Question

How many bonds could each of the following chelating ligands form with a metal ion? a. acetylacetone (acacH), a common ligand in organometallic catalysts: 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: a. Acetylacetone (acacH) ligand can form 2 bonds with a metal ion. b. Diethylenetriamine ligand can form 3 bonds with a metal ion. c. Salen ligand can form 4 bonds with a metal ion. d. Porphine ligand can form 4 bonds with a metal ion.

Step-by-step solution

a. Acetylacetone (acacH) ligand

To determine the number of bonds that acetylacetone can form with a metal ion, we need to identify the donor atoms in the molecule. Acetylacetone is an enolized 1,3-diketone, containing an oxygen atom at each end of the structure. The oxygen atoms are the donor atoms. Since acetylacetone has 2 oxygen atoms, it can form 2 bonds with a metal ion.

b. Diethylenetriamine ligand

Let's identify the donor atoms in the chemical formula of diethylenetriamine: \[ \mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \] In this formula, we can see there are three nitrogen atoms (NH₂, NH, and NH₂). Each nitrogen atom contains a lone pair of electrons, making them donor atoms in the coordination with metal ions. Thus, diethylenetriamine can form 3 bonds with a metal ion.

c. Salen ligand

Salen is a ligand derived from salicylaldehyde and ethylenediamine. The donor atoms in a salen ligand consist of 2 phenolic oxygen atoms and 2 imine nitrogen atoms. Consequently, a salen ligand can form 4 bonds with a metal ion.

d. Porphine ligand

Porphine is a tetrapyrrole ligand consisting of 4 pyrrole rings connected by methine bridge groups. The donor atoms in porphine are the 4 nitrogen atoms, one in each pyrrole ring. Therefore, porphine can form 4 bonds with a metal ion.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of chemistry that deals with compounds comprising metal ions surrounded by various ligands. Ligands are ions or molecules that can have one or more points of attachment to a metal ion, called coordination sites. The specific arrangement of the ligands attached to the central ion is known as the coordination sphere.

Ligands contribute to the stability and reactivity of the metal centers, thus deciphering the number of bonds they can form is essential for understanding the structure and reactivity of coordination complexes. Chelating ligands, such as acetylacetone and diethylenetriamine, have multiple donor atoms that can form rings with the metal ion, enhancing the stability of the complex through the chelate effect.

Organometallic Catalysts

Organometallic catalysts are complexes consisting of organic ligands bound to metal atoms. These catalysts are pivotal in various industrial chemical processes due to their ability to accelerate reactions. For example, acetylacetone (acacH) can form bonds with metals to create organometallic catalysts used in hydrogenation and polymerization reactions.

Chiral organometallic catalysts like those derived from salen ligands play a vital role in asymmetric synthesis, providing a means to create optically active compounds. These catalysts' performance is intrinsically linked to their ligand architecture, where the number and arrangement of donor atoms can significantly impact their catalytic activity and selectivity.

Donor Atoms

Donor atoms are the locations within a ligand where the ligand donates a pair of electrons to form a bond with a metal ion. Common donor atoms include nitrogen, oxygen, and sulfur, each possessing a lone pair of electrons that can be shared with a metal. Deepening our understanding, consider diethylenetriamine, which has three nitrogen donor atoms. Each nitrogen supplies a lone pair for bonding, allowing for multiple coordination sites with a single metal ion.

This polydentate nature of chelating ligands leads to more stable coordination compounds. Recognizing donor atoms is fundamental in predicting the number of possible bonds, as well as the strength and geometry of the resulting complex.

Metal-Ligand Bonding

Metal-ligand bonding is the interaction between the central metal ion and ligands in a coordination complex. This bond formation is a result of electron pair donation from the ligand to the vacant d-orbitals of the metal ion, which can be explained by the Lewis acid-base theory, with metals acting as Lewis acids and ligands as Lewis bases.

For instance, the porphine ligand, which forms four bonds, does so due to the four nitrogen atoms of its pyrrole rings acting as donor sites. The nature of these bonds is critical to the functioning of biological systems, as seen with heme groups in hemoglobin, where metal-ligand bonding allows for the reversible binding of oxygen, thereby transporting it throughout the body.