Question

By writing formulas or drawing structures related to any one of these three complexes, $$ \begin{aligned} &{\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2}\right] \mathrm{Cl}} \\ &{\left[\mathrm{Pd}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{ONO})_{2}\right]} \\\ &\mathrm{cis}-\left[\mathrm{V}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+} \end{aligned} $$ illustrate (a) geometric isomerism, (b) linkage isomerism, (c) optical isomerism, (d) coordination-sphere isomerism.

Short answer

To summarize, the three complexes illustrate the following types of isomerism: 1. \(\require{chemformula}\left[ \mathrm{Co{(\chemformula{NH3})_{4} (\chemformula{Br})_{2}}] \, (\chemformula{Cl}) \right]\): Geometric isomerism (cis and trans isomers) 2. \(\require{chemformula}\left[ \mathrm{Pd{(\chemformula{NH3})_{2} (\chemformula{ONO})_{2}}} \right]\): Linkage isomerism (ONO ligand bonding through N or O) 3. \(\require{chemformula}\mathrm{cis\, [V{(\chemformula{en})_{2} (\chemformula{Cl})_{2}}} \, (\chemformula{+})\): Geometric isomerism (cis and trans isomers) and optical isomerism (L and D enantiomers)

Step-by-step solution

First Complex: [Co(NH3)4Br2]Cl

This complex has a cobalt ion with four NH3 ligands and two Br ligands, surrounded by a chloride ion. Its structure can be illustrated as follows: \[ \require{chemformula} \[ \mathrm{\, [Co{(\chemformula{NH3})_{4} (\chemformula{Br})_{2}}] \, (\chemformula{Cl}) \] \] Now, let's identify the isomerism present in this complex: (a) Geometric Isomerism: In this complex, geometric isomerism is observed due to the arrangement of NH3 and Br ligands surrounding the central Co atom. There are two possible geometric isomers: a cis-isomer and a trans-isomer. In the cis arrangement, both Br ligands are adjacent to each other, while in the trans arrangement, the Br ligands and NH3 ligands are opposite from each other. (b) Linkage Isomerism: No linkage isomerism is observed in this complex, as the ligands NH3 and Br only have one possible site to bond with the central Co atom. (c) Optical Isomerism: No optical isomerism is observed in this complex, as it doesn't have a chiral center, and it is not possible to form a non-superimposable mirror image of the complex due to the symmetry of the ligands around the central Co atom. (d) Coordination-Sphere Isomerism: No coordination-sphere isomerism is observed in this complex, as there is no ligand that can be replaced by the counter ion without changing the compound's identity.

Second Complex: [Pd(NH3)2(ONO)2]

This complex has a palladium ion with two NH3 ligands and two ONO ligands. Its structure can be illustrated as follows: \[ \require{chemformula} \[ \mathrm{\, [Pd{(\chemformula{NH3})_{2} (\chemformula{ONO})_{2}}} \] \] Now, let's identify the isomerism present in this complex: (a) Geometric Isomerism: No geometric isomerism is observed in this complex, as there is only one possible arrangement of the NH3 ligands and ONO ligands around the central Pd atom. (b) Linkage Isomerism: This complex exhibits linkage isomerism due to the ONO ligands. The ONO ligand can bond to the Pd ion through either the nitrogen (N) or the oxygen (O) atom. Thus, there are two different linkage isomers for this complex: one with the ligand bonding through N, and one with the ligand bonding through O. (c) Optical Isomerism: No optical isomerism is observed in this complex, as there is no chiral center and no possibility to form a non-superimposable mirror image of the complex. (d) Coordination-Sphere Isomerism: No coordination-sphere isomerism is observed in this complex.

Third Complex: cis-[V(en)2Cl2]+

This complex has a vanadium ion with two ethylenediamine (en) ligands and two Cl ligands. Its cis-configuration means that the two Cl ligands are adjacent to each other. \[ \require{chemformula} \[ \mathrm{\, cis\, [V{(\chemformula{en})_{2} (\chemformula{Cl})_{2}}} \, (\chemformula{+}) \] \] Now, let's identify the isomerism present in this complex: (a) Geometric Isomerism: This complex exhibits geometric isomerism, as there are two possible arrangements for the ethylenediamine ligands and Cl ligands around the central V atom: cis and trans isomers. We are given the cis-isomer. (b) Linkage Isomerism: No linkage isomerism is observed in this complex, as the ethylenediamine ligands and Cl ligands can only bond through one site to the central V atom. (c) Optical Isomerism: The cis-[V(en)2Cl2]+ complex does exhibit optical isomerism. Due to the geometry of the complex, it has a chiral center, and thus there are both left-handed (L) and right-handed (D) enantiomers that are non-superimposable mirror images. (d) Coordination-Sphere Isomerism: No coordination-sphere isomerism is observed in this complex.

Key concepts

Geometric Isomerism

Geometric isomerism arises when there are different spatial arrangements of ligands around a central metal atom. In the coordination complex [Co(NH₃)₄Br₂]Cl, cobalt is surrounded by six ligands - four ammonia (NH₃) and two bromide (Br) ions. This setup allows for geometric isomerism because the two Br ions can either be adjacent (cis isomer) or opposite each other (trans isomer).

• **Cis Isomer**: The Br ligands are next to each other in the molecular structure. This arrangement impacts the properties and reactivity of the complex.
• **Trans Isomer**: The Br ligands are positioned across from each other. This configuration often results in different properties compared to the cis isomer.

This type of isomerism is crucial in determining the physical and chemical properties of coordination complexes such as stability and color.

Linkage Isomerism

Linkage isomerism occurs when a ligand can coordinate to the metal in more than one way. It is seen in complexes such as [Pd(NH₃)₂(ONO)₂]. The ligand involved here is the ONO (nitrito) group that can attach to the central palladium metal ion through different atoms:

• **Through Nitrogen (N)**: The ONO group can bind to the palladium using the nitrogen atom. This form is called nitro.
• **Through Oxygen (O)**: Alternatively, the binding can occur via one of the oxygen atoms, forming nitrito.

These variations result in distinct compounds despite having the same chemical composition; thus, they represent linkage isomerism. This property is important as the differing linkage sites can lead to different reactions and usages in chemical processes.

Optical Isomerism

Optical isomerism is a fascinating phenomenon where complexes exhibit chirality. This means the complex can exist in two forms that are non-superimposable mirror images – much like left and right hands. The complex cis-[V(en)₂Cl₂]+ displays this type of isomerism.

• **Chiral Center**: The vanadium center, with ethylenediamine (en) ligands and chloride ions, orchestrates a 3D arrangement facilitating chirality.
• **Enantiomers**: These are the two mirror images, known as left-handed (L) and right-handed (D) forms. They have the same chemical properties but rotate plane-polarized light differently.

Optical isomers have significant implications in fields like pharmaceuticals, where different enantiomers can have vastly different biological activities.