Question

Which complexes exhibit geometric isomerism? $$\begin{array}{l}{\text { a. }\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{OH})\right]^{2+}} \\\ {\text { b. }\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}} \\ {\text { c. }\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{2}\right]^{+}} \\\ {\text { d. }\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}_{3}\right]^{-}} \\ {\text { e. }\left[\mathrm{Pt}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{CN})_{2}\right]}\end{array}$$

Short answer

Complexes b \[\mathrm{Cr}(\mathrm{en})_2 \mathrm{Cl}_2\]^{+}, c \[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})(\mathrm{NH}_3)_3\mathrm{Cl}_2\]^{+}, and e \[\mathrm{Pt}(\mathrm{H}_2\mathrm{O})_2(\mathrm{CN})_2\] exhibit geometric isomerism.

Step-by-step solution

Understand Geometric Isomerism

Geometric isomerism occurs in coordination complexes when there is an opportunity for ligands to occupy different relative positions around the central metal atom. This is seen in complexes with coordination number 4 in a square planar or 6 in an octahedral arrangement, where the ligands can be arranged differently around the central atom. For geometric isomerism to occur, there must be at least two different types of ligands in such a way that there can be non-superimposable configurations.

Examine Complex a - \[\mathrm{Cr}(\mathrm{NH}_3)_5(\mathrm{OH})\]^{2+}

This complex has a coordination number of 6 and is octahedral. However, there is only one type of monodentate ligand (ammine) present, along with a hydroxo ligand. This creates a symmetrical arrangement with no possibility for cis or trans isomers. Therefore, this complex does not exhibit geometric isomerism.

Examine Complex b - \[\mathrm{Cr}(\mathrm{en})_2 \mathrm{Cl}_2\]^{+}

This complex has a coordination number of 6. It contains bidentate ethylenediamine (en) ligands and two chloride ligands. The bidentate ligands enforce a specific spatial arrangement that allows for the possibility of cis/trans isomerism of the chloride ligands, hence this complex does exhibit geometric isomerism.

Examine Complex c - \[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})(\mathrm{NH}_3)_3\mathrm{Cl}_2\]^{+}

This complex also has a coordination number of 6. It possesses different types of monodentate ligands, which allow for different spatial arrangements, resulting in possible geometric isomerism. Thus, this complex can exhibit geometric isomerism.

Examine Complex d - \[\mathrm{Pt}(\mathrm{NH}_3)\mathrm{Cl}_3\]^{-}

This complex has a coordination number of 4, and assuming a square planar geometry, three chloride ligands and one ammine ligand could potentially be arranged in differing cis or trans forms. However, because three ligands are identical, there are no different arrangements possible that result in unique isomers. Therefore, this complex does not exhibit geometric isomerism.

Examine Complex e - \[\mathrm{Pt}(\mathrm{H}_2\mathrm{O})_2(\mathrm{CN})_2\]

This complex also has a coordination number of 4, which can lead to square planar geometry. The presence of two different types of ligands (aqua and cyano) allows for cis and trans arrangements around the central metal atom. Therefore, this complex can exhibit geometric isomerism.

Key concepts

Coordination Complexes

Coordination complexes are compounds formed by central metal atoms or ions bonded to a group of surrounding molecules or ions called ligands. These ligands donate electron pairs to the metal, forming coordination bonds. The number of these bonds is known as the coordination number. Coordination complexes play significant roles in various fields, including biochemistry where they are essential in processes like oxygen transport and enzyme reactions.

In the context of geometric isomerism, coordination complexes with certain coordination numbers and types of ligands can exhibit different spatial arrangements of those ligands, leading to the existence of isomers. For example, a complex with a square planar or octahedral arrangement and at least two types of ligands can demonstrate such isomerism.

Ligands

Ligands are ions or molecules that bind to a central metal atom or ion in a coordination complex. Each ligand has at least one donor atom with a pair of electrons used to form a bond with the metal. Ligands are classified based on the number of bonding sites: monodentate ligands have one donor site, bidentate ligands have two, and polydentate ligands, known as chelates, have multiple bonding sites.

Ligands determine the geometry and the possible isomerism in coordination complexes. For instance, in geometric isomerism, especially cis/trans isomerism, monodentate ligands with varying types can occupy different relative positions around the central metal atom, leading to different isomers.

Octahedral Arrangement

An octahedral arrangement in coordination complexes refers to a geometric structure where six ligands symmetrically surround a central metal atom, forming octahedral geometry. All positions are equivalent in an idealized octahedron, but the ligands can still be arranged differently when there are at least two different types of ligands.

In such an arrangement, geometric isomerism becomes possible, as exhibited by the cis and trans forms. Specifically, in the octahedral complexes with cis/trans isomerism, ligands can be adjacent (cis) or opposite (trans) to each other. Not all octahedral complexes show this behaviour; it's reliant on the presence of different ligands and their unique spatial arrangements.

Cis/Trans Isomerism

Cis/trans isomerism is a form of geometric isomerism where the molecules differ in the position of atoms or groups relative to a double bond, a ring structure, or ligands in coordination complexes. In coordination chemistry, cis isomers have identical or similar ligands adjacent to each other, while trans isomers have them opposite each other in the coordination sphere.

This type of isomerism can significantly affect the properties of a complex, including its reactivity, color, and biological activity. It's crucial for students to understand that the mere presence of different ligands doesn't guarantee isomerism; the spatial arrangement must also enable distinguishable non-superimposable configurations to consider the presence of cis/trans isomerism.

Coordination Number

The coordination number of a metal in a coordination complex refers to the number of ligand donor atoms that are directly bonded to the metal. It's an important aspect affecting the geometry of a coordination complex. Common coordination numbers include 4 and 6, corresponding typically to square planar and octahedral geometries, respectively.

The coordination number plays a crucial role in determining whether a complex can exhibit geometric isomerism. For example, a coordination number of 6 can lead to octahedral complexes, which potentially can show cis/trans isomerism, while a coordination number of 4 in square planar complexes can also present this type of isomerism. It's essential to recognize that the specific arrangement of ligands directly influences the existence and type of isomers possible.