Question

Consider the following three complexes: (Complex 1) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2}\right] \mathrm{Cl}\) \(\left(\right.\) Complex 2) \(\left[\mathrm{Pd}\left(\mathrm{NH}_{5}\right)_{2}(\mathrm{ONO})_{2}\right]\) (Complex 3) \(\left[\mathrm{V}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\). Which of the three complexes can have (a) geometric isomers, (b) linkage isomers, (c) optical isomers, (d) coordinationsphere isomers?

Short answer

In summary: (a) Geometric isomers are possible for all three complexes. (b) Linkage isomers can only occur in Complex 2. (c) Optical isomers are possible for Complex 1 (cis-isomer only) and Complex 3. (d) Coordination sphere isomers are not possible for any of the complexes.

Step-by-step solution

Complex 1: Analysis

Complex 1 is \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\mathrm{Br}_{2}\right] \mathrm{Cl}\). The coordination sphere consists of four ammine ligands (\(\mathrm{NH}_3\)) and two bromine ligands (\(\mathrm{Br}\)), with a chlorine counter-ion (\(\mathrm{Cl}\)). The coordination number is 6, suggesting an octahedral geometry.

Complex 1: Geometric Isomers

In an octahedral complex with two different types of ligands (four \(\mathrm{NH}_3\) and two \(\mathrm{Br}\)), geometric isomerism is possible. The complex can have both cis and trans isomers.

Complex 1: Linkage Isomers

There are no ligands in Complex 1 that can bind to the central metal ion through two different atoms. Therefore, linkage isomerism is not possible for this complex.

Complex 1: Optical Isomers

Since Complex 1 has a plane of symmetry in its trans isomer, it cannot have optical isomers. Only the cis isomer exhibits optical isomerism, due to the lack of a symmetry plane.

Complex 1: Coordination Sphere Isomers

Coordination sphere isomerism is not possible for Complex 1, as the ligands within the coordination sphere cannot be replaced or rearranged with any other species.

Complex 2: Analysis

Complex 2 is \(\left[\mathrm{Pd}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{ONO})_{2}\right]\). It has a square planar geometry with the central metal atom surrounded by two ammine ligands (\(\mathrm{NH}_3\)) and two nitrito ligands (ONO).

Complex 2: Geometric Isomers

Since Complex 2 has a square planar geometry with two different types of ligands, geometric isomerism (cis and trans isomers) is possible.

Complex 2: Linkage Isomers

The nitrito ligand (ONO) in Complex 2 can bind to the central metal atom either through the nitrogen atom or the oxygen atom. As a result, linkage isomerism is possible for this complex.

Complex 2: Optical Isomers

Complex 2 does not have a chiral center, and thus cannot exhibit optical isomerism.

Complex 2: Coordination Sphere Isomers

Coordination sphere isomerism is not possible for Complex 2, as the ligands within the coordination sphere cannot be replaced or rearranged with any other species.

Complex 3: Analysis

Complex 3 is \(\left[\mathrm{V}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\). It has an octahedral geometry, with each ethylenediamine ligand (en) surrounding the central metal atom and two chlorine ligands.

Complex 3: Geometric Isomers

Since Complex 3 has two different types of ligands in an octahedral geometry, geometric isomerism (cis and trans isomers) is possible.

Complex 3: Linkage Isomers

There are no ligands in Complex 3 that can bind to the central metal ion through two different atoms, so linkage isomerism is not possible.

Complex 3: Optical Isomers

Ethylenediamine ligands are bidentate and can create a chiral center in the complex. Therefore, Complex 3 can exhibit optical isomerism.

Complex 3: Coordination Sphere Isomers

Coordination sphere isomerism is not possible for Complex 3, as the species within the coordination sphere cannot be replaced or rearranged with any other components. Based on the analysis, we can conclude that: - Complex 1: Geometric isomers and Optical isomers (cis-isomer only) - Complex 2: Geometric isomers and Linkage isomers - Complex 3: Geometric isomers and Optical isomers

Key concepts

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, is common in coordination complexes. This type of isomerism arises when a compound has the same formula but different spatial arrangements of the ligands. In an octahedral complex with six coordination sites, such as Compound 1 \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\mathrm{Br}_{2}\right] \mathrm{Cl}\), geometric isomers can form when it has at least two different types of ligands.

Imagine the central metal as the center of a sphere, with the ligands positioned at the vertices of an octahedron. The ligands can be arranged such that similar ligands are adjacent (cis) or opposite (trans) to each other. Likewise, in Compound 3, \(\left[\mathrm{V}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\), the arrangement of the two chloride ligands determines the isomerism. However, for square planar complexes like Compound 2, \(\left[\mathrm{Pd}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{ONO})_{2}\right]\), geometric isomerism results from the sides or opposite positions of the different ligands on the square plane.

Linkage Isomerism

Linkage isomerism occurs when a ligand is capable of coordinating to the metal center through different atoms. This type of isomerism is illustrated by Compound 2, \(\left[\mathrm{Pd}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{ONO})_{2}\right]\), where the nitrito ligand (\(\mathrm{ONO}\)) can attach to the palladium via the nitrogen atom (nitro isomer) or an oxygen atom (nitrito isomer).

The properties of linkage isomers can differ significantly because the point of attachment to the central metal can change the overall electronic distribution within the complex. In the case of nitrito ligand, the switch between the 'nitro-' and 'nitrito-' forms can affect the complex's reactivity and other chemical properties.

Optical Isomerism

Optical isomerism in coordination complexes arises when a compound exists in two non-superimposable mirror image forms, much like a person's left and right hands. These isomers, known as enantiomers, have identical physical properties except for the direction in which they rotate plane-polarized light. A molecule must lack a plane of symmetry to display optical isomerism. For example, the cis isomer of Complex 1, \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\mathrm{Br}_{2}\right] \mathrm{Cl}\), can exhibit optical activity. In contrast, Complex 3, \(\left[\mathrm{V}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\), showcases optical isomerism due to its chelating ethylenediamine ligands, which create a chiral center at the vanadium ion.

Coordination Chemistry

Coordination chemistry involves the study of compounds formed between metal ions and ligands, where ligands are ions or molecules that can donate a pair of electrons to the metal. This interaction results in complex formation, characteristics of which are described by concepts such as coordination number, coordination polyhedron, and the type of ligands.

The coordination number is the number of ligand donor atoms to which the metal is directly bonded. For instance, in the complexes mentioned above, the coordination numbers range from 4 to 6, resulting in different geometries such as octahedral and square planar. The geometrical arrangement of the ligands around the central atom is called the coordination polyhedron. Understanding these factors is crucial as they influence the complex's properties such as color, magnetic behavior, and reactivity, which have vast applications in various fields including catalysis, material science, and biomedicine.