Problem 20 - Chemistry and Chemical Reactivity

Question

One of the following nitrogen compounds or ions is not capable of serving as a ligand: \(\mathrm{NH}_{4}^{+}, \mathrm{NH}_{3}\) \(\mathrm{NH}_{2}^{-} .\) Identify this species, and explain your answer.

Step-by-Step Solution

Answer

The species \(\mathrm{NH}_{4}^{+}\) cannot serve as a ligand because it lacks a lone electron pair.

Step 1: Define a Ligand

A ligand is a molecule or ion that can donate at least one pair of electrons to a central metal atom or ion, forming a coordinate bond. This means a ligand should have at least one pair of non-bonding electrons.

Step 2: Examine Each Species for Lone Pairs

Let's look at each species for the presence of non-bonding electron pairs:- The ammonium ion, \(\mathrm{NH}_{4}^{+}\), has no non-bonding electron pairs because all electrons are involved in bonding, and the nitrogen carries a positive charge.- Ammonia, \(\mathrm{NH}_{3}\), has a lone pair of electrons, making it capable of donating electrons.- The amide ion, \(\mathrm{NH}_{2}^{-}\), also has a lone pair of electrons and can donate to form a bond.

Step 3: Identify the Non-Ligand Species

From the examination, \(\mathrm{NH}_{4}^{+}\) lacks a lone electron pair and, therefore, cannot act as a ligand by donating electrons. In comparison, both \(\mathrm{NH}_{3}\) and \(\mathrm{NH}_{2}^{-}\) have lone pairs and can serve as ligands.

Key Concepts

Coordinate Bond

A coordinate bond, also referred to as a dative covalent bond, is a type of chemical bond where both electrons in the bond originate from the same atom. This is different from a typical covalent bond, where each atom provides one electron.
A coordinate bond forms when a ligand donates a pair of electrons to an empty orbital of a central metal atom or ion. This bond is crucial in complex formation.

Key characteristics include:

• Involvement of electron pair donation from a ligand to a metal center.
• Common in transition metal complexes.
• Often denoted with an arrow (→) pointing from the donor atom to the acceptor atom.

Understanding coordinate bonds helps explain why certain species can act as ligands, emphasizing the importance of electron pair availability.

Electron Pair Donation

In chemistry, electron pair donation refers to the process by which a molecule or ion provides a pair of electrons to an electron-deficient species, thereby forming a bond.
This donation is essential for the formation of coordinate bonds, as it allows ligands to attach to metal centers.

Important points include:

• Electron-rich species, like ligands, typically have lone pairs available.
• Electron donation leads to complex formation in coordination chemistry.
• This concept is vital to understanding the reactivity and bonding capabilities of ligands.

By grasping electron pair donation, we can determine which species can act as ligands and understand their role in coordination compounds.

Ammonium Ion

The ammonium ion, \(\mathrm{NH}_{4}^{+}\), is a positively charged ion with no lone electron pairs.
This ion forms by the protonation of ammonia, involving the donation of a hydrogen ion (H⁺).

Key features include:

• Has a tetrahedral geometry with four hydrogen atoms bonded to nitrogen.
• Lacks lone pairs on nitrogen, making it unable to form coordinate bonds.
• Positively charged, denoted as \(\mathrm{NH}_{4}^{+}\).

This absence of lone electron pairs is what prevents \(\mathrm{NH}_{4}^{+}\) from acting as a ligand, distinguishing it from other nitrogen-based species.

Lone Electron Pairs

Lone electron pairs are pairs of valence electrons not involved in bonding.
These pairs are crucial in determining the ability of molecules or ions to participate in forming coordinate bonds.

Characteristics include:

• Located on atoms like nitrogen, oxygen, and halogens.
• Can be donated to electron-deficient species, forming a bond.
• Essential for the behavior of molecules in chemical reactions.

The presence of lone electron pairs in molecules like \(\mathrm{NH}_3\) and \(\mathrm{NH}_{2}^{-}\) allows them to act as ligands, unlike \(\mathrm{NH}_{4}^{+}\), which lacks such pairs.