Problem 29 - Chemistry The Central Science In Si Units

Question

Polydentate ligands can vary in the number of coordination positions they occupy. In each of the following, identify the polydentate ligand present and indicate the probable number of coordination positions it occupies: (a) \(\mathrm{Cr}(\mathrm{EDTA})^{-}\) (b) \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Br}_{2}\) (c) \(\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{3}\) (d) \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{o}\) -phen \()(\mathrm{CN})_{4}\right]\)

Step-by-Step Solution

Answer

(a) EDTA - 6 positions, (b) Oxalate - 2 positions, (c) en - 2 positions, (d) o-phen - 2 positions.

Step 1: Identify Polydentate Ligands for Each Compound

Analyze each compound to identify the polydentate ligand present. Each compound has specific ligands that can coordinate to metal ions through multiple sites.

Step 2: Determine Coordination Positions for (a) Cr(EDTA)^-

EDTA is a well-known hexadentate ligand, which means it can occupy six coordination positions. Hence, in \( \mathrm{Cr}( ext{EDTA})^{-} \), EDTA occupies six positions.

Step 3: Determine Coordination Positions for (b) [Ni(C2O4)(H2O)2]Br2

Oxalate \( (\mathrm{C}_2\mathrm{O}_4)^{2-} \) acts as a bidentate ligand, occupying two coordination positions. \( (\mathrm{H}_2\mathrm{O}) \) is monodentate, so each occupies one position. Thus, oxalate occupies two positions.

Step 4: Determine Coordination Positions for (c) [Ru(en)(NH3)4]Cl3

Ethylenediamine (en) is a bidentate ligand, which occupies two coordination positions. Therefore, in \( [\mathrm{Ru}(\mathrm{en})(\mathrm{NH}_3)_4]^{3+} \), the ethylenediamine occupies two positions.

Step 5: Determine Coordination Positions for (d) K2[Fe(o-phen)(CN)4]

o-phen (1,10-phenanthroline) is a bidentate ligand, and it occupies two coordination positions. \( [\mathrm{Fe}(\mathrm{o}\text{-phen})(\mathrm{CN})_4]^{2-} \) has o-phen occupying two positions.

Key Concepts

Coordination Chemistry

Coordination chemistry is the study of compounds formed between metal ions and ligands. Ligands are molecules or ions that can donate pairs of electrons to a metal ion, creating a coordination complex. A key concept within this field is the coordinate covalent bond, which forms when the ligand donates electron pairs to the metal center.
The metal ions generally act as Lewis acids by accepting these electron pairs, while the ligands act as Lewis bases. Depending on how many coordination sites a ligand can occupy on a metal, they are classified as monodentate, bidentate, or polydentate. This concept is essential because it affects the geometry and stability of coordination complexes.

• Coordination complex: A structure consisting of a central metal ion bonded to surrounding ligands.
• Coordinate covalent bond: A bond formed when a ligand donates both electrons involved in the bond.
• Stability: Influenced by the type and number of ligands, as well as the metal ion involved.

This understanding helps in predicting the behavior and reactivity of complexes in various chemical environments.

Ligand Coordination

Ligand coordination refers to the manner in which ligands attach to metal ions within a coordination complex. Each ligand can occupy one or more binding sites on the metal, depending on the number of donor atoms it contains. These donor atoms are critical as they form coordinate bonds with the metal ion.
Ligands are categorized based on their denticity, which describes the number of donor sites a ligand uses to bond to the metal ion. Polydentate ligands, such as hexadentate or bidentate, play a significant role in forming stable coordination complexes.

• Monodentate: Ligands that bind through a single donor atom, like chloride and water.
• Bidentate: Ligands that attach through two donor atoms, increasing the stability of the complex.
• Hexadentate: Rare ligands like EDTA that engage six sites, enveloping the metal ion and greatly enhancing complex stability.

Understanding ligand coordination is crucial in applications ranging from catalysis to biomedical fields, where complex stability and formation are pivotal.

Hexadentate Ligands

Hexadentate ligands, such as ethylenediaminetetraacetic acid (EDTA), are capable of forming six bonds with a metal ion. This high level of coordination results in extremely stable complexes. EDTA, a common hexadentate ligand, utilizes four carboxylate groups and two amine groups to coordinate to the central metal.
Hexadentate ligands are commonly used in chelation therapy to remove metal ions from the body. Their ability to wrap around a metal ion makes them excellent at sequestering metals, as they form a stable and less toxic complex that can be excreted more easily.

• Sequestration: Hexadentate ligands envelop the metal ion, making it easy to remove.
• Applications: Used in industrial cleaning agents and medical procedures to bind metal ions tightly.
• Examples: EDTA is widely used in both laboratory and industrial applications due to its efficiency in binding metals.

By understanding the power of hexadentate ligands, one can appreciate their importance in both chemical and biological systems.

Bidentate Ligands

Bidentate ligands are an important class of ligands in coordination chemistry, characterized by their ability to form two bonds with a single metal ion. These ligands enhance the stability of coordination complexes through the chelate effect—a phenomenon where having multiple bonds between the metal and ligand increases complex stability.
One common bidentate ligand is ethylenediamine (en), which has two nitrogen atoms that can each donate an electron pair to the metal. Another example is oxalate, which uses its oxygen atoms to bind with the metal ion.

• Stability: Bidentate ligands form a ring structure with the metal, contributing to the chelate effect which stabilizes the complex.
• Flexibility: Allows for diverse geometries in metal complexes, offering insights into reactivity and configuration.
• Common examples: Ethylenediamine and oxalate, known for forming double bonds with a metal center.

The role of bidentate ligands extends from enhancing complex stability to facilitating biological processes, making them pivotal in both chemical and biological settings.