Question

Consider the following three complexes: \(\left(\right.\) Complex 1) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{SCN}\right]^{2+}\) \(\left(\right.\) Complex 2) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]^{2+}\) \(\left(\right.\) Complex 3) \(\mathrm{CoClBr} \cdot 5 \mathrm{NH}_{3}\) Which of the three complexes can have (a) geometric isomers, (b) linkage isomers, (c) optical isomers, (d) coordination-sphere isomers?

Short answer

In summary: (a) Geometric Isomers: Complex 1 (b) Linkage Isomers: Complex 1 (c) Optical Isomers: Complex 1 and Complex 3 (d) Coordination-Sphere Isomers: Complex 3

Step-by-step solution

Geometric Isomers

Geometric isomers occur when two complexes have the same composition but differ in the arrangement of their ligands in space. For a complex to exhibit geometric isomerism, it must have an ambidentate ligand, meaning the ligand can bind through two different atoms. Let's analyze each complex to see if they can form geometric isomers: 1) Complex 1: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{SCN}\right]^{2+}\) - This complex has an ambidentate ligand (SCN^-). The ligand can bind to Co either through N (thiocyanate) or through S (isothiocyanate). Hence, Complex 1 can form geometric isomers. 2) Complex 2: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]^{2+}\) - The ligands in this complex (NH3 and Cl^-) are not ambidentate. Therefore, Complex 2 cannot have geometric isomers. 3) Complex 3: \(\mathrm{CoClBr} \cdot 5 \mathrm{NH}_{3}\) - The ligands in this complex (Cl^- and Br^-) are not ambidentate. Therefore, Complex 3 cannot have geometric isomers.

Linkage Isomers

Linkage isomers can be observed in a complex with an ambidentate ligand when the ligand alternates between the atoms it can bind with the metal, forming different structures. Let's check for linkage isomers in the given complexes: 1) Complex 1: As mentioned above, this complex has an ambidentate ligand (SCN^-). Hence, Complex 1 can have linkage isomers. 2) Complex 2 and Complex 3: Both the complexes have no ambidentate ligands. Therefore, neither Complex 2 nor Complex 3 can have linkage isomers.

Optical Isomers

Optical isomers are mirror-image isomers that are non-superimposable. They exhibit chirality, meaning they can rotate plane-polarized light in opposite directions. To have optical isomers, a complex must lack an internal plane of symmetry. Let's look at our three complexes: 1) Complex 1: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{SCN}\right]^{2+}\) - This complex lacks a plane of symmetry. Therefore, Complex 1 can have optical isomers. 2) Complex 2: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]^{2+}\) - This complex has a plane of symmetry and hence, cannot have optical isomers. 3) Complex 3: \(\mathrm{CoClBr} \cdot 5 \mathrm{NH}_{3}\) - This complex lacks a plane of symmetry. Therefore, Complex 3 can have optical isomers.

Coordination-Sphere Isomers

Coordination-sphere isomers occur when two complexes have the same composition but differ in the configuration between the central metal atom and its ligands. These isomers arise when a compound contains both coordination and non-coordination groups. Let's check for coordination-sphere isomers in the given complexes: 1) Complex 1: This complex has no non-coordination groups. Hence, Complex 1 cannot have coordination-sphere isomers. 2) Complex 2: This complex has no non-coordination groups. Hence, Complex 2 cannot have coordination-sphere isomers. 3) Complex 3: \(\mathrm{CoClBr} \cdot 5 \mathrm{NH}_{3}\) - This complex can have coordination-sphere isomers because it contains a coordination group (CoClBr) and a non-coordination group (5NH3). In summary: (a) Geometric Isomers: Complex 1 (b) Linkage Isomers: Complex 1 (c) Optical Isomers: Complex 1 and Complex 3 (d) Coordination-Sphere Isomers: Complex 3

Key concepts

Geometric Isomerism

Geometric isomerism in coordination compounds is fascinating and occurs when complexes have the same chemical formula but differ in the spatial arrangement of their ligands. This isomerism is most common in square planar and octahedral complexes. For a coordination compound to exhibit geometric isomerism, it typically needs ambidentate ligands.

• Ambidentate ligands are special because they have multiple atoms that can bind to a central metal atom.
• An example of such a ligand is the thiocyanate ion (\(\text{SCN}^-\)), which can connect to the metal through either the sulfur (\(\text{S}\)) or the nitrogen (\(\text{N}\)).

In our given situation, Complex 1 (\(\left[\text{Co}\left(\text{NH}_3\right)_5 \text{SCN}\right]^{2+}\)) can have geometric isomers due to its ambidentate ligand (\(\text{SCN}^-\)), which allows different binding orientations. As the arrangement of ligands leads to distinct compounds, these variations are not just theoretical; they can have different physical and chemical properties.

Linkage Isomerism

Linkage isomerism is a unique feature of coordination compounds that contains ambidentate ligands, where the ligand can attach through different atoms. This results in structurally distinct complexes that share the same molecular formula but differ in connectivity.

• A quintessential example is SCN\(^-\), where the ligand can form a coordination bond either through sulfur (\(\text{S}\)) or through nitrogen (\(\text{N}\)).
• This leads to different isomers known as thiocyanate and isothiocyanate complexes.

In the complexes provided, only Complex 1 (\(\left[\text{Co}\left(\text{NH}_3\right)_5 \text{SCN}\right]^{2+}\)) can exhibit linkage isomerism. It’s the versatility of the SCN\(^-\) ligand which permits such structural variations, impacting the properties of these compounds. Understanding linkage isomerism aids in predicting and manipulating the reactivity and stability of coordination compounds.

Optical Isomerism

Optical isomerism is present in coordination compounds that have non-superimposable mirror images, known as enantiomers, much like left and right hands. These isomers are chiral and can rotate plane-polarized light in different directions, either dextrorotatory or levorotatory.

• For optical isomerism to exist, the complex must lack an internal plane of symmetry.
• Typically, tetrahedral or octahedral complexes exhibit such isomerism under specific conditions.

In our analysis, Complex 1 (\(\left[\text{Co}\left(\text{NH}_3\right)_5 \text{SCN}\right]^{2+}\)) and Complex 3 (\(\text{CoClBr} \cdot 5 \text{NH}_3\)) both lack a plane of symmetry and thus can exhibit optical isomerism. This property is significant in the study of biological systems where enzyme activity and drug interactions are influenced by chirality, highlighting the importance of this phenomenon in coordination chemistry.

Coordination-Sphere Isomerism

Coordination-sphere isomerism, also known as solvate isomerism, appears when complexes have the same formula but vary in the groups within and outside their coordination sphere. This kind of isomerism usually arises from the exchange of ligands between the coordination sphere and the ionic sphere or other molecules in the crystal lattice.

• Coordination sphere refers to the central metal ion and its directly attached ligands.
• These isomers can arise from different arrangements of coordinated and uncoordinated ions or molecules.

In the given complexes, Complex 3 (\(\text{CoClBr} \cdot 5 \text{NH}_3\)) can form such isomers because it has distinct coordination and non-coordination groups, allowing for the rearrangement of these components. This type of isomerism plays a crucial role in the synthesis and stability of compounds, reflecting how minor changes in structure can lead to differing properties.