Question

The complex ion \(\mathrm{Ru}(\text { phen })_{3}{ }^{2+}\) has been used as a probe for the structure of DNA. (Phen is a bidentate ligand.) a. What type of isomerism is found in \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) ? b. \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) is diamagnetic (as are all complex ions of \(\mathrm{Ru}^{2+}\) ). Draw the crystal field diagram for the \(d\) orbitals in this complex ion.

Short answer

a. The type of isomerism found in \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) is geometrical isomerism, specifically facial (fac) and meridional (mer) isomerism. b. The crystal field diagram for the d orbitals in the \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) complex ion is as follows: ─── eg ─── ↑↑ dx2-y2 ↑↑ d(z2) _________ ─── t2g ─── ↑↑ d_xy ↑↑ d_xz ↑↑ d_yz

Step-by-step solution

Understand Isomerism Types

Isomerism refers to the existence of two or more chemical species with the same molecular formula but different arrangements of atoms or groups within their respective structures. Common types of isomerism in coordination chemistry are: 1. Structural isomerism: When isomers have different connections or bonds between atoms (e.g., linkage isomerism, coordination isomerism, and ionization isomerism). 2. Stereoisomerism: When isomers have different spatial arrangements of atoms or groups (e.g., geometrical isomerism and optical isomerism).

Analyze the Complex Ion

The given complex ion has a central metal atom - Ruthenium (Ru) and three bidentate ligands - phenanthroline (phen). The metal ion \(\mathrm{Ru^{2+}}\) is surrounded by three bidentate ligands, each contributing two electron pairs, making the coordination number 6.

Determine the Type of Isomerism

Since each phenanthroline ligand has two bonding sites, the complex cannot change its connections or bonds without rearranging the geometry. Thus, no structural isomerism is observed. However, it can form stereoisomers, specifically geometrical isomers. Geometrical isomers in a complex ion with a coordination number of 6 can have two forms or geometries - octahedral and square planar. The \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) complex has an octahedral geometry due to its 3 bidentate ligands. In the case of octahedral complexes, typically those formed by tris-chelated complexes, the isomerism is called facial (fac) and meridional (mer) isomerism. a. The type of isomerism found in \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) is geometrical isomerism, specifically facial (fac) and meridional (mer) isomerism.

Draw Crystal Field Diagram for d Orbitals in the Complex

In octahedral complexes, the crystal field splits the d orbitals into two sets with different energies: three with lower energy (t2g orbitals: dxy, dxz, dyz) and two with higher energy (eg orbitals: dx2-y2, d(z2)). To draw the crystal field diagram, we should remember that \(\mathrm{Ru}^{2+}\) has a [Kr]4d6 electron configuration. b. Since the complex is diamagnetic, it means all d orbitals are fully filled, or in other words, all electrons are paired. Below is the crystal field diagram for the d orbitals in this complex ion: ─── eg ─── ↑↑ dx2-y2 ↑↑ d(z2) _________ ─── t2g ─── ↑↑ d_xy ↑↑ d_xz ↑↑ d_yz In conclusion, the given complex ion \(\mathrm{Ru}(\mathrm{phen})_{3}{ }^{2+}\) exhibits geometrical isomerism, specifically fac and mer isomers due to its octahedral geometry. The crystal field diagram shows that it is diamagnetic, with all d orbital electrons being paired.

Key concepts

Octahedral Complexes

In coordination chemistry, octahedral complexes are one of the most common geometric arrangements. This spatial configuration occurs when a metal ion is surrounded by six ligands, forming an octahedral shape. The complex \(\mathrm{Ru} (\text{phen})_3^{2+}\) is a perfect example of an octahedral complex. Here, each phenanthroline ligand, which is bidentate, coordinates with the ruthenium metal ion, creating this six-coordinate structure.
Octahedral complexes can exhibit isomerism, especially geometrical isomerism. Geometrically, these complexes may present two types of isomers: **facial (fac)** and **meridional (mer)**. In **facial isomerism**, three identical ligands form a triangular face. In **meridional isomerism**, three ligands form a plane that includes the metal center. Because of these geometrical arrangements, octahedral complexes like \(\mathrm{Ru}(\mathrm{phen})_3^{2+}\) can have distinct isomeric forms.
Octahedral complexes play an important role in diverse fields, such as bioinorganic chemistry and materials science, making their study essential for understanding complex chemical behaviors.

Crystal Field Theory

Crystal Field Theory (CFT) is an essential concept in understanding the electronic structure of transition metal complexes. It provides a model to explain how ligands affect the energies of the d orbitals of the central metal ion. When ligands approach a metal ion, they interact with its d orbitals, splitting them into different energy levels.
In **octahedral complexes**, like \(\mathrm{Ru}(\mathrm{phen})_3^{2+}\), the incoming ligands create a field that splits the five d orbitals into two groups. The **lower energy group** consists of the \(\text{t}_{2g}\) orbitals (d_xy, d_xz, d_yz), while the **higher energy group** comprises the \(\text{e}_g\) orbitals (d_{x^2-y^2}, d_{z^2}).
A crucial point in CFT is the **electron configuration** in these orbitals. In \(\mathrm{Ru}(\mathrm{phen})_3^{2+}\), which is known to be diamagnetic, all electrons in d orbitals are paired, resulting in no unpaired electrons. This specific pairing is responsible for its magnetic properties. Understanding CFT helps in predicting and explaining the color, magnetism, and reactivity of metal complexes.

Bidentate Ligands

Bidentate ligands are ligands that can form two bonds with a central metal ion. Each bidentate ligand possesses two donor atoms capable of donating a pair of electrons each to bind to the metal ion. In the case of \(\mathrm{Ru}(\text{phen})_3^{2+}\), phenanthroline is the bidentate ligand.
These ligands are important because they create more stable complexes by forming ring structures, known as "chelate rings," with the metal ion. This formation is often referred to as the "chelate effect," which increases the stability of a complex compared to those formed with monodentate ligands.
In applications such as catalysis or therapeutic treatments, bidentate ligands are favored due to the stability they impart to metal complexes. Their ability to stably occupy coordination sites affects the physicochemical properties of the complexes they form, making them especially significant in designing compounds with specific functions.