Problem 54 - Chemistry The Molecular Science

Question

Identify the coordination number of the metal ion in these coordination complexes. (a) \(\left[\mathrm{FeCl}_{4}\right]^{-}\) (b) \(\left[\mathrm{PtBr}_{4}\right]^{2-}\) (c) \(\left[\mathrm{Mn}(\mathrm{en})_{3}\right]^{2+}\) (d) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{H}_{2} \mathrm{O}\right]^{3+}\)

Step-by-Step Solution

Answer

(a) 4, (b) 4, (c) 6, (d) 6

Step 1: Understand Coordination Number

The coordination number is the total number of ligand atoms that are bonded to the metal ion in a coordination complex. Ligands can be monodentate (bonding through a single atom), bidentate (bonding through two atoms), or polydentate (bonding through more than two atoms).

Step 2: Analyze Complex (a)

In the complex \(\left[\mathrm{FeCl}_{4}\right]^{-}\), there are four chloride (Cl\(^-\)) ligands. Each Cl\(^-\) is a monodentate ligand, so it bonds to the iron (Fe) through one site. This gives a coordination number of 4.

Step 3: Analyze Complex (b)

In the complex \(\left[\mathrm{PtBr}_{4}\right]^{2-}\), there are four bromide (Br\(^-\)) ligands. Each Br\(^-\) is a monodentate ligand, bonding to the platinum (Pt) through one site each. Therefore, the coordination number is 4.

Step 4: Analyze Complex (c)

In the complex \(\left[\mathrm{Mn}(\mathrm{en})_{3}\right]^{2+}\), \(\mathrm{en}\) is ethylenediamine, a bidentate ligand that bonds through two sites. With three \(\mathrm{en}\) ligands, the coordination number is \(3 \times 2 = 6\).

Step 5: Analyze Complex (d)

In the complex \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{H}_{2}\mathrm{O}\right]^{3+}\), there are five \(\mathrm{NH}_{3}\) ligands and one \(\mathrm{H}_{2}\mathrm{O}\) ligand. All are monodentate ligands, bonding to chromium through one site each, giving a coordination number of \(5 + 1 = 6\).

Key Concepts

Coordination Number

In coordination chemistry, the coordination number is a fundamental concept. It refers to the number of ligand atoms that are directly bonded to a central metal ion within a coordination complex.

These ligands can vary widely in how they attach. Each ligand, whether monodentate or bidentate, contributes to the coordination number.

• Monodentate ligands attach through a single atom, contributing one to the coordination number.
• Bidentate ligands attach through two atoms, donating two to the coordination number.
• Polydentate ligands, which are referenced but uncommon in simpler problems, attach through multiple atoms.

Understanding the coordination number is crucial for predicting the structure and properties of coordination complexes.

Monodentate Ligands

Monodentate ligands are the simplest type of ligands in coordination chemistry. They bond to a central metal ion through a single atom.

This single point of attachment makes them straightforward to work with when determining coordination numbers. Common examples include chloride (Cl"), bromide (Br"), and ammonia ( H3).

• In the complex FeCl4]_, each chloride ion (Cl") acts as a monodentate ligand, contributing one point of attachment.
• Similarly, in PtBr4]2-, each bromide ion (Br") functions as a monodentate ligand.

These examples illustrate how monodentate ligands contribute to simpler structures with fewer points of attachment, easily translating to low coordination numbers.

Bidentate Ligands

Bidentate ligands are unique because they connect to the central metal ion through two different atoms. This dual attachment increases stability and usually enhances the overall coordination number of a complex.

Ethylenediamine (en) is a classic example of a bidentate ligand.

• In the complex Mn(en)3])2+, each ethylenediamine ligand attaches through two donor atoms. With three ethylenediamine ligands, a total of six coordination sites are utilized.

This dual bonding is advantageous because it forms chelate rings, which provide extra stability to the coordination complex through a phenomenon known as the chelate effect.

Transition Metal Complexes

Transition metal complexes consist of a central metal ion surrounded by various ligands. These complexes are fundamental in various chemical processes and technologies.

Transition metals are known for their ability to form stable complexes with a wide range of ligands due to their variable oxidation states and special orbital arrangements, allowing them to achieve various coordination geometries.

• The complex Cr(NH3)5 H2O]3+ shows transition metals' versatility by having six monodentate ligands, resulting in an octahedral shape commonly observed in coordination chemistry.

The diversity and complexity of these transition metal complexes are central to their widespread use in industrial catalysts, biological molecules like hemoglobin, and materials science.