Problem 29 - Chemistry The Central Science

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) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}(o\) -phen \()\right] \mathrm{Cl}_{3}\) (b) \(\left[\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right] \mathrm{Br}\) (c) \(\left[\mathrm{Cr}(\mathrm{EDTA})\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{-}\) (d) \(\left[\mathrm{Zn}(\mathrm{en})_{2}\right]\left(\mathrm{ClO}_{4}\right)_{2}\)

Step-by-Step Solution

Answer

In the given complexes: (a) The polydentate ligand is o-phenanthroline (o-phen), which is a bidentate ligand occupying two coordination positions. (b) The polydentate ligand is oxalate ion (\(\mathrm{C}_{2}\mathrm{O}_{4}\)), which is a bidentate ligand occupying two coordination positions. (c) The polydentate ligand is ethylenediaminetetraacetic acid (EDTA), which is a hexadentate ligand occupying six coordination positions. (d) The polydentate ligand is ethylenediamine (en), which is a bidentate ligand occupying two coordination positions on the metal ion.

Step 1: (a) Identify the polydentate ligand and its probable coordination positions

In the complex \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}(o\) -phen \()\right] \mathrm{Cl}_{3}\), the polydentate ligand is "o-phen" which stands for o-phenanthroline. o-phenanthroline can act as a bidentate ligand, meaning it occupies two coordination positions on the metal ion. The nitrogen atoms in the heterocyclic aromatic ring system are the donor atoms that bind to the metal ion.

Step 2: (b) Identify the polydentate ligand and its probable coordination positions

In the complex \(\left[\mathrm{Cr}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}\right] \mathrm{Br}\), the polydentate ligand is \(\mathrm{C}_{2}\mathrm{O}_{4}\) which is the oxalate ion. Oxalate ion can act as a bidentate ligand, as it has two oxygen atoms with lone pairs that can coordinate to the metal ion. The oxalate ion occupies two coordination positions on the metal ion.

Step 3: (c) Identify the polydentate ligand and its probable coordination positions

In the complex \(\left[\mathrm{Cr}(\mathrm{EDTA})\left(\mathrm{H}_{2}\mathrm{O}\right)\right]^{-}\), the polydentate ligand is "EDTA" which stands for ethylenediaminetetraacetic acid. EDTA is a hexadentate ligand, meaning it occupies six coordination positions on the metal ion. The donor atoms in EDTA are the two nitrogen atoms and four oxygen atoms (attached to the carboxylate groups).

Step 4: (d) Identify the polydentate ligand and its probable coordination positions

In the complex \(\left[\mathrm{Zn}(\mathrm{en})_{2}\right]\left(\mathrm{ClO}_{4}\right)_{2}\), the polydentate ligand is "en" which stands for ethylenediamine. Ethylenediamine is a bidentate ligand, as it has two nitrogen atoms with lone pairs that can coordinate to the metal ion. The ethylenediamine ligand occupies two coordination positions on the metal ion. There are two en ligands in this complex, each taking two coordination positions, for a total of four.

Key Concepts

Polydentate Ligands

Polydentate ligands are fascinating components in coordination chemistry. These ligands possess multiple donor atoms, allowing them to bind to a metal atom at several sites. This creates complex structures known as chelates.

Polydentate ligands can significantly increase the stability of metal complexes due to the chelate effect. This is because forming rings within the complex makes it less likely for the metal to dissociate from the ligand.

• Monodentate ligands bind through a single atom, such as the chloride ion (Cl dsup-1).
• Bidentate ligands coordinate through two atoms, providing additional stability.
• Tridentate, tetradentate, and hexadentate ligands bind through three, four, or six atoms respectively, creating more robust complexes.

Understanding the nature of polydentate ligands is crucial in various applications, including catalysis, materials science, and medicine.

Coordination Number

The coordination number is a critical concept in coordination chemistry, describing how many ligand atoms are directly bonded to the central metal atom. This number can vary widely depending on the metal and ligands involved, typically ranging from 2 to 9.

A higher coordination number often means a more stable and complex structure. Here’s a basic outline of coordination:

• 2-4: Linear or square planar geometries, often found in simple metal ions.
• 6: Octahedral geometry, common for transition metals.
• 8-9: More complex geometries, usually seen in larger metal ions.

The coordination number affects the geometry and properties of the metal complex, influencing its reactivity and biological activity.

Metal Complexes

Metal complexes are structured entities formed when metal ions bind with ligands. These complexes play an essential role in many biological and synthetic processes.

In a metal complex, the central metal atom or ion serves as the focus, surrounded by ligands that supply electron pairs for binding. This results in diverse geometrical arrangements, depending on the coordination number and the nature of the ligands involved.

• Chemical Stability: Metal complexes' stability depends on the type of ligands and metal used. Polydentate ligands generally form more stable complexes due to the chelate effect.
• Biological Importance: Metal complexes are present in various biological systems, such as hemoglobin, where an iron complex is crucial for oxygen transport.
• Catalytic Applications: They are essential in catalysis for industrial processes, helping convert raw materials into valuable products efficiently.

Understanding metal complexes is vital for grasping how they impact both industrial applications and natural biological processes.

Bidentate Ligands

Bidentate ligands are a specific type of polydentate ligand capable of binding to a central metal atom via two distinct sites. This dual binding capability often leads to more stable complexes due to the formation of a chelate ring.

Ethylenediamine ("en") is a classic example of a bidentate ligand, as seen in the zinc complex ( [Zn(en) ₂ ] (ClO ₄ ) ₂ ). Here are some features of bidentate ligands:

• Coordination Versatility: Bidentate ligands can form more stable compounds compared to monodentate counterparts due to their ability to occupy two coordination sites.
• Application Range: Commonly used in stabilizing metal ions for analytical and therapeutic purposes.
• Structural Importance: Create rings with the metal center, enhancing complex robusticity and thermal stability.

By using bidentate ligands, chemists can tailor metal complexes for specific applications, enhancing stability and functionality.