Ligands: Videos & Practice Problems

Ligands are molecules or ions that act as Lewis bases, donating lone pairs to metal cations, which function as Lewis acids. Ligands can be neutral, like ammonia and water, or negatively charged, such as hydroxide and cyanide ions. The reaction between a ligand and a metal cation forms an adduct, where the overall charge equals the sum of the charges of the metal cation and the ligands. For example, a cadmium ion with four water ligands results in a 2+ charge adduct, illustrating the coordination chemistry principles at play.

Ligand Types

Ligands are essential components in coordination chemistry, acting as molecules or ions that function as Lewis bases by donating at least one lone pair of electrons to a metal cation, which behaves as a Lewis acid by accepting that lone pair. Understanding the types of ligands is crucial, as they can be categorized into neutral and negatively charged (anionic) ligands. While cationic ligands exist, they are not typically discussed in general chemistry contexts.

Neutral ligands include molecules such as ammonia (NH₃), water (H₂O), and carbon monoxide (CO). Ammonia has a lone pair on the nitrogen atom that can be donated to a metal cation. Water can also donate a lone pair from one of its hydrogen atoms. Carbon monoxide, although neutral overall, has a formal charge distribution where carbon carries a negative charge and oxygen a positive charge; thus, it is the carbon atom that donates its lone pair to the metal cation.

On the other hand, anionic ligands carry a negative charge and include hydroxide ions (OH^-), halides (X^-, where X represents halogens such as fluorine, chlorine, bromine, or iodine), cyanide ions (CN^-), and thiocyanate ions (SCN^-). For instance, in the cyanide ion, the negative charge is on the carbon atom, which donates its lone pair. Similarly, in thiocyanate, the nitrogen can also donate its lone pairs due to its formal negative charge.

In summary, ligands are crucial as they donate lone pairs to metal cations, and they can be classified as either neutral or negatively charged. This classification helps in understanding their behavior and interactions in various chemical reactions.

Ligands Example

In coordination chemistry, ligands are classified based on their charge. Neutral ligands do not carry any charge, while negatively charged ligands do. Among the options provided—bromide, hydrogen sulfide, ammonium ion, hydroxide ion, and cyanide ion—only hydrogen sulfide (H₂S) qualifies as a neutral ligand.

To analyze the options, the ammonium ion (NH₄⁺) is positively charged, disqualifying it as a neutral ligand. Bromide (Br^-), hydroxide (OH^-), and cyanide (CN^-) are all negatively charged ligands, which also excludes them from being neutral. In contrast, hydrogen sulfide, similar to water (H₂O), is neutral and can effectively act as a ligand in coordination complexes.

Thus, hydrogen sulfide is the correct answer, representing a neutral ligand in this context.

Ligand Reaction

In coordination chemistry, the interaction between a metal cation and ligands is fundamental to understanding adduct formation. An adduct is formed when a Lewis base, represented by a ligand, donates a lone pair of electrons to a Lewis acid, which is the metal cation. This process can be visualized through the example of a cadmium ion (Cd²⁺) interacting with water (H₂O) molecules.

In this scenario, the cadmium ion acts as the Lewis acid due to its ability to accept electron pairs, while water serves as the Lewis base by donating one of its lone pairs. It is important to note that multiple water molecules can coordinate to a single cadmium ion, enhancing the complexity of the adduct.

For instance, if four water molecules coordinate with one cadmium ion, the resulting adduct can be represented as [Cd(H₂O)₄]²⁺. Here, the cadmium ion has a +2 charge, while each water molecule is neutral (0 charge). Therefore, the overall charge of the adduct is calculated by summing the charges of the metal cation and the ligands, resulting in a total charge of +2 for the adduct.

This example illustrates the essential concept of adduct formation in coordination compounds, highlighting the roles of Lewis acids and bases in creating stable complexes.

Ligands Example

When a nickel(III) ion combines with two bromide ions, the resulting adduct product can be determined by analyzing the charges involved. The nickel(III) ion, denoted as Ni³⁺, has a charge of +3. Each bromide ion (Br⁻) carries a charge of -1, and since there are two bromide ions, their total charge is -2.To find the overall charge of the adduct, we can sum the charges:\[\text{Total charge} = \text{Charge of Ni}^{3+} + 2 \times \text{Charge of Br}^{-} = +3 + 2 \times (-1) = +3 - 2 = +1\]This results in an overall charge of +1 for the adduct. Since the adduct has a net charge, it is represented within brackets to indicate its ionic nature. Therefore, the adduct product formed from the combination of one nickel(III) ion and two bromide ions is expressed as:\[\text{[NiBr}_2\text{]}^{+}\]This notation signifies that the compound is an ionic adduct with a net charge of +1.

Determine the charge of the metal cation in the given adduct product below:

• A. +1
• B. 0
• C. –2
• D. +3
• E. +2

Answer: (E)

Determine the type of ligand connected to the calcium ion.

• A. Neutral
• B. Anionic
• C. Cationic
• D. Not enough information given.

Answer: (A)

Determine the overall charge of the adduct when the aluminum ion combines with 2 bromides and 1 chlorides.

• A. +1
• B. 0
• C. –2
• D. –1
• E. +2

Answer: (B)

Here’s what students ask on this topic:

What are ligands in chemistry?

Ligands are molecules or ions that act as Lewis bases by donating at least one lone pair of electrons to a metal cation, which acts as a Lewis acid. This interaction forms a coordination complex. Ligands can be neutral, such as ammonia (NH₃) and water (H₂O), or negatively charged, such as hydroxide ion (OH^-) and cyanide ion (CN^-). The lone pair donation from the ligand to the metal cation is crucial in the formation of these complexes.

What is the difference between neutral and negatively charged ligands?

Neutral ligands, such as ammonia (NH₃) and water (H₂O), have no overall charge and donate lone pairs from neutral atoms. Negatively charged ligands, such as hydroxide ion (OH^-) and cyanide ion (CN^-), carry a negative charge and donate lone pairs from atoms with a negative formal charge. Both types of ligands act as Lewis bases by donating lone pairs to metal cations, but their charge affects the overall charge of the resulting coordination complex.

How do ligands interact with metal cations?

Ligands interact with metal cations by donating lone pairs of electrons to the metal, which acts as a Lewis acid. This donation forms a coordinate covalent bond between the ligand and the metal cation. The resulting product is called an adduct or coordination complex. The overall charge of the adduct is the sum of the charges of the metal cation and the ligands. For example, a cadmium ion (Cd²⁺) can form an adduct with four water molecules, resulting in a complex with a 2+ overall charge.

What is an adduct in ligand chemistry?

An adduct in ligand chemistry is the product formed by the interaction of a ligand (Lewis base) and a metal cation (Lewis acid). The ligand donates a lone pair of electrons to the metal cation, forming a coordinate covalent bond. The overall charge of the adduct is the sum of the charges of the metal cation and the ligands. For example, the adduct formed between a cadmium ion (Cd²⁺) and four water molecules has an overall charge of 2+.

Can you provide an example of a ligand reaction?

An example of a ligand reaction is the formation of a coordination complex between a cadmium ion (Cd²⁺) and water molecules. In this reaction, the cadmium ion acts as a Lewis acid, accepting lone pairs from water molecules, which act as Lewis bases. The reaction can be represented as:

Cd²⁺ + 4H₂O → [Cd(H₂O)₄]²⁺

In this complex, four water molecules donate lone pairs to the cadmium ion, resulting in an adduct with an overall charge of 2+.