Problem 11 - Chemistry in 30 days

Question

Stability constants for some copper complexes are given below \(\mathrm{Cu}^{+2}+4 \mathrm{NH}_{3} \rightleftharpoons\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+2} \quad \mathrm{~K}=4.5 \times 10^{11}\) \(\mathrm{Cu}^{+2}+4 \mathrm{CN}^{-} \rightleftharpoons\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{-2} \quad \mathrm{~K}=2.0 \times 10^{2}\) \(\mathrm{Cu}^{+2}+2 \mathrm{sen} \rightleftharpoons\left[\mathrm{Cu}(\mathrm{en})_{2}\right]^{+2} \quad \mathrm{~K}=3.0 \times 10^{15}\) \(\mathrm{Cu}^{+2}+4 \mathrm{H}_{2} \mathrm{O} \rightleftharpoons\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{l}^{+2} \quad \mathrm{~K}=9.5 \times 10^{8}\right.\) Which is the strongest ligand? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{CN}^{-}\) (c) en (d) \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

The strongest ligand is en.

Step 1: Understand the Concept of Stability Constant

The stability constant (\(K\)) measures the equilibrium between a metal ion and ligands in a complex. A higher stability constant indicates a more stable complex formation and, thus, a stronger ligand.

Step 2: Compare Stability Constants

Look at the given stability constants for each ligand: - \(\mathrm{NH}_{3}\) has \(K = 4.5 \times 10^{11}\)- \(\mathrm{CN}^{-}\) has \(K = 2.0 \times 10^{2}\)- \(\mathrm{en}\) has \(K = 3.0 \times 10^{15}\)- \(\mathrm{H}_{2} \mathrm{O}\) has \(K = 9.5 \times 10^{8}\)Compare these values to see which is the largest.

Step 3: Identify the Largest Stability Constant

The largest stability constant is \(K = 3.0 \times 10^{15}\), which is associated with the ligand \(\mathrm{en}\). This indicates that \(\mathrm{en}\) forms the most stable complex with \(\mathrm{Cu}^{+2}\).

Step 4: Conclusion

Based on the stability constants, the strongest ligand is \(\mathrm{en}\) because it forms the most stable complex with copper (II) ions compared to the other given ligands.

Key Concepts

Copper Complexes

Copper ions (\(\mathrm{Cu}^{+2}\)) have a fascinating capability to bind with different chemicals to form copper complexes. The process is governed by how well the copper ion can combine with other molecules, known as ligands. These ligands can be ammonia (\(\mathrm{NH}_3\)), cyanide (\(\mathrm{CN}^-\)), ethylenediamine (en), or water (\(\mathrm{H}_2\mathrm{O}\)).
Copper forms complexes by surrounding itself with these ligands through coordinate bonds, leading to structures that are often stable and colorful.

• These complexes occur because copper ions have a positive charge and attract ligands, which are usually negatively charged or neutral but rich in electron pairs.
• The strength and formation of these complexes are measured using stability constants, which represent the equilibrium in the complex formation.

Ligand Strength

Ligand strength refers to the ability of a ligand to bind effectively and strongly with a central metal ion like copper. Not all ligands bind with equal strength. The stability constant (\(K\)) is a measure indicating how strongly a ligand can bind to the metal ion.

• A higher \(K\) implies a more stable complex, thus indicating a stronger ligand.
• The stability constant varies among different ligands due to their nature and properties.
• For example, ethylenediamine (en) has a higher stability constant compared to ammonia (\(\mathrm{NH}_3\)) or water (\(\mathrm{H}_2\mathrm{O}\)) when binding with copper.
• This means that en is a stronger ligand, showing that it can hold the copper ion more firmly than the other ligands.

Complex Formation

Complex formation in chemistry involves creating a stable structure by a central metal ion and surrounding ligands. In the case of copper complexes, this is significant because copper ions form extremely stable structures with certain ligands.

• Formation occurs through coordinate covalent bonds, where the ligand donates a pair of electrons to the ion.
• The nature of the ligand, including its size, charge, and electron availability, affects how well the complex forms.
• Ligands like en form very stable copper complexes due to effective binding, shown by their high stability constants.
• This is crucial in processes such as catalysis and material synthesis, where copper complexes are employed.

Equilibrium Chemistry

Equilibrium chemistry studies the balance in a reaction where the formation and dissociation of complexes occur simultaneously. In copper complexes, equilibrium is reached when the formation of complexes and the reverse process of breaking them down happen at the same rate.

• The stability constant \(K\) reflects this balance by quantifying how much product versus reactant is present at equilibrium.
• Higher stability constants indicate that the complex formation is favored and that a large proportion of reactants are converting into products.
• This aspect of equilibrium chemistry helps in deciding the strength and stability of metal ligand bonds.
• In practical applications, understanding these equilibria enables better manipulation in chemical reactions involving copper complexes.