Question

Because of the Lewis base properties of ether oxygen atoms, crown ethers are excellent complexing agents for \(\mathrm{Na}^{+}, \mathrm{K}^{+}\), and \(\mathrm{NH}_{4}{ }^{+}\). What kind of molecule might serve as a complexing agent for \(\mathrm{Cl}^{-}\)or \(\mathrm{Br}^{-}\)?

Short answer

Answer: A suitable complexing agent for \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\) could be a transition metal ion with vacant orbitals or a molecule containing a central atom with a partial positive charge that can accept electron pairs donated by the anions. Examples include transition metals like \(\mathrm{Fe}^{3+}\), \(\mathrm{Cu}^{2+}\), or \(\mathrm{Pd}^{2+}\), and electronegative central atoms such as boranes or phosphines (e.g., \(\mathrm{BH}_{3}\), \(\mathrm{B(CH_{3})_{3}}\), or \(\mathrm{PH}_{3}\)).

Step-by-step solution

Understand Lewis Acids and Bases

In order to determine a suitable molecule for complexing \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\), we must first understand the Lewis Acid-Base concept. A Lewis Acid is a molecule or ion that can accept electron pairs, while a Lewis Base is a molecule or ion that can donate electron pairs. In the given example, crown ethers serve as Lewis Bases, donating electron pairs to cations like \(\mathrm{Na}^{+}, \mathrm{K}^{+}\), and \(\mathrm{NH}_{4}{ }^{+}\), which act as Lewis Acids by accepting electron pairs.

Identify the Anions Properties

The anions \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\) have a negative charge due to the presence of one extra electron. These negatively charged ions are interested in donating their electron pair or forming a bond with a molecule that can accept their electron pair.

Determine the Type of Molecule

To serve as a complexing agent for \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\), the molecule should have the capacity to accept electron pairs donated by the anions. In other words, we are looking for a molecule that can act as a Lewis Acid. Ideal candidates for Lewis Acids include molecules or ions with a positive charge, vacant orbitals, or strong electronegative atoms that have the ability to attract and accommodate the electron pair from the anions.

Possible Complexing Agents

One possible candidate for complexing \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\) is a molecule containing a central metal ion with vacant orbitals, such as transition metals. These metal ions have the ability to accommodate and accept donated electron pairs from anions. Examples of transition metals that could be used as suitable complexing agents for these anions include \(\mathrm{Fe}^{3+}\), \(\mathrm{Cu}^{2+}\), or \(\mathrm{Pd}^{2+}\). Another potential complexing agent is a molecule containing a central atom with a partial positive charge. These molecules often have highly electronegative atoms, such as boron or phosphorus, which will have an electron-deficient center that can accept electron pairs from anions. Examples of such molecules include boron-based molecules like boranes (e.g., \(\mathrm{BH}_{3}\) or \(\mathrm{B(CH_{3})_{3}}\) ) and phosphorus-containing molecules like phosphine (e.g. \(\mathrm{PH}_{3}\)). In summary, a suitable complexing agent for \(\mathrm{Cl}^{-}\) and \(\mathrm{Br}^{-}\) could be a transition metal ion with vacant orbitals or a molecule containing a central atom with a partial positive charge that can accept the electron pairs donated by the anions.