Question

Determine if each of the following complexes exhibits geometric isomerism. If geometric isomers exist, determine how many there are. (a) [ Rh(bipy) \((o-\) phen \()_{2} ]^{3+},\) \((\mathbf{b})\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}(\mathrm{bipy}) \mathrm{Br}\right]^{2+},(\mathbf{c})\) square-planar \(\left[\mathrm{Pd}(\mathrm{en})(\mathrm{CN})_{2}\right].\)

Short answer

The short answer based on the step-by-step solution is as follows: a) [Rh(bipy)(o-phen)$_{2}$]$^{3+}$ does not exhibit geometric isomerism as there is only one possible arrangement of ligands since bipy and o-phen ligands are identical. b) [Co(NH$_{3}$)$_{3}$(bipy)Br]$^{2+}$ exhibits geometric isomerism and has two geometric isomers, cis and trans. c) [Pd(en)(CN)$_{2}$] does not exhibit geometric isomerism as there is only one possible arrangement that can be made for the ligands around the Pd ion in square-planar geometry.

Step-by-step solution

a) [Rh(bipy)(o-phen)2]3+

: First, let's analyze the structure of the given complex. Rhodium, the central metal ion, is coordinated by one bipy ligand (bipy = bidentate ligand 2,2'-Bipyridine) and two o-phen ligands (o-phen = o-Phenanthroline, also a bidentate ligand). The structure can be represented by octahedral geometry. Now, let's find if there are any geometrical isomers possible for this complex. The isomers would differ in the arrangement of the ligands around the central metal. In this complex, there is only one possible arrangement of ligands since the bipy and o-phen ligands are identical. Thus, the given complex does not exhibit geometric isomerism.

b) [Co(NH3)3(bipy)Br]2+

: In this complex, Co is the central metal ion, which is coordinated by three NH3 ligands (monodentates), one bipy ligand (bidentate), and one Br ligand (monodentate). The structure can be represented by octahedral geometry as well. To find if there are any geometrical isomers possible for this complex, we need to check for different arrangements of the NH3, bipy, and Br ligands. Two geometrical isomers are possible for this complex: 1. Cis isomer: The Br ligand and one nitrogen atom of the bipy ligand are adjacent to each other in the structure. 2. Trans isomer: The Br ligand and one nitrogen atom of the bipy ligand are opposite to each other in the structure. Hence, the given complex exhibits geometric isomerism, and there are two geometric isomers possible in this case.

c) [Pd(en)(CN)2]

: In the last given complex, Pd is the central metal ion, which is coordinated by one en ligand (en = ethylenediamine, a bidentate ligand) and two CN ligands (monodentate). The structure can be represented by square-planar geometry. To check for geometrical isomers, we need to analyze the possible arrangements of the en and CN ligands. In this case, there is only one possible arrangement that can be made for the ligands around the Pd ion, since the en ligands occupy opposite positions in the square planar geometry, and the CN ligands also occupy opposite positions. Thus, there are no geometrical isomers for the given complex, and it does not exhibit geometric isomerism.

Key concepts

Coordination Compounds

When dealing with coordination compounds, it's essential to understand they are made up of a central metal atom or ion surrounded by molecules or ions known as ligands. These ligands can be either neutral molecules like water (\(\text{H}_2\text{O}\)) or ions such as chloride (\(\text{Cl}^-\)). Coordination compounds play a significant role in various fields such as chemistry, biology, and industry.

The central metal atom can form multiple bonds with the ligands, creating what is known as a coordination sphere. The number of these coordinate bonds is called the coordination number, which varies depending on the metal and the ligands involved. Understanding these basics is essential for exploring geometric isomerism in coordination compounds.

Octahedral Geometry

Octahedral geometry is a common spatial arrangement in coordination chemistry, where six ligands are symmetrically arranged around a central metal ion. This symmetrical structure resembles an octahedron, a shape with eight faces.

A classic example of octahedral geometry would be a complex like \([\text{Co(NH}_3)_6]^{3+}\). In this scenario, all ligands have positions that are equivalent to each other. However, when the ligands are not all the same, as in \([\text{Co(NH}_3)_3(\text{bipy})\text{Br}]^{2+}\), different spatial arrangements or isomers can arise, such as cis and trans isomers. The octahedral arrangement can lead to potential geometric isomers, which differ by the arrangement of ligands in space rather than their connectivity.

Square-Planar Geometry

Square-planar geometry is another type of arrangement, typically found in metal complexes where four ligands are equidistant in a square around the central metal ion. This planarity is typical in complexes of d\(^{8}\) metal ions, like Pd(II) and Pt(II).

For instance, in a square-planar complex such as \([\text{Pd}(\text{en})(\text{CN})_2]\), the ligands are coordinated in a flat square around the metal ion. While these types of complexes can exhibit geometric isomerism, it depends on the composition. If the ligands are different, isomers can occur. In the case of \([\text{Pd}(\text{en})(\text{CN})_2]\), the symmetry and ligand arrangement means there’s only one possible structure, thus no geometric isomers exist for this compound.

Bidentate Ligand

Bidentate ligands are special because they bind to the central metal at two points, providing increased stability to the coordination complex. These ligands have two separate donor atoms that attach to the metal ion. A well-known example of a bidentate ligand is ethylenediamine (en), which can form two bonds with a metal center.

Bidentate ligands like 2,2'-bipyridine (bipy) and phenanthroline can influence the geometry and possible isomers of a complex. They often lead to more rigid structures because both ends link to the central ion, reducing the number of possible isomers compared to unidentate ligands. In our example, the presence of bidentate ligands in \([\text{Rh(bipy)(o-phen)}_2]^{3+}\) means there’s only one stable arrangement, precluding geometric isomerism.

Cis-Trans Isomerism

Cis-trans isomerism is particularly important in coordination chemistry. It involves complexes where identical ligands can either be adjacent to each other (cis) or opposite each other (trans). This kind of isomerism can significantly affect the chemical and physical properties of the compounds.

A classic example is the cis and trans forms of \([\text{Co(NH}_3)_3(\text{bipy})\text{Br}]^{2+}\). In the cis form, the Br and part of bipy are next to each other, while in the trans form, they are across from one another. This spatial difference defines cis and trans isomers and can lead to unique reactivity and different colors or solubilities.

Identifying and understanding cis-trans isomerism requires analyzing the spatial arrangement within the complexes and recognizing how such variations affect the compound's function and application.