Question

Indicate what type of isomerism may be found in each of the following cases. If no isomerism is possible, so indicate. (a) \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right]^{+}\) (b) \(\left[\operatorname{Co}(\text { ox })_{2} \operatorname{Br}(\text { SCN })\right]^{3-}\) (c) \(\left[\mathrm{NiCl}_{4}(\mathrm{en})\right]^{2-}\) (d) \([\mathrm{PtBrCl}(\text { ox })]^{-}\) (e) \(\left[\operatorname{Cr}(\text { Cl })_{3}(\text { det })\right],\) det is \(\mathrm{H}_{2} \mathrm{N}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{NH}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{NH}_{2}\)

Short answer

For the given cases, (a) exhibits geometrical isomerism, (b) exhibits linkage isomerism, (c) doesn't exhibit any isomerism, (d) can exhibit geometrical isomerism, and (e) doesn't exhibit any isomerism.

Step-by-step solution

Case (a)

In \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right]^{+}\), each ethylenediamine (en) is a bidentate ligand hence it can lead to two types of isomers: facial (fac) and meridional (mer). Hence, geometrical isomerism is possible.

Case (b)

In \(\left[\operatorname{Co}(\text { ox })_{2} \operatorname{Br}(\text { SCN })\right]^{3-}\), the coordination compound has different ligands attached to the metal ion, which means isomerisation may occur. Specifically, this compound can exhibit linkage isomerism, as the thiocyanate ligand (SCN) can bind through either the sulphur or the nitrogen atom.

Case (c)

For \(\left[\mathrm{NiCl}_{4}(\mathrm{en})\right]^{2-}\), the complex has identical ligands (Chlorines). This means that the compound does not exhibit isomerism as there are no different ways that the ligands can be arranged.

Case (d)

In \([\mathrm{PtBrCl}(\text { ox })]^{-}\), the coordination compound has different ligands attached to the metal ion, hence it can exhibit geometrical isomerism due to different spatial arrangements of the ligands.

Case (e)

The \(\left[\operatorname{Cr}(\text { Cl })_{3}(\text { det })\right]\) complex contains a coordination complex – Chromium (Cr) – which is bonded to three chlorine (Cl) atoms and with det (H2N(CH2)2NH(CH2)2NH2) as a ligand. Here, no type of isomerism is possible as there are no different arrangements of the ligands around the metal ion.

Key concepts

Geometrical Isomerism

Understanding geometrical isomerism is fundamental when studying coordination compounds. This type of isomerism occurs due to the different possible spatial arrangements of ligands around the central metal atom.

Imagine a square, where the corners are occupied by ligands or groups of atoms; if we rearrange these corners, keeping the center (the metal) fixed, we may end up with distinct shapes - this is essentially geometrical isomerism. In octahedral complexes, which have six ligands, two common forms are facial (fac) and meridional (mer). In fac isomers, three identical ligands occupy one face of the octahedron, forming a triangle, while in mer isomers, they form a sort of 'T' shape across the middle.

This phenomenon was identified in the mentioned exercise case (a) for the complex \(\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right]^{+}\), where ethylenediamine (en) acts as a bidentate ligand, allowing for these two distinct arrangements. Additionally, in case (d), \(\left[\mathrm{PtBrCl}(\text { ox })\right]^{-}\), the presence of different ligands also means geometrical isomerism is possible, as there is more than one way to order the ligands around the metal ion.

Linkage Isomerism

Another fascinating aspect of coordination chemistry is linkage isomerism. This type of isomerism arises when a ligand can coordinate to the central metal atom in more than one way. This is typically seen with ambidentate ligands, which have two different atoms capable of donating an electron pair to the metal.

For example, in the exercise case (b), the thiocyanate ion (SCN-) can bind through the sulfur (S) or the nitrogen (N) atom. When SCN- coordinates through the sulfur atom, it's referred to as a thiocyanato-S complex, while coordination through the nitrogen atom results in a thiocyanato-N complex. Such flexibility in bonding leads to different compounds, which may have different properties.

The presence of linkage isomerism enhances the complexity and richness of coordination chemistry, providing a wide array of compounds from a single ligand based on its bonding preferences.

Stereochemistry in Coordination Complexes

Stereochemistry in coordination complexes deals with the arrangement of ligands in three-dimensional space around the central metal atom. It has profound implications on the chemical behavior and reactivity of complexes. In coordination chemistry, the spatial arrangement of ligands can influence the compound's color, magnetic properties, reactivity, and biological activity.

In the context of the exercise, understanding stereochemistry helps to explain why some complexes exhibit isomerism while others do not. For instance, case (c) \(\left[\mathrm{NiCl}_{4}(\mathrm{en})\right]^{2-}\) lacks isomerism because it has identical ligands, and flipping or rotating them does not create a new arrangement.

The stereochemistry of coordination complexes is dictated by factors such as the size of the ligands, the electronic configuration of the metal center, and steric hindrance, which altogether determine how ligands can be positioned in space. These considerations are critical for predicting and understanding the properties of coordination complexes.