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: 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

a. Acetylacetone (acacH) can form 2 bonds with a metal ion. b. Diethylenetriamine can form 3 bonds with a metal ion. c. Salen can form 4 bonds with a metal ion. d. Porphine can form 4 bonds with a metal ion.

Step-by-step solution

a. Acetylacetone (acacH)

Acetylacetone has two oxygen atoms with lone pairs, which can donate electron density to the metal ion. The two oxygen atoms are connected by a carbon chain, forming a 5-membered ring with the metal ion when they bind to it. Thus, acetylacetone can form 2 bonds with a metal ion.

b. Diethylenetriamine

Diethylenetriamine has three nitrogen atoms with lone pairs that can donate electron density to the metal ion. The nitrogen atoms are connected by ethylene (-CH2-CH2-) spacers. This ligand can coordinate with the metal ion through each of the three nitrogen atoms, forming a total of 3 bonds with a metal ion.

c. Salen

Salen is a dianionic ligand with four potential coordination sites, two on each of the phenolic oxygen atoms and two on the imine nitrogen atoms. When the salen ligand binds to a metal ion, it often forms a planar, four-coordinate complex. Therefore, salen can form 4 bonds with a metal ion.

d. Porphine

Porphine is a tetrapyrrolic macrocycle with four pyrrole nitrogen atoms that can coordinate to a central metal ion. The structure of porphine creates a square-planar or square-pyramidal geometry with the metal ion in the center. The macrocycle acts as a tetradentate ligand, with each of the four nitrogen atoms donating electron density to the metal ion. As a result, porphine can form 4 bonds with a metal ion.

Key concepts

Organometallic Catalysts

Organometallic catalysts are a fascinating area of chemistry where organic compounds and metallic elements work together. This field combines the power of organic molecules with the catalytic abilities of metals to facilitate chemical reactions. Overall, the main goal is to speed up reactions and make them more efficient. These catalysts are used widely in both industrial and laboratory settings due to their effectiveness.
One crucial component of organometallic catalysts is the ligand, like acetylacetone, which directly influences their functioning. Ligands can stabilize the metal and modify its electronic properties, which is essential for the catalyst's activity. For example:

• Ligands can create specific steric environments, shaping how the catalyst approaches reactants.
• They can adjust the electron density of the metal center, affecting how it participates in reactions.

Understanding these interactions helps chemists design better catalysts that are more selective and efficient.

Coordination Chemistry

Coordination chemistry deals with the study of molecules that form complexes with metal ions. It involves understanding how these metals interact with surrounding molecules, called ligands, which donate electron pairs to the metal. This bonding forms complex structures with unique properties.
A central aspect of coordination chemistry is the coordination number, representing the number of bonds a metal can form. This is influenced by the electron availability of the ligands and the metal's capacity to accommodate them. For example, porphine interacts as a tetradentate ligand by providing four nitrogen atoms to bond with a metal ion, optimizing coordination.
Examples of important applications include:

• Medicine: Metal complexes play essential roles in drugs.
• Catalysis: Used to accelerate chemical reactions.
• Biological systems: Metalloenzymes are critical for life processes.

These diverse examples highlight the significance of coordination chemistry in various fields.

Ligand Coordination

Ligand coordination is central to forming stable and functional metal complexes. It involves the rules and interactions that govern how ligands attach to a central metal ion. Ligands donate pairs of electrons to the metal, creating coordinate covalent bonds.
Different ligands have various affinities for metals, impacting the geometry of the resulting complex. In this way, diethylenetriamine, for example, can form bonds with a metal through each of its nitrogen atoms. It's essential to understand:

• The type of bond: Coordinate bonds are pivotal in metal-ligand stability.
• Geometric arrangement: The shape of a complex influences its chemical and physical properties.
• Electron donation: Quantity and location of electron pairs matter for effective coordination.

Effective ligand coordination is crucial for achieving the desired chemical characteristics and reactivity of metal complexes.

Poly dentate Ligands

Poly dentate ligands are powerful tools in coordination chemistry. A polydentate ligand has multiple electron-donating sites, allowing it to bind a metal ion at more than one point, forming stable complexes. This type of ligand can wrap around a metal ion, strongly securing it in place.
Salen, for example, acts as a tetradentate ligand by donating four coordinating pairs of electrons from its oxygen and nitrogen atoms. These multi-attachment abilities of polydentate ligands have several advantages:

• Stability: Stronger and more stable complexes compared to monodentate ligands.
• Chelation: Leads to the formation of ring-like structures, which enhance stability.
• Selective metal binding: Facilitates selective formation of specific metal complexes, useful in various applications.

Understanding how polydentate ligands function and their benefits helps in designing more efficient and selective metal-based complexes.