Question

How many bonds could each of the following chelating ligands form with a metal ion? a. acetylacetone (acacH), a common ligand in organometal:atalysts: b. diethylenetriamine, used in a variety of industrial processes: $$ \mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} $$ c. salen, a common ligand for chiral organometallic catalysts: d. porphine, often used in supermolecular chemistry as well as catalysis; biologically, porphine is the basis for many different types of porphyrin- containing proteins, including heme proteins:

Short answer

In summary, the number of bonds each chelating ligand can form with a metal ion are: a. Acetylacetone (acacH): 2 bonds b. Diethylenetriamine (dien): 3 bonds c. Salen: 4 bonds d. Porphine: 4 bonds

Step-by-step solution

a. Acetylacetone (acacH)

Acetylacetone (acacH) has two donor atoms: one oxygen (O) atom on each side of the ligand. Each oxygen atom has two lone pairs of electrons, which can form bonds with a metal ion when one of them is donated to the ion. Therefore, acetylacetone can form two bonds with a metal ion since it has two donor atoms.

b. Diethylenetriamine (dien)

Diethylenetriamine (dien) has three donor atoms, which are the nitrogen (N) atoms in the molecule's structure: \( \mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) Each nitrogen atom has a lone pair of electrons available for donation to a metal ion, which allows them to form bonds. Thus, diethylenetriamine can form a total of three bonds with a metal ion.

c. Salen

Salen is a more complex ligand with two nitrogen and two oxygen donor atoms. The nitrogen atoms are present as amines, while the oxygen atoms are present as part of a phenolic ring structure. Each donor atom has a lone pair of electrons that can be donated to a metal ion, allowing them to form bonds. Therefore, salen can form a total of four bonds with a metal ion.

d. Porphine

Porphine is a large macrocyclic ligand, which forms the core structure of the more famous porphyrin ligands. Porphine has four nitrogen donor atoms, forming the core of the molecule. Each nitrogen atom has a lone pair of electrons that can be donated to a metal ion, allowing for the formation of coordination bonds. Consequently, porphine can form a total of four bonds with a metal ion.

Key concepts

Acetylacetone

Acetylacetone is a fascinating chelating ligand that plays a distinct role in organometallic chemistry. It acts as a bidentate ligand, which means it can form two bonds with a metal ion. This is due to the presence of two oxygen atoms, each located on opposite sides of the ligand's molecular structure. These oxygen atoms are significant because they each hold lone pairs of electrons.

When acetylacetone interacts with a metal ion, these lone pairs can be donated, forming strong coordination bonds. This ability allows acetylacetone to stabilize metal ions effectively, which is crucial in catalysts where maintaining a stable configuration enhances reaction outcomes. Understanding how acetylacetone interacts with metal ions can help predict its behavior in different chemical environments.

Diethylenetriamine

Diethylenetriamine, often abbreviated as dien, is a versatile chelating ligand popular in industrial applications. Unlike acetylacetone, diethylenetriamine is a tridentate ligand. This means it can form three bonds with a metal ion. The main contributors to this bonding ability are the three nitrogen atoms in its molecular structure.

Each nitrogen atom contains a lone pair of electrons, ready to be shared with a metal ion, creating a stable metal-ligand complex. Such stability is beneficial in industrial processes, where robust metal chelation is often required to improve the efficiency and selectivity of chemical reactions. Diagramming the alignment of diethylenetriamine with a metal ion can enhance understanding of its three-point attachment mechanism.

Salen

Salen is a complex chelating ligand characterized by both nitrogen and oxygen donor atoms. This ligand is highly valued, especially in asymmetric synthesis and chiral catalysis. Salen operates as a tetradentate ligand, capable of forming four bonds with a metal ion.

The structure of salen includes two nitrogen atoms within its amine groups and two oxygen atoms from its phenolic components. Each of these donor atoms holds a lone pair of electrons, making them ready to form durable coordination bonds with metal ions. Salen's unique structure allows it to create stable and rigid complexes, which are advantageous in processes requiring high precision and specific chirality.

Porphine

Porphine is a macrocyclic ligand notable for its role in biomolecular chemistry. It forms the basic structure for porphyrins, which are crucial in many biological processes, such as oxygen transport and photosynthesis. Porphine is a tetradentate ligand with four nitrogen donor atoms set in its core.

Each nitrogen atom has a lone pair of electrons capable of coordinating with a metal ion. This property endows porphine with the ability to form four strong bonds, creating a stable ring-like metal complex. The natural arrangement and electron donation capability of porphine make it an excellent ligand for constructing metalloproteins and exploring catalytic mechanisms in nature and synthetic chemistry.