Question

Many molecules with central atoms from Period 3 or higher take part in Lewis acid-base reactions in which the central atom expands its valence shell. \(\mathrm{SnCl}_{4}\) reacts with \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) as follows: (a) Identify the Lewis acid and the Lewis base in the reaction. (b) Give the \(n l\) designation of the sublevel of the central atom in the acid before it accepts the lone pair.

Short answer

Lewis acid: \(\text{SnCl}_4\); Lewis base: \((\text{CH}_3)_3 \text{N}\); Sublevel before accepting lone pair: \(5s\,5p\).

Step-by-step solution

Determine the Lewis Acid

A Lewis acid is a substance that accepts a pair of electrons. In the reaction between \(\text{SnCl}_4\) and \((\text{CH}_3)_3 \text{N}\), \(\text{SnCl}_4\) accepts a lone pair of electrons from \((\text{CH}_3)_3 \text{N}\). Therefore, \(\text{SnCl}_4\) is the Lewis acid.

Determine the Lewis Base

A Lewis base is a substance that donates a pair of electrons. In this reaction, \((\text{CH}_3)_3 \text{N}\) donates its lone pair of electrons to \(\text{SnCl}_4\). Therefore, \((\text{CH}_3)_3 \text{N}\) is the Lewis base.

Determine the Sublevel of Central Atom in Lewis Acid

Before accepting the lone pair, the central atom in \(\text{SnCl}_4\) is \(\text{Sn}\). Tin (Sn) is found in Period 5 and is in Group 14 of the periodic table. The sublevel of Sn in \(\text{SnCl}_4\) is the \(5s\,5p\)-orbitals.

Key concepts

Lewis acids

A Lewis acid is any substance that accepts a pair of electrons. In the reaction between \(\text{SnCl}_4\) and \((\text{CH}_3)_3 \text{N}\), the \(\text{SnCl}_4\) accepts an electron pair from the nitrogen atom in the \((\text{CH}_3)_3 \text{N}\). This makes \(\text{SnCl}_4\) the Lewis acid.
Understanding Lewis acids is crucial because they play a big part in various chemical reactions, especially those involving metals and complexes.
Lewis acids are often found among the compounds of transition metals, main group elements, and even some non-metals in higher oxidation states.

• \(\text{BF}_3\)
• \(\text{AlCl}_3\)
• \(\text{FeCl}_3\)

Lewis bases

A Lewis base is a substance that donates an electron pair to a Lewis acid. In our reaction, \((\text{CH}_3)_3 \text{N}\) donates its lone pair of electrons to \(\text{SnCl}_4\).
This means that \((\text{CH}_3)_3 \text{N}\) is the Lewis base.
Lewis bases are generally compounds with lone pairs of electrons that can be easily donated.
They act as nucleophiles in many chemical reactions, participating in the formation of bonds by donating their lone pairs.

• \(\text{NH}_3\)
• \(\text{H}_2\text{O}\)
• \(\text{OH}^-\)

Valence shell expansion

Valence shell expansion refers to an atom in a molecule increasing the number of electrons in its valence shell beyond the typical octet rule.
In our case, the central atom in \(\text{SnCl}_4\) expands its valence shell when it accepts an electron pair from the \((\text{CH}_3)_3 \text{N}\).
Atoms from Period 3 or higher on the periodic table can expand their valence shells due to available d-orbitals. This ability allows them to form more bonds and make complex structures.
Such expansion is a notable exception to the octet rule, and it is essential for understanding many chemical bonding scenarios in advanced inorganic chemistry.

Electron pair donation

Electron pair donation is a core concept in Lewis acid-base reactions. The Lewis base donates an electron pair to the Lewis acid. In the reaction between \(\text{SnCl}_4\) and \((\text{CH}_3)_3 \text{N}\), the electron pair is donated by the nitrogen atom in \((\text{CH}_3)_3 \text{N}\).
This type of interaction is crucial for the formation of coordinate covalent bonds, where both electrons in the bond come from the same atom (the Lewis base). Such bonds are essential in the chemistry of complex ions and metal-ligand interactions.

• A strong Lewis base donates electrons easily.
• Lewis bases usually contain lone pairs of electrons.

Periodic table sublevels

Knowledge of periodic table sublevels is vital for understanding the distribution of electrons in atoms. Sublevels include s, p, d, and f orbitals, which fill up in a specific order following the Aufbau principle.
In the case of \(\text{SnCl}_4\), the central atom \(\text{Sn}\) (Tin), belongs to Period 5 and Group 14. Before Sn accepts the lone pair, its valence electrons are in the \(5s\) and \(5p\) sublevels.
Understanding the electron configuration helps in predicting and explaining chemical behavior, bond formation, and reactivity of elements.