Question

Comment on the possibility of isomer formation for each of the following complexes (the ligand tpy is \(2,2^{\prime}: 6^{\prime}, 2^{\prime \prime}=\) terpyridine, 20.27 ): (a) \(\left[\mathrm{Ru}(\mathrm{py})_{3} \mathrm{Cl}_{3}\right]\) (b) \(\left[\mathrm{Ru}(\mathrm{bpy})_{2} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Ru}(\mathrm{tpy}) \mathrm{Cl}_{3}\right]\)

Short answer

(a) has fac and mer isomers, (b) has cis and trans isomers, (c) has no isomers.

Step-by-step solution

Analyze the Coordination Environment of (a)

The complex \([\mathrm{Ru}(\mathrm{py})_{3} \mathrm{Cl}_{3}]\) consists of three pyridine ligands and three chloride ions. The coordination sphere of the metal is 6, suggesting octahedral geometry. In octahedral complexes with partial identical ligands, isomers can form through the replacement or positionshift of these ligands. Considering facial (fac) and meridional (mer) isomers, it is possible to have both fac and mer isomers.

Determine Isomers for (b)

The complex \([\mathrm{Ru}(\mathrm{bpy})_{2} \mathrm{Cl}_{2}]^{+}\) has two bidentate 2,2'-bipyridine ligands and two chloride ions, leading to a stable octahedral structure. Two bpy ligands must be cis to each other. Therefore, this complex can exist in cis and trans geometric isomers, depending on the positions of the two chloride ligands relative to each other.

Analyze Isomer Possibilities for (c)

The complex \([\mathrm{Ru}(\mathrm{tpy}) \mathrm{Cl}_{3}]\) contains terpyridine, a tridentate ligand occupying one face of the octahedron. The remaining three positions are occupied by chloride ions. This fixed, facial binding with tpy leads to a single fixed arrangement of ligands, indicating that no isomers are possible for this complex.

Key concepts

Octahedral Geometry

Octahedral geometry is a common shape found in coordination complexes, particularly when a metal is surrounded by six ligands. This is a three-dimensional shape that resembles two pyramids stuck together at their bases. Understanding this geometry is crucial because it influences the formation of isomers. In an octahedral complex, there are 90-degree angles between adjacent ligands, which allows for various possible arrangements of these ligands. This flexibility can result in different isomers.

• It is a highly symmetrical structure.
• Allows room for up to six ligands.
• Facilitates different spatial arrangements of ligands, leading to isomer formation.

Facial and Meridional Isomers

Facial and meridional isomers are specific types of geometric isomers found in octahedral complexes. When a coordination complex has three identical ligands, these can be arranged in two distinct ways: facial (fac) and meridional (mer) isomers. This naming comes from the positions of ligands relative to each other in the octahedral geometry.
For facial isomers, the identical ligands occupy one face of the octahedral structure. This means that they are adjacent to each other, forming a triangular face of the octahedron. In contrast, meridional isomers have the ligands arranged such that two are opposite each other around the central metal, and the third is in between on a different axis.

• Facial Isomers: Ligands are aligned along one face of the octahedron.
• Meridional Isomers: Ligands are arranged in a more linear fashion, across different axes.
• The distinction affects the complex's properties and reactivity.

Bidentate Ligands

Bidentate ligands are ligands that can attach to a central metal atom or ion at two points. This feature enhances the stability of the coordination complex by forming a ring system that completes the coordination sphere of the metal. When considering isomer formation, bidentate ligands can significantly influence the outcome due to their unique binding modes.
Bidentate ligands often lead to different geometric isomers because they can bridge or "hug" the metal more effectively. In the example of ([ ext{Ru}( ext{bpy})_2 ext{Cl}_2]^+), the bidentate 2,2'-bipyridine (bpy) ligands create a rigid structure, dictating the positioning of other ligands around the metal.

• Typically result in more stable complexes.
• Influence the coordination sphere extensively due to their dual attachment.
• Can enforce the formation of specific isomers, like cis or trans, based on their binding preferences.

Geometric Isomers

Geometric isomers are types of stereoisomers where ligands have different spatial arrangements around a central atom. These isomers are common in octahedral or square planar coordination complexes, where distinct positions relative to the metal center can lead to unique structures and properties. In coordination chemistry, geometric isomers can significantly influence a compound's reactivity, color, and even its physiological effects.
In the example of ([ ext{Ru}( ext{bpy})_2 ext{Cl}_2]^+), cis-trans isomerism is observed. The two chloride ions can either be adjacent (cis) or opposite each other (trans). This results in distinct shapes that can influence how the compound interacts with its environment or other molecules.

• Leads to distinct chemical and physical properties.
• Important for applications in catalysis and drugs, where isomer type can affect effectiveness.
• Occurs due to rigid ligand structures or specific spatial arrangements.