Question

Identify the Lewis acid and the Lewis base in the following reactions: (a) \(5 \mathrm{CO}(g)+\mathrm{Fe}(s) \longrightarrow \mathrm{Fe}(\mathrm{CO})_{5}(l)\) (b) \(\mathrm{NH}_{3}(g)+\mathrm{BCl}_{3}(g) \longrightarrow \mathrm{Cl}_{3} \mathrm{BNH}_{3}(s)\) (c) \(\mathrm{Hg}^{2+}(a q)+4 \mathrm{I}^{-}(a q) \longrightarrow \mathrm{HgI}_{4}^{2-}(a q)\)

Short answer

(a) Lewis acid: Fe, Lewis base: CO; (b) Lewis acid: BCl₃, Lewis base: NH₃; (c) Lewis acid: Hg²⁺, Lewis base: I⁻.

Step-by-step solution

Understanding Lewis acids and bases

According to Lewis theory, a Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. In each reaction, identify the species that fulfills these roles.

Analyze reaction (a)

In reaction (a), the complex formation between carbon monoxide (CO) and iron (Fe) is taking place. CO has lone pairs on the carbon atom and acts as a Lewis base by donating these pairs to the iron atom. The iron atom, which accepts these electron pairs to form a coordination complex, acts as a Lewis acid.

Analyze reaction (b)

In reaction (b), ammonia (NH₃) has a lone pair of electrons on the nitrogen atom, which it donates to the boron atom in boron trichloride (BCl₃). Since BCl₃ accepts this pair, BCl₃ is the Lewis acid and NH₃ is the Lewis base.

Analyze reaction (c)

In reaction (c), the iodide ions (I⁻) provide electron pairs to bind with the mercury ion (Hg²⁺), forming the complex HgI₄²⁻. The iodide ions act as Lewis bases donating electron pairs, while Hg²⁺ is the Lewis acid accepting electron pairs.

Key concepts

Electron Pair Acceptor

In chemistry, an electron pair acceptor is a key component of Lewis acids. According to Lewis theory, a Lewis acid is defined as a substance that can accept a pair of electrons from a Lewis base. This concept is crucial in understanding how certain chemical reactions occur, especially those involving coordination complexes.

For instance, in reaction (a), the iron (Fe) atom acts as an electron pair acceptor. This means that Fe takes electron pairs from carbon monoxide (CO), forming a stable coordination complex known as iron pentacarbonyl, \( ext{Fe(CO)}_{5}\). Similarly, in reaction (c), the mercury ion \( ext{Hg}^{2+} \) accepts electron pairs from iodide ions \( ext{I}^{-} \) to form a stable complex \( ext{HgI}_{4}^{2-} \).

The role of electron pair acceptors is fundamentally important in creating new chemical bonds by incorporating an extra electron pair into a molecule. Thus, they are central to the formation of many coordination complexes.

Electron Pair Donor

A Lewis base, on the other hand, serves as an electron pair donor. It provides a pair of electrons that another atom or ion, the Lewis acid, can accept. This exchange is crucial in forming chemical compounds, especially coordination complexes.

In reaction (a), carbon monoxide (CO) acts as a Lewis base. It donates an electron pair to the iron atom, enabling the formation of iron pentacarbonyl. In reaction (b), ammonia (\( ext{NH}_{3} \)) acts as an electron pair donor. It donates a lone pair of electrons from its nitrogen atom to boron trichloride (\( ext{BCl}_{3} \)). This donation of electrons facilitates the formation of \( ext{Cl}_{3} ext{BNH}_{3} \).

Understanding the role of electron pair donors helps elucidate how complexes and new compounds form by sharing electron pairs. When these bases share their pairs, they enable an array of chemical transformations.

Coordination Complex

A coordination complex is a fascinating concept in chemistry where molecules or ions are bound to a central metal atom through the donation of electron pairs. These complexes form via the interaction between Lewis acids and bases; the base donates an electron pair which the acid accepts.

In reaction (a), iron pentacarbonyl \( ext{Fe(CO)}_{5}\) is a result of forming a coordination complex, created by CO molecules donating electron pairs to the iron atom. In reaction (c), the complex \( ext{HgI}_{4}^{2-} \) arises when iodide ions \( ext{I}^{-}\) provide electron pairs to the mercury ion \( ext{Hg}^{2+} \).

Coordination complexes are significant in both inorganic and organic chemistry due to their stability and the unique properties they impart to compounds. They are central in many applications, including catalysis, material science, and bioinorganic chemistry.

Lewis Structures

Understanding Lewis structures is paramount to grasping the interactions between electron pair acceptors and donors. Lewis structures are diagrams that show the bonding between atoms and the lone pairs of electrons in a molecule. They provide a visual representation which helps in understanding how molecules and ions interact.

For reaction (b), the Lewis structure of ammonia (\( ext{NH}_{3} \)) shows a lone pair on the nitrogen atom. Similarly, the Lewis structure of boron trichloride (\( ext{BCl}_{3} \)) shows boron ready to accept an electron pair. These structures explain how Lewis acid-base reactions create new bonds.

Drawing and interpreting Lewis structures allows students to visualize the transfer of electron pairs. This understanding facilitates a deeper comprehension of reaction mechanisms and bonding, which are the foundation of chemistry.