid with an incomplete octet, making it capable of accepting electron pairs. When it forms \( \mathrm{H}_{3} \mathrm{BNMe}_{3} \), it accepts a pair of electrons from the nitrogen of \( \mathrm{NMe}_{3} \), thereby completing its octet and resulting in a tetrahedral geometry around the \( \mathrm{B} \) atom.

Key concepts

Molecular Geometry

Many molecules and ions exhibit shapes that are determined by the arrangements of their atoms in space, a concept known as molecular geometry. Understanding the shape of a molecule is crucial because it can affect the molecule's reactivity, polarity, phase of matter, color, magnetism, biological activity, and more. - **Linear:** A simple geometric shape where atoms align in a straight line. This is seen in molecules like carbon dioxide ( [] CO_2[]) and the azide ion ( [] [N3]^- []), where the bonds are arranged in a 180-degree angle. - **Bent or Angular:** This type occurs due to lone pair-bond pair repulsion. Water (H_2O) is a classic example, but compounds like [N_5]^- can also show such geometry due to central atom repulsions. - **Octahedral and Distorted Octahedral:** Ideal for 6 bonded atoms around a central atom, resulting in 90-degree bond angles. Xenon hexafluoride ( [] XeF_6[]) demonstrates a distorted version due to lone pair interactions altering the ideal symmetry. The more understanding we gain of a molecule's shape, the more effectively we can predict and explain its behavior.

Lone Pairs

Lone pairs refer to a pair of valence electrons that are not shared with another atom in a molecule. These unshared electrons can have a significant impact on the geometry and properties of molecules. - **Distortion in Structure:** Lone pairs repel bonded pairs of electrons, causing the molecular geometry to adjust and become asymmetrical. In XeF_6 the presence of a lone pair on xenon leads to a distorted octahedral shape. - **Influence on Bond Angles:** These electrons also affect bond angles. For example, ammonia ( SHS_H_3HS[] []) is trigonal pyramidal instead of planar due to the repulsion between the lone pair and the bond pairs. - **Effect on Polarity:** A molecule with lone pairs tends to have polar characteristics, resulting in a dipole moment if the polarities do not cancel each other out. Ammonia is slightly polar due to its lone pair. Lone pairs play an invisible yet vital role in determining how a molecule behaves, including its reactivity and interaction with other molecules.

Resonance Structures

Resonance structures are a way of describing the delocalization of electrons in molecules where the bonding cannot be expressed by a single Lewis structure. They help in representing the real, averaged bonding scenario within certain molecules or ions. - **Stability through Resonance:** Compounds like the azide ion, [N_3]^-[], can have multiple valid Lewis structures. The electrons are shared across the entire molecule, stabilizing it due to delocalization. - **Bond Length Equality:** For azide, this sharing results in equal N-N bond lengths. - **Representation of Real Structure:** Individual resonance structures do not truly exist; rather, the real structure is a resonance hybrid that exhibits characteristics of all contributing structures. Understanding resonance structures is crucial in evaluating molecules with certain types of chemical reactivity and stability.

Lewis Acids

A Lewis acid is a chemical compound that can accept an electron pair. This concept extends beyond the traditional understanding of acids as substances that produce hydrogen ions in solution. - **Electron Pair Acceptor:** Compounds like BH_3 are quintessential examples. BH_3 does not have a full octet, making it eager to accept electrons and form new bonds. - **Interaction with Lewis Bases:** When BH_3 meets a Lewis base such as NMe_3 , they combine to form compounds like H_3 BNMe_3[]. - **Tetrahedral Result:** The electron acceptance results in a complete octet and alters the molecular geometry around B from planar to tetrahedral due to the added electron pair. Recognizing molecules acting as Lewis acids lets us comprehend their role in forming new substances, catalyzing reactions, and other chemical processes.